Patrick Chase
8-Jan-2000, 19:29
Hi;
This is a continuation of a tangential discussion from the "Pros and cons of fie ld cameras" thread. In that thread, I wrote:
"Consider the simple manipulation of using front rise to prevent convergence. Co nvergence is a perspective effect, and front rise can be used to control converg ence. This example therefore proves that camera movements CAN alter perspective. Obviously, there are some limitations: In particular, camera movements cannot a lter the near/far size relationships of objects which are overlapped in the imag e (though you CAN alter the aparrent size relationships of objects which are sep arated in the image...)"
Larry Shearer then replied:
"Not that I am trying to cause more confusion for Alan then is already present,b ut Patrick how do you control convergence of a scene by raising the front standa rd? Alright I have to drop a name here e,ummm Ansel Adams states on more than once in "The Camera" that "True perspecti ve depends only upon the camera-to-subject distance." (page 106)."
Depends on what you define as "perspective". When Ansel Adams says "perspective" , he is defining it as the size relationship between near and far objects which overlap in the scene. He's right, but only within the narrow limits of his own d efinition. To see where that definition breaks down, consider the following exam ple: You have a near object on the left side of the image, and a far object on t he right. You want to change the relative sizes of those object without moving t he camera. Though Adams' comment appears to suggest that you cannot accomplish t his task, it is in reality a very simple matter of swinging the back standard su ch that the film plane moves away from the object you want rendered larger (you' ll then probably need to swing the front standard as well to establish focus). T he reason this works is because changing the relationship between the film plane and subject planes allows you to change the distance relationships between off- axis subjects, the lens, and the film plane without moving the camera. The key t hing to note here is that for this trick to work, there has to be separation in the image between the objects whose relative sizes you are trying to alter. If t hey're overlapped, then you're hosed.
Whether you call what I've described above "altering perspective" or not is enti rely a matter of semantics.
Just to be perversely confusing, I'll also throw out a case where camera movemen ts truly cannot alter perspective: When photographing with a fisheye lens.
To understand the answer to your specific question about correcting convergence, you need to understand how rectilinear lenses work: A rectilinear lens renders parallel lines in the subject as parallel in the image if and only if those line s lie in a plane which is parallel to the film. To use movements to eliminate co nvergence, you simply set the back of the camera parallel to whatever it is you' re photographing (for example, the side of a building) and then use rise/fall an d shift to establish the composition you want. Note that what I've described abo ve controls covergence on both the horizontal and vertical axes; If you're only worried about one or the other, then you only need to set the film plane paralle l to the subject along that one axis. Adams definitely explains this to some deg ree in "The Camera".
You might want to check out the Stroebel and Simmons books (Stroebel is encyclop edic; Simmons is more readable) on large-format technique.
-- Patrick
This is a continuation of a tangential discussion from the "Pros and cons of fie ld cameras" thread. In that thread, I wrote:
"Consider the simple manipulation of using front rise to prevent convergence. Co nvergence is a perspective effect, and front rise can be used to control converg ence. This example therefore proves that camera movements CAN alter perspective. Obviously, there are some limitations: In particular, camera movements cannot a lter the near/far size relationships of objects which are overlapped in the imag e (though you CAN alter the aparrent size relationships of objects which are sep arated in the image...)"
Larry Shearer then replied:
"Not that I am trying to cause more confusion for Alan then is already present,b ut Patrick how do you control convergence of a scene by raising the front standa rd? Alright I have to drop a name here e,ummm Ansel Adams states on more than once in "The Camera" that "True perspecti ve depends only upon the camera-to-subject distance." (page 106)."
Depends on what you define as "perspective". When Ansel Adams says "perspective" , he is defining it as the size relationship between near and far objects which overlap in the scene. He's right, but only within the narrow limits of his own d efinition. To see where that definition breaks down, consider the following exam ple: You have a near object on the left side of the image, and a far object on t he right. You want to change the relative sizes of those object without moving t he camera. Though Adams' comment appears to suggest that you cannot accomplish t his task, it is in reality a very simple matter of swinging the back standard su ch that the film plane moves away from the object you want rendered larger (you' ll then probably need to swing the front standard as well to establish focus). T he reason this works is because changing the relationship between the film plane and subject planes allows you to change the distance relationships between off- axis subjects, the lens, and the film plane without moving the camera. The key t hing to note here is that for this trick to work, there has to be separation in the image between the objects whose relative sizes you are trying to alter. If t hey're overlapped, then you're hosed.
Whether you call what I've described above "altering perspective" or not is enti rely a matter of semantics.
Just to be perversely confusing, I'll also throw out a case where camera movemen ts truly cannot alter perspective: When photographing with a fisheye lens.
To understand the answer to your specific question about correcting convergence, you need to understand how rectilinear lenses work: A rectilinear lens renders parallel lines in the subject as parallel in the image if and only if those line s lie in a plane which is parallel to the film. To use movements to eliminate co nvergence, you simply set the back of the camera parallel to whatever it is you' re photographing (for example, the side of a building) and then use rise/fall an d shift to establish the composition you want. Note that what I've described abo ve controls covergence on both the horizontal and vertical axes; If you're only worried about one or the other, then you only need to set the film plane paralle l to the subject along that one axis. Adams definitely explains this to some deg ree in "The Camera".
You might want to check out the Stroebel and Simmons books (Stroebel is encyclop edic; Simmons is more readable) on large-format technique.
-- Patrick