goamules
5-Jul-2014, 09:18
This may help people get more accurate maximum F-stop, lens speed measurements. The actual formula is often misinterpreted, and people mistakenly measure the physical iris inside the lens. Actually, the formula is Focal Length divided by Entrance Pupil. There is a difference, because the EP will take any magnification of the front glass into account.
Wrong method - measuring the physical iris itself will yield F4.4, (16 inch focal length divided by 3.6 inches).
https://farm6.staticflickr.com/5558/14393319220_72092b8b51_o.jpg
Correct method - measuring the apparent aperture looking through the front glass gives the Entrance Pupil. This yields F4.2, somewhat faster. This method is difficult because parallax makes it hard to see both ends of your ruler and make sure you are just measuring the circle of light you can see. I.E. you try to measure the internal iris seen through the glass. Do not simply measure the glass.
https://farm3.staticflickr.com/2934/14579983355_8dab3f6488_o.jpg
Also wrong method - measuring the physical glass does not account for vignetting caused by the internal barrel and the iris. This yields F3.5.
https://farm4.staticflickr.com/3882/14393319970_2f1630ba5c_o.jpg
The measurement that is technically correct for this lens is F4.2. What you will find is no lens ever (well, seldom) matches it's max speed rating exactly. Nor do the measurements for each aperture setting match exactly. Perhaps someone with more optical engineering than I can explain why. But this is the way I've been measuring old lenses (especially old Petzvals) for years.
https://farm3.staticflickr.com/2897/14556981486_211bd08160_o.jpg
Wrong method - measuring the physical iris itself will yield F4.4, (16 inch focal length divided by 3.6 inches).
https://farm6.staticflickr.com/5558/14393319220_72092b8b51_o.jpg
Correct method - measuring the apparent aperture looking through the front glass gives the Entrance Pupil. This yields F4.2, somewhat faster. This method is difficult because parallax makes it hard to see both ends of your ruler and make sure you are just measuring the circle of light you can see. I.E. you try to measure the internal iris seen through the glass. Do not simply measure the glass.
https://farm3.staticflickr.com/2934/14579983355_8dab3f6488_o.jpg
Also wrong method - measuring the physical glass does not account for vignetting caused by the internal barrel and the iris. This yields F3.5.
https://farm4.staticflickr.com/3882/14393319970_2f1630ba5c_o.jpg
The measurement that is technically correct for this lens is F4.2. What you will find is no lens ever (well, seldom) matches it's max speed rating exactly. Nor do the measurements for each aperture setting match exactly. Perhaps someone with more optical engineering than I can explain why. But this is the way I've been measuring old lenses (especially old Petzvals) for years.
https://farm3.staticflickr.com/2897/14556981486_211bd08160_o.jpg