I have a tophat extender which I have attached to my 210mm lens so that I can use the lens on my Ebony SW45.
I have looked through Stroebel's book but cannot find an answer to my query, which is:
What effect does increasing the image (film to lens) distance have, optically. Does it artificially alter the focal length of the lens, angle of view, coverage, depth of field etc etc? I am merely curious as to whether different compensatory exposure adjustments have to be made in the event of using the lens at long bellows extensions, or indeed in any other situations. Or is the difference too negligible to worry about
A query from one not steeped in the laws of optics!
John Fontana
All the tophat does is move the optical center of the lens relative to the front standard, enabling you to use a longer lens with a shorter bellows. You'll still have to compensate for exposure based on total extension (bellows plus tophat). If you have a 3" tophat, and a 210 (8 1/4") lens, you are focused at infinity with 5" of bellows extension. At 1:1 magnification, you'd have 16" of total extension (13" of bellows plus the 3" of your tophat), and have to add two stops to your exposure.
A short answer would be "don't worry about any of it."
The focal length and angle of view are inherent to the lens (design). Coverage is determined by the angle of view and changes depending on how far/close you focus, i.e. lens-film distance (imagine a cone of light projected from the rear of the lens toward the film). Depth of field is...well, it's more complicated but not affected except by the longer focal length of the lens. Bellows factor shouldn't be a factor (pun not intended) as you can focus a 210mm lens only close to infinity with SW45 (190mm bellows) and a tophat. DoF and BF would be a significant consideration if you do macro with 47mm SA or something.
If the appearance of the tophat is deceiving you, just ignore it and imagine your camera is now equipped with a longer bellows.
Another useful link in this forum is: http://www.largeformatphotography.info/articles/IntroToDoF.pdf
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