Choice for the Circle of Confusion?

[neil poulsen]

I'm starting a project to calculate my own DOF tables. And of course, it all comes down to one's choice for the circle of confusion, "c".

What's a typical choice for "c"? I'm getting my formulae from an article that's decades old (not sure of the source), and they recommend using c=0.033mm. Does this value sound about right? (Not sure.) I would probably select a single value of "c" for the negative. If I decide to take into account enlargement factors for different formats, I can use multiples of this value.

I plan to calculate separate tables for each lens, and for each f-stop. Each table would include the near and far distances of acceptable sharpness, and the hyperfocal length.

[Dan Fromm]

How much do you want to enlarge? That's what determines how fuzzy is too fuzzy.

[Tin Can]

This is interesting Neil.

I now need to shoot hyper-focal all the time in the field.

I plan to buy a digital rangefinder and calibrate scales on my field cameras.

Eyesight is requiring point and shoot LF as I learned yesterday shooting in high glare conditions. Even my 20X reversed Nikon 50 mm lens that works in studio was useless in the field.

[Jim Jones]

To make universal DOF tables we should consider all possible uses of all the interpretations of all the images of all the subjects we may photograph. That's quite a task. Better yet, study several of the experts who have researched and written extensively on the subject, such as Harold Merklinger. Then you will be able to derive rules of thumb that let you solve many DOF problems, perhaps without tables or calculators. Other photographers may prefer different solutions on the same occasion. For example, the hyperfocal distance is C multiplied by the diameter of the lens entrance pupil (the aperture as viewed through the front of the lens). C = maybe 1000 for casual work, perhaps less for a cell phone, and 2000 or more for critically sharp images. Another one: when using a lens for images to be viewed at a distance that provides correct perspective, stopping the aperture down below an entrance pupil of about 1/4 inch or 6mm invites unsharpness due to diffraction. If these rules sound too simple, remember that the mathematics of DOF involve only two parameters: the lens entrance pupil and the distance to the subject. Complicating this are the diverse preferences of photographers. If the rules of thumb cited above seem too simple, perhaps I erred in formulating them without the use of a calculator about 50 years ago, or perhaps the reader hasn't fully absorbed all that experts have written on DOF. It might be more convenient for most photographers to just rely on existing charts.

[Tin Can]

Jim, I just checked my Calter II 150mm f5.6 lens at f22 which I shot yesterday (https://www.largeformatphotography.info/forum/showthread.php?57170-In-Praise-of-5x7-Post-em-!) with the 2 5x7 I posted today. Close to 1/4". My scans look not sharp. I don't really care about scans and my technique is weak.

I will 'see' how contact prints look tomorrow.

[Doremus Scudder]

I have found the article on focusing the view camera right here on the LF home page to be quite helpful.

Here's the link: https://www.largeformatphotography.info/fstop.html

Best,

Doremus

[neil poulsen]

I have found the article on focusing the view camera right here on the LF home page to be quite helpful.

Here's the link: https://www.largeformatphotography.info/fstop.html

Best,

Doremus

Doremus,

Thanks! I didn't realize these recommendations were available.

c=0.033 for the negative it is. The article recommends 0.1mm for 4x5, and I figure that I enlarge by about 3X for this format. So, that's essentially two sources that make the same recommendation.

[Emmanuel BIGLER]

Hi Neil

Very often, recommendations for the choice of a circle of confusion as found on the Internet come with no justification nor indications of the validity for the recommended value.

0.033 mm is the classical value for 35 mm photography and DOF scales for 35mm standard lenses have been computed for decades based on this value.
The justification and rules to choose you own value for "c" are quite simple, and from a basic value, it is easy to modify if according to your specific needs.

First come the angular resolution of the human eye, a common value is an angle of 2 minutes of arc, this is approx 1/1720 radian 1/(30*180/3.14)
This is already quite stringent: 2 minutes of arc for a graduation means that each tick on a metric ruler with graduations in mm, as seen from a distance of 1.72 m, is separated by an angle of 1/1720.

Well, let's admit this starting value of 1/1720.

Now we look at a 8x10" print from a distance equal to the diagonal of the image format, i.e. about 30 cm / 12 inches.
The circle of confusion on the print is 300 mm / 1720 = 0.17 mm
Hence with a contact print recorded on a 8x10" film, in those viewing conditions, c=0.17mm.
For a 4x5", enlarged 2x, c = .17/2 = .085 mm
More generally, if D is the diagonal of the format, c = D/1720
Take D = 43 mm for 35 mm [24x36 mm] photography, we get 43/1720 = 25 microns
Take D = 50 mm like the focal lenght of so many 35 mm cameras, we get 50/1720 = 30 microns.

Now you are all set, if you work with the 4x5 format with 8x10" prints and only allow the public to look at your work from a distance of 30 cm /12 inches, take c = about 0.09 mm, but if you still allow a distance of 30 cm but on a bigger 16x20" print, you'll have to choose c = 0.09/2 = 0.045 = 45 microns. DOF will be very shallow under those conditions!

[neil poulsen]

Hi Neil

Very often, recommendations for the choice of a circle of confusion as found on the Internet come with no justification nor indications of the validity for the recommended value. . . .

Emmanuel,

Thanks very much; I can see this logic. It becomes apparent how depth of field extends originally from the print, and then to the negative. I wasn't very clear on that.

[Drew Wiley]

Decent pocket-sized electronic distance meters can now be had for $200 or so. More expensive ones with all kinds of bells n whistles and very long distance ranges are not necessary, cause you'll be in optical infinity territory way out there anyway. But don't go cheap. Anything for a hundred bucks or so will be an inaccurate worthless toy.

[Pere Casals]

I'm starting a project to calculate my own DOF tables.

It is perfect having the own DOF tables, but anyway this can be complemented by a good DOF app:

192744


Also think that calculated CoC / DOF are theoric and each lenses its particular behaviour. In cinematography they use through focus MTF charts.


IMHO it should be understood how DOF CoC is compared to the CoC of the lens in the perfect focus.

[MAubrey]

The original 0.033 for 35mm film assumes an 8x10 final print. So...like Dan says...


How much do you want to enlarge? That's what determines how fuzzy is too fuzzy.

[Bob Salomon]

The original 0.033 for 35mm film assumes an 8x10 final print. So...like Dan says...

And how far away will it be viewed at?

[Dan Fromm]

To summarize this discussion, in post #8 above Emmanuel Bigler told the OP, and very politely too, that it had asked too narrow a question. Emmanuel's answer provides a procedure for finding CoC given negative size, print size and the distance at which the print is to be viewed. CoC is clearly not a fixed number, one per film format. It depends on how much the negative is to be enlarged and the distance from which the print is viewed. Change either of these and the CoC will change. Ain't no one CoC that fits all situations.

From this it follows that in spite of the tremendous effort that's been put into DoF tables -- see A. A. Blaker's book Applied Depth of Field -- and calculators such as DOFMaster the enterprise is more than a little silly. Why calculate when you can (a) see a contact print's DoF on the ground glass and (b) see how enlargement will shrink it by swapping in a longer lens? Yes, I know, we already encumber ourselves with too much too heavy gear. That's what assistants are for.

[Bob Salomon]

To summarize this discussion, in post #8 above Emmanuel Bigler told the OP, and very politely too, that it had asked too narrow a question. Emmanuel's answer provides a procedure for finding CoC given negative size, print size and the distance at which the print is to be viewed. CoC is clearly not a fixed number, one per film format. It depends on how much the negative is to be enlarged and the distance from which the print is viewed. Change either of these and the CoC will change. Ain't no one CoC that fits all situations.

From this it follows that in spite of the tremendous effort that's been put into DoF tables -- see A. A. Blaker's book Applied Depth of Field -- and calculators such as DOFMaster the enterprise is more than a little silly. Why calculate when you can (a) see a contact print's DoF on the ground glass and (b) see how enlargement will shrink it by swapping in a longer lens? Yes, I know, we already encumber ourselves with too much too heavy gear. That's what assistants are for.
You can also see DOF at various magnifications on the gg depending on the magnification of the loupe that you use. However you would only see it in small parts of the gg that the loupe covers.

[Drew Wiley]

It seems he's having eyesight problems that make full use of a loupe unrealistic, Bob. Otherwise, all this circle of confusion talk could just be thrown out the window.

[Pere Casals]

You can also see DOF at various magnifications on the gg depending on the magnification of the loupe that you use. However you would only see it in small parts of the gg that the loupe covers.

Yes, but GG cannot display by far (because grain) all Image Quality that can be recorded on film, so we won't be able to judge the finest effects.



Emmanuel's answer provides a procedure for finding CoC given negative size, print size and the distance at which the print is to be viewed. CoC is clearly not a fixed number, one per film format. It depends on how much the negative is to be enlarged and the distance from which the print is viewed. Change either of these and the CoC will change. Ain't no one CoC that fits all situations.

Yes... first step is what Emmanuel pointed to reduce the conceptual complexity, if we consider a print size we also know what diffraction we may add to also extend DOF to not damage required IQ in the focus plane.

There is a balance, as we extend DOF to improve the rendering of certain spots we also stop the aperture, adding diffraction. Really, it's not that easy (or possible) to make an optimal shot if not knowing the print size,

Of course it's quite important to know what's the critical CoC depending on negative size vs print size. The enlargement factor tells how the "in negative CoC" also will be enlarged, so we only need to say what CoC we want on the print and dividing by the enlargement factor to have our CoC on film.

A problem is that often we don't know how large we'll print.


Then next step would be understanding how, for a very big print, we damage IQ in the focus plane as we extend DOF by stopping, in that case we should consider our lens performance.

And, of course, we have the Scheimpflug, which adds additional complexity to any calcualtion.


IMHO this is a very interesting matter, LF has insane IQ, but managing to have an optimal shot it's not that easy, anyway we may have an IQ overkill.

[Bob Salomon]

“Yes, but GG cannot display by far (because grain) all Image Quality that can be recorded on film, so we won't be able to judge the finest effects.”

What finest detail would bother you? By definition DOF is the “apparently” sharp area in front and behind the subject. That’s all it is.

[Pere Casals]

“Yes, but GG cannot display by far (because grain) all Image Quality that can be recorded on film, so we won't be able to judge the finest effects.”

What finest detail would bother you? By definition DOF is the “apparently” sharp area in front and behind the subject. That’s all it is.


Bob, the problem if that we won't see in what extent we damage ultimate IQ in the focus plane when stopping for DOF. Say we stop f/32, diffraction limit is 50cycles/mm but it also damages contrast at 25 cycles/mm. We won't see this in the GG, because GG graininess won't allow it.

Also if we have to inspect the GG at f/32 then we have a very dim image.


Also we may not be able to judge how sharp is the “apparently” sharp area, also because film records more than the GG allows to see.


A way to learn it without wasting film is attaching a DSLR (without lens) in the camera back, (I did it by fixing a macro extension ring in a Sinar lens board, and placing the resulting Nikon F mount in the Norma back.) A DX consumer DSLR sports 250 pix/mm which is good to explore DOF vs focus plane IQ.

That way does not substitute tables, apps and calcultions, but's a first hand direct view of the real effects.

[Bob Salomon]

Bob, the problem if that we won't see in what extent we damage ultimate IQ in the focus plane when stopping for DOF. Say we stop f/32, diffraction limit is 50cycles/mm but it also damages contrast at 25 cycles/mm. We won't see this in the GG, because GG graininess won't allow it.

Also if we have to inspect the GG at f/32 then we have a very dim image.


Also we may not be able to judge how sharp is the “apparently” sharp area, also because film records more than the GG allows to see.


A way to learn it without wasting film is attaching a DSLR (without lens) in the camera back, (I did it by fixing a macro extension ring in a Sinar lens board, and placing the resulting Nikon F mount in the Norma back.) A DX consumer DSLR sports 250 pix/mm which is good to explore DOF vs focus plane IQ.

That way does not substitute tables, apps and calcultions, but's a first hand direct view of the real effects.

Why do you keep reinventing the wheel? Photographers controlled DOF accurately and easily well before digital came about.
Their are numerous manual DOF devices for calculating and controlling DOF like the Rodenstock pocket calculator, DOF control rings on Sinar and Linhof cameras and the Rodenstock pocket one also computes Scheimpflug.

More importantly these systems calculate the proper position for the rear standard to focus on the correct point for maximum DOF. Eliminating the beginner mistake of focusing on the subject and losing most of the possible DOF and then stopping down too far to regain the lost area but ending in diffraction!

Get your self the pocket Rodenstock and do it easily, no batteries, no electronics, no rulers needed!

[Pere Casals]

Why do you keep reinventing the wheel? Photographers controlled DOF accurately and easily well before digital came about.

Bob, at all I'm not saying that attachig a DSLR in the back is the way to adjust DOF in the field, just I'm saying that's a very useful exercise to learn very well that matter from direct view of the real effects. Me, I learned a lot about what matters and what not with this exercise.

First hand, I can say that this way helped me to understand/learn easily and fast the DOF/diffraction effects.

It is a WYSIWYG, as we stop we inmediately see in the monitor the blurring effect of diffraction in the focus plane, and the sharpening effect in the DOF, so it's useful to learn the trade-off and the good balance.





Get your self the pocket Rodenstock and do it easily, no batteries, no electronics, no rulers needed!

Well, this is very good advice, no doubt.

[Larry Gebhardt]

I agree with Pere that looking at large format lenses with a high res DSLR can be a useful exercise. For example it showed me that the 180mm Renstock Sironar S 180mm at f/8 is as good as the native glass I have for the Sony A7RIII (42MP) at a pixel level. That goes against a lot of what I've seen published. The Nikon 180mm lens is also as good (as measurable by the test) in the center, but seems to have some field curvature as you move towards the edges. You can of course also see the effect of stopping down and resolution does indeed drop as you stop down, as we all know with out needing to test it.

But I don't see much benefit to the test in a practical manner for composing large format photos, other than letting you know you want to use a large aperture that still meets your depth of field needs. If I know I need F/32 to get the whole subject in the acceptable DOF then that's what I need. I also know there will be upper limits to how large a print I can make that's "acceptably sharp". That balance is the heart of this thread.

I really like the approach outlined in the article, https://www.largeformatphotography.info/fstop.html, linked by Doremus Scudder in answer #6. For me that's all I want to consider while shooting.

[Tin Can]

It's get confusing as some wrote in this old LFPF thread. (https://www.largeformatphotography.info/forum/showthread.php?24471-Rodenstock-depth-of-Field-Calculator-and-Sinar-Norma&p=229474&viewfull=1#post229474)

Bob often suggests the Rodenstock Depth of Field Calculator (https://www.bhphotovideo.com/c/product/155145-REG/Rodenstock_260700_Depth_of_Field_Calculator.html) which is available at that link, but seemingly out of production back in 2007 as indicated in the early post.

I cannot find out how big the circular slide rule is, one says it's made of plastic. Any thoughts on durability?

I may have difficulty reading the fine print on the slide rule in the field.

As I recall from my brief Sinar P experience there are scales on a P to use for deciding ideal focus directly on GG the near and far points.

Does a Norma have this function also?

The slide rule costs more than my reliable new Walmart iPhone SE which has zoom to read fine print enabling an App to be read anywhere, even in the dark.

Plus, having the slide rule instructions seems very important.

I grew up using slide rules...the only one I have now calculates compound interest and mortgages.

So we ask for more discussion.

[neil poulsen]

Why do you keep reinventing the wheel? . . .


There will always be an endeavor to integrate the new with the old. Even if a given effort isn't fruitful, at least different avenues are being explored. And, who's to say what other possibilities might not emerge?

[Pere Casals]

that looking at large format lenses with a high res DSLR can be a useful exercise

It also can be useful to check exposure. If lacking a shutter tester we may set B position in the DSLR (that's in the back of the view camera) and firing the LF shutter, then comparing the image with the the LF shutter open in B and firing the DSLR. Also it can check the aperture calibration, and it would also work as a TTL spot meter for the view camera that accounts for bellows extension and lens transmission, working like a probe.

Again this would not be much for the field, but it would be useful to check/adjust gear.




I really like the approach outlined in the article, https://www.largeformatphotography.info/fstop.html, linked by Doremus Scudder in answer #6. For me that's all I want to consider while shooting.

+1

But I'd add using a DOF App in the smartphone and the pocket Rodenstock calculator, it can be found used.

[Bob Salomon]

It's get confusing as some wrote in this old LFPF thread. (https://www.largeformatphotography.info/forum/showthread.php?24471-Rodenstock-depth-of-Field-Calculator-and-Sinar-Norma&p=229474&viewfull=1#post229474)

Bob often suggests the Rodenstock Depth of Field Calculator (https://www.bhphotovideo.com/c/product/155145-REG/Rodenstock_260700_Depth_of_Field_Calculator.html) which is available at that link, but seemingly out of production back in 2007 as indicated in the early post.

I cannot find out how big the circular slide rule is, one says it's made of plastic. Any thoughts on durability?

I may have difficulty reading the fine print on the slide rule in the field.

As I recall from my brief Sinar P experience there are scales on a P to use for deciding ideal focus directly on GG the near and far points.

Does a Norma have this function also?

The slide rule costs more than my reliable new Walmart iPhone SE which has zoom to read fine print enabling an App to be read anywhere, even in the dark.

Plus, having the slide rule instructions seems very important.

I grew up using slide rules...the only one I have now calculates compound interest and mortgages.

So we ask for more discussion.

It is now called the Linos calculator as Rodenstock Precision Optical was sold to Linos and then Linos was sold to Qioptiq.

The scale is about the width of a dress shirt pocket as the calculator easily fits in one, in its case. One side calculates DOF at several scales for 35mm to 810. So it can also calculate DOF for DSLR FF Cameras. It also calculates the angle of the camera to the subject.

The opposite side calculates Scheimpflug for the same film sizes and also camera angles.
Lastly the calculator will indicate how much exposure correction might be required, depending on the chosen image scale.

The scales are printed in very legible type on the calculators and are easily readable even with my 78 year old eyes with or without my glasses.

While it comes with a detailed instruction sheet the designer, Dr, Schön, numbered each step to do the calculation determination and even his some pictograms to show what each step is doing on the calculator.

Unlike dragging a DSLR out with you to do the DOF the Linos weighs about 2 or 3 ozs and fits flat in your shirt pocket. In all the years that we distributed them we never had one wear out nor had we ever had to replace one. They are extremely durable and easily cleaned off, if necessary. Rodenstock also supplied them OEM for other European companies like Sinar and Linhof without reports of problems, save for people who didn’t read the instruction sheet. But simple, short telephone conversations fixed those problems.

[Bob Salomon]

It also can be useful to check exposure. If lacking a shutter tester we may set B position in the DSLR (that's in the back of the view camera) and firing the LF shutter, then comparing the image with the the LF shutter open in B and firing the DSLR. Also it can check the aperture calibration, and it would also work as a TTL spot meter for the view camera that accounts for bellows extension and lens transmission, working like a probe.

Again this would not be much for the field, but it would be useful to check/adjust gear.





+1

But I'd add using a DOF App in the smartphone and the pocket Rodenstock calculator, it can be found used.

It would seem that if you wanted to use a DSLR on a view camera as a spot meter then you would be constantly moving the view camera to meter different spots on the scene.
It would be much easier to just use a spot meter in the first place.

[Tin Can]

Good answer Bob. I like things that fit in a shirt pocket.

My eyes used to focus on anything very close, such as a GG, but since my cataracts were removed and far sight 10 diopters lenses installed in both eyes, close up now requires at least 3X readers.

And 20X loupe seems best, but I hate using my Nikon AF 50mm f1.4 as field loupe. It will get damaged.

I am going to start another loupe thread.



It is now called the Linos calculator as Rodenstock Precision Optical was sold to Linos and then Linos was sold to Qioptiq.

The scale is about the length of a dress shirt pocket as the calculator easily fits in one, in its case. One side calculates DOF at several scales for 35mm to 810. So it can also calculate DOF for DSLR FF Cameras. It also calculates the angle of the camera to the subject.

The opposite side calculates Scheimpflug for the same film sizes and also camera angles.
Lastly the calculator will indicate how much exposure correction might be required, depending on the chosen image scale.

The scales are printed in very legible type on the calculators and are easily readable even with my 78 year old eyes with or without my glasses.

While it comes with a detailed instruction sheet the designer, Dr, Schön, numbered each step to do the calculation determination and even his some pictograms to show what each step is doing on the calculator.

Unlike dragging a DSLR out with you to do the DOF the Linos weighs about 2 or 3 ozs and fits flat in your shirt pocket. In all the years that we distributed them we never had one wear out nor had we ever had to replace one. They are extremely durable and easily cleaned off, if necessary. Rodenstock also supplied them OEM for other European companies like Sinar and Linhof without reports of problems, save for people who didn’t read the instruction sheet. But simple, short telephone conversations fixed those problems.

[Bernice Loui]

Challenge becomes translating the target circle of confusion into meaningful image elements for any given image to be made?


Bernice

[Pere Casals]

It would seem that if you wanted to use a DSLR on a view camera as a spot meter then you would be constantly moving the view camera to meter different spots on the scene.
It would be much easier to just use a spot meter in the first place.


Of course, a good spot meter is a perfect choice, but I've not one.

Using a DSLR/SLR in the back of a view camera is not for everyday, IMHO, but it allows to calibrate and check many things. In fact a (near for free) Nikon F65 in the back is a perfect Probe meter accounting all, lens transmission, aperture calibration, fall-off, bellows extension... It has a viewfinder so it's very convenient to navigate in the framing with the shift-rise.

Also I can use the DSLR/SLR (+zoom) as if it was a Director's Scope that will tell if framing fits in the negative (Hor and Vert eq focals in the zoom), so when I haul the cambo 8x10 and my insanely heavy Bilora is just to shot, this way I avoid dragging around all those irons to find the good shot.

Also, as DOF rules are the same, in the zoom I may place the same focal and aperture, and I can check what will happen in the 8x10 shot. So I focus to the intended distance, and with focus blocked I can explore and (DSLR) shot the rest of the field, by zomming in the rear screen of the DSLR I see how the rest will be compared to the object in the focus plane (if no tilt).

I'm not saying that this has to be the regular procedure, at all, just it's useful for a learner like me.




Challenge becomes translating the target circle of confusion into meaningful image elements for any given image to be made?

At all, we can make top notch art with an smartphone in auto mode.


An artist may take advantage from softness, from sharpness or from both at the same time. But there is nothing wrong in mastering DOF management, if one wants that.

[Bob Salomon]

Of course, a good spot meter is a perfect choice, but I've not one.

Using a DSLR/SLR in the back of a view camera is not for everyday, IMHO, but it allows to calibrate and check many things. In fact a (near for free) Nikon F65 in the back is a perfect Probe meter accounting all, lens transmission, aperture calibration, fall-off, bellows extension... It has a viewfinder so it's very convenient to navigate in the framing with the shift-rise.

Also I can use the DSLR/SLR (+zoom) as if it was a Director's Scope that will tell if framing fits in the negative (Hor and Vert eq focals in the zoom), so when I haul the cambo 8x10 and my insanely heavy Bilora is just to shot, this way I avoid dragging around all those irons to find the good shot.

Also, as DOF rules are the same, in the zoom I may place the same focal and aperture, and I can check what will happen in the 8x10 shot. So I focus to the intended distance, and with focus blocked I can explore and (DSLR) shot the rest of the field, by zomming in the rear screen of the DSLR I see how the rest will be compared to the object in the focus plane (if no tilt).

I'm not saying that this has to be the regular procedure, at all, just it's useful for a learner like me.





At all, we can make top notch art with an smartphone in auto mode.


An artist may take advantage from softness, from sharpness or from both at the same time. But there is nothing wrong in mastering DOF management, if one wants that.

As a learner you excel at making things far more difficult and complicated then they really need to be!

For decades their have been very easy and accurate means of metering very small spots to the entire gg. One common system was the Prontor Probe with a Gossen Profi Six or Lunapro, as well as some other meters that metered in the film plane, this probe is what Sinar and Linhof sold.
Or by metering through the gg and, if in place the Fresnel, with most any meter that accepted either a microscope adapter or a gg adapter.

They were easy to use and dead accurate.

[Pere Casals]

As a learner you excel at making things far more difficult and complicated then they really need to be!

:) Sure !!!

A noetic approach usually fails, but it also can deliver inventions.



For decades their have been very easy and accurate means of metering very small spots to the entire gg. One common system was the Prontor Probe with a Gossen Profi Six or Lunapro, as well as some other meters that metered in the film plane, this probe is what Sinar and Linhof sold.
Or by metering through the gg and, if in place the Fresnel, with most any meter that accepted either a microscope adapter or a gg adapter.

They were easy to use and dead accurate.

Of course, but what I had at hand was slr and a dslr cameras. For negatives I had no problem, but with velvia is different. I adjusted my exposure style making bracketings with a Nikon F5, by placing the F5 in the back I could adjust the metering style for the view camera.

Don't think that placing say a $15 Nikon F65 in the back is difficult, it's just a plate with a macro extension or reverse ring bolted. The F65 spot meter is absolutely dead on, and you have a viewfinder with an eyecup, you don't need the darkcloth. That solution is easy and totally accurate, technically good, but of course it would be a total surprise to see John Sexton using that, it would be viral !

Well, this is testing TTL metering in a view camera for $15

[neil poulsen]

This has all been very interesting, and I've gained a better understanding of circles of confusion. (This name really kills me.:)) I appreciate everyone's contribution to this thread. With that said, I'm thinking of the following.

It becomes apparent that choosing a practical circle of confusion comes down to determining the viewing distance from the print and knowing the human eye's angular resolution.

I like aspects of Tuan's article.

https://www.largeformatphotography.info/fstop.html

For one thing, it gives a reasonable derivation of c=0.033, which has been used historically for 35mm film. It's based on making a 6"x8" enlargement, and then viewing it at a "reading distance" of about 10". Kingslake [Lenses in Photography, 1963] has suggested that a "normal" viewing distance of a print is approximately the diagonal of the print. 10" is exactly the diagonal of a 6x8 print. This is also consistent with what Emmanual has suggested, that we view an 8x10 contact print at about 12". In fact, that's very close to the diagonal of the image area of an 8x10 negative.

As to the angular resolution, Tuan's article states that we can perceive at best, 0.2mm difference in two lines at a reading distance of 25cm. (About 10".) This corresponds to an angular resolution of 1/1250 radians. In comparison, Emmanual has suggested an angular resolution of 1/1720 radians, which he states is quite a stringent value. Kingslake has suggested that, under research conditions, the angular resolution is 1/2300 radians. But he counters that, practically speaking, it's closer to 1/1000 radians. So, I tend to lean towards 1/1250 radians as being somewhat conservative, but not stringently so.

It's been implied in this thread and in Tuan's article, that the circle of confusion should depend on print size. But, I'm uncomfortable with this. Taken to the extreme, that would suggest that we would view a 16x20 print at the same distance that we view an 8x10, or a 6x8 print. I think that, as we view larger and larger prints, we tend to step back. Of course, one can view any print up close; but, I don't wish to base a circle of confusion on this type of close examination. On the other hand, it seems that circles of confusion should definitely be based on film format size.

Given an angular resolution of 1/1250 radians and a viewing distance of 25cm, one gets a working circle of confusion of 0.2mm. Using multipliers for different formats, I can now construct DOF tables that give near and far distances of acceptable sharpness for each lens at each each f-stop. Each table would consist of three pairs of numbers, near and far for medium format, for 4x5, and for 8x10.

[reddesert]

Eyesight is requiring point and shoot LF as I learned yesterday shooting in high glare conditions. Even my 20X reversed Nikon 50 mm lens that works in studio was useless in the field.

Randy, since this came up a couple of times: a 50mm lens is not a 20x loupe. It's a 5x loupe. The magnification of a loupe is defined as 250mm/focal length. (The idea being, I guess, that a person with good eyesight can focus the unaided eye at 250mm, and the loupe magnifies X times that.) If the 50mm lens isn't close enough for you, maybe try an 8x or 10x loupe.

You might be thinking of diopters: diopters are 1000mm/focal length, so a 50mm lens is a +20 diopter.

[Tin Can]

Thanks for the correction. You are correct I contused diopters.

Going through boxes searching for a lesser 50mm to use. Tiny Repronar 5cm with a spacer tube may work.


Somewhere I have a couple 50mm enlarging lens to try. The good one stays in the enlarger.






Randy, since this came up a couple of times: a 50mm lens is not a 20x loupe. It's a 5x loupe. The magnification of a loupe is defined as 250mm/focal length. (The idea being, I guess, that a person with good eyesight can focus the unaided eye at 250mm, and the loupe magnifies X times that.) If the 50mm lens isn't close enough for you, maybe try an 8x or 10x loupe.

You might be thinking of diopters: diopters are 1000mm/focal length, so a 50mm lens is a +20 diopter.

[Drew Wiley]

It's amazing how much confusion can be packed into a tiny little circle! I can understand the concept of circle of confusion if a halftone image, along with text, is printed in a book intended to be read from a given distance. But never once in my own photographic work have I seen any need whatsoever to engage that nebulous concept in order to efficiently manage either depth of field issues or detail. Just one more unnecessary convoluted hurdle.

[Pere Casals]

1/1250 1/1720 1/2300 1/1000 radians.

It should be noted that eye resolving power is not constant.

> Of course it depeds on the subject, a 20/20 subject has good eyes.

> It depends on the target contrast, an slide may show 1:4000 contrast, a paper may show 1:100, but in an image our fine detail may have 1:2 contrast.

> It depends on illumination, our eye varies the f/ aperture in the iris depending on light, the eye also has an optimal aperture.

[Emmanuel BIGLER]

From Neil:
...that the circle of confusion should depend on print size. But, I'm uncomfortable with this.

Hi Neil.
You should not be uncomfortable with the idea of varying diameters for circles of confusion.

Another historical example. For decades, amateurs used 6x9 [cm] rollfilm cameras with a 100-105 mm lens.
If you reverse-engineer DOF scales on those venerable cameras (see just below), you'll find that very often DOF scales were computed for a circle of confusion of 100 microns, i.e. D/1000 or f/1000 (those cameras had a fixed standard lens with f=D).
And this did make sense, because most often images were only contact prints of 6x9 cm size for the family album, viewed at a distance of about 30 cm.
If you use a modern 100 mm view camera lens, with resolutions limits well above 60 cy/mm, those 100 microns are really fuzzy with respect to what you can record with a 100 mm lens. If you allow your prints to be scrutinized to the the tiniest details, then you should choose your CoC according the to the ultimate lens resolution capability :-)
But if you only make 2"x3" contact prints, 100 microns will be the good CoC and you'll get plenty of DoF!


--------

Reverse-engineering an engraved DOF scale is straightforward
Choose the smallest available aperture e.g. 22, rotate the focusing ring so that the infinity sign is facing "22".
In the middle of the DOF scale you can read the hyperfocal length H and in principle facing the other "22" marking you can read H/2.
This allows you to find H(N) the hyperfocal distance for the f-number N.
Coming back to basic maths of DOF scales with H(N)=f2/(Nc), you get
c = f2/(N H(N))

I have measured the following values for "c" (sorry this is off-topic medium format, but illustrates the above demonstration)

Rolleiflex T TLR lens tessar 3.5-75mm H(22)=5m ==> coc=51 microns

Zeiss Hasselblad Planar CF100 model 1982 : H(22)=6m coc=75microns strange value; not very stringent !
Zeiss Hasselblad Distagon C 50 Sync-Compur: H(22)=2.2m coc=50microns standard classical for 6x6
Zeiss Hasselblad Sonnar C 250 chrome Synchro-Compur model ~ 196x : H(45)=24m ==> coc=60microns

From Wildi's book :
Zeiss Hasselblad Planar 2,8-80 (C?)/Wildi : H(22)=4.9 coc=60microns OK but lens is credited to pass about 100 cy/mm ...

[Jac@stafford.net]

What a bundle of silly, irrelevant key-board metrics this has become. You have a lens, make a few exposures at various apertures, print and decide. Real life experience - what a concept!

[Tin Can]

I can no longer trust what I see. Nothing is sharp...




What a bundle of silly, irrelevant key-board metrics this has become. You have a lens, make a few exposures at various apertures, print and decide.

[Jac@stafford.net]

I can no longer trust what I see. Nothing is sharp...

FWIW, my focusing eye is worthless. At some point is is good to ask ourselves for whom do we make images that we cannot experience. I am about done with it all.

[Pere Casals]

And this did make sense, because most often images were only contact prints of 6x9 cm size for the family album, viewed at a distance of about 30 cm.


Yes.. the print size critical to find the optimal aperture.

If I understand it correctly, if we know the print size (and viewing distance) then we know the aperture that starts damaging IQ in the focus plane, so we can extent DOF until that point without damaging the print.

With the same viewing distance, as the print is larger we can stop less before we damage IQ in scene's the focus plane.


But when we take a shot we may not know what print size we'll have... if the image is very good then we may want a big print.

[Tin Can]

Never quit! i am working on set up for full blind LF photography.

I lurk on Blind Ham Radio forums to prepare for the inevitable.

Maybe 2 more years of day driving, unless something improves.

No night driving for a while...

I still shoot righty with my left eye...and my brother was opposite.
Father confused us with his both eyes open approach. Which never worked for me. So I shot even shotgun with left eye. Which he considered idiocy. He was very good at knocking down 3 birds quickly. I stopped hunting with him. RIP


FWIW, my focusing eye is worthless. At some point is is good to ask ourselves for whom do we make images that we cannot experience. I am about done with it all.

[Drew Wiley]

I can certainly sympathize with people still trying to do what they love yet with diminishing sight. It takes courage as well as some kind of plan. But there's a major museum dedicated to how Monet painted things as he actually saw them with impaired sight during old age. I'm not suggesting to do things the same manner, but that where there is a will, there is a way.

[MAubrey]

And how far away will it be viewed at?

Details.

[Tin Can]

Most people crowd around one of my favorite paintings, so many do it's hard to see it at all. They go to the rope. Fools!

But if I stop on the stairs just at the room entrance this 8X10 ft painting looks best at 30 ft.
(https://www.artic.edu/artworks/27992/a-sunday-on-la-grande-jatte-1884)




Details.

[Drew Wiley]

Was Seurat the inventor of pixels or grainy color film?

[neil poulsen]

Yes.. the print size critical to find the optimal aperture. . . .

The constant is the circle of confusion, on which the COC is based. As people back away from an increase in print size (to "better" see the print), the COC must also increase.

What I'm suggesting is that, as the enlargement factor increases, one needs to decrease the circle of confusion proportionately. But if the viewer backs up proportionally to the increase in print size, then one needs to increase the circle of confusion by the same proportion. The decrease above in the COC offsets the simultaneous increase in the COC.

Of course, this assumes that the viewer backs up in proportion to the increase in print size. As an approximation, I'm willing to make this assumption.

[neil poulsen]

Hi Neil.
You should not be uncomfortable with the idea of varying diameters for circles of confusion. . . .

Emmanuel,

Thanks again. This is interesting.

I know that the COC needs to change as the format changes.

But, I can also see adjusting the COC for different lenses. Although, I'm not sure how to evaluate a given lens to make this change.

[Pere Casals]

But if the viewer backs up proportionally to the increase in print size, then one needs to increase the circle of confusion by the same proportion.


Yes... the question is if we want a big print to be perfect at reading distance, or if it's enough if the print is seen perfect at the "ususal" viewing distance.


If the print a at distance that we see it all, without moving head, then the print requires around 60MPix efective, not mattering the print size, so required circle on negative is constant. If we want to inspect the print at reading distance then, obviously, we require an smaller CoC in teh negative as print size increases.


A human eye can see around 8MPix effective, but resolving power is very concentred in the fovea, so we can see aroung 60MPix effective if we don't move the head but we move the eyes exploring the field of view we can see without moving the head.


To me a big print does not require the same perfection at reading distance than an small one, but LF allows to make monster prints that are perfect at reading distance, those prints are impressive !!!!

[Drew Wiley]

Whhhhat, Neil? I know what you're implying regarding circle of confusion; but some of us might like our prints worthy of viewing both up close and at a distance. I'm stating this for sake of argument, and not just in relation to the poster's special requirements or your response to it. But it is how I actually approach the issue. I simply don't believe in any of that "normal viewing distance" nonsense. Do we need to place a gold nylon rope around our framed work, along with a security guard, to keep people at a prescribed distance? You stated things as if, the bigger the print is, the less sharp we need to make it, which of course would be a misrepresentation of what you probably meant; but it still came across that way.

[Doremus Scudder]

I approach the CoC issue from another perspective. The goal when making negatives is to find the optimum compromise between depth-of-field and diffraction, i.e., finding the aperture that gets the job done while providing the most possibility for enlargement. Scenes with a very shallow depth-of-field end up allowing for a great deal of enlargement, but often a scene will require a very small aperture to get everything acceptably sharp, thereby limiting the degree to which it can be enlarged. I made tables years ago, using a rather small CoC, which show the maximum acceptable (for me) print size for the aperture used. This has served me well over the years.

FWIW, f/32 for 4x5 work makes stunning 11x14-inch prints and enlarges well to 16x20. Even the occasional 20x24 print works if the viewing distance is not too close. The proof is really in the enlargement itself.

Best,

Doremus

[Drew Wiley]

As formats get bigger and lenses longer, and as images get enlarged to greater sizes, depth of field issues require more and more strategic thought. There are numerous factors and options. But a blanket approach or formula might prove counterproductive, both in a mechanical and esthetic sense. With long-lens 8x10 work particularly, which I tend to enlarge, I find that it is important to prioritize what parts of the scene need to be in acute focus, and which are subsidiary, in a support role, so to speak. I'd generally rather do that than just throw a blanket over the whole thing in terms of theoretical diffraction limits, increasingly confusing circles of confusion, and so forth. Contact printers might logically approach the problem from a different angle. But when I contact print, it's apt to be with the same negs that I enlarge, and intend to be versatile.

[neil poulsen]

. . . I made tables years ago, using a rather small CoC, which show the maximum acceptable (for me) print size for the aperture used. This has served me well over the years. . . .

Thanks Doremus! . . . Of course, that's the "obvious" solution. (Not so obvious to me, I guess.:)) As it is, I will have tables for each of three formats. I couldn't see, in addition, having tables for multiple prints sizes. It also helps that, I tend to print small.

I appreciate everyone's input. This has been quite a productive thread.

[Tin Can]

Bob,

I have an original NOS Rodenstock DOF Calculator in hand, with case and 2 pages of fine print instructions. In German and English.

I am studying!

Thank you Alan!

And Bob Salomon


It is now called the Linos calculator as Rodenstock Precision Optical was sold to Linos and then Linos was sold to Qioptiq.

The scale is about the width of a dress shirt pocket as the calculator easily fits in one, in its case. One side calculates DOF at several scales for 35mm to 810. So it can also calculate DOF for DSLR FF Cameras. It also calculates the angle of the camera to the subject.

The opposite side calculates Scheimpflug for the same film sizes and also camera angles.
Lastly the calculator will indicate how much exposure correction might be required, depending on the chosen image scale.

The scales are printed in very legible type on the calculators and are easily readable even with my 78 year old eyes with or without my glasses.

While it comes with a detailed instruction sheet the designer, Dr, Schön, numbered each step to do the calculation determination and even his some pictograms to show what each step is doing on the calculator.

Unlike dragging a DSLR out with you to do the DOF the Linos weighs about 2 or 3 ozs and fits flat in your shirt pocket. In all the years that we distributed them we never had one wear out nor had we ever had to replace one. They are extremely durable and easily cleaned off, if necessary. Rodenstock also supplied them OEM for other European companies like Sinar and Linhof without reports of problems, save for people who didn’t read the instruction sheet. But simple, short telephone conversations fixed those problems.

[Bob Salomon]

Bob,

I have an original NOS Rodenstock DOF Calculator in hand, with case and 2 pages of fine print instructions. In German and English.

I am studying!

Thank you Alan!

And Bob Salomon

Enjoy!