Hello There - I am new to the forum. To answer your question on how to address the bellow extension as it affects exposure... this is what I came up with. The F stop number is a calculation of the diameter of the len's design. F means fraction in this case. So F8 means 1/8 of one. One being the actual size of the lens opening which would equal the focal length of the lens. The focal length being the distance between the lens and the film plane when it is focused on infinity. That means F8 is only F8 when the lens is focused on infinity. So a 380mm lens will produce a 380mm length of the bellows when focused at infinity. Thus whatever you focus on just measure the length of the lens to film plane to calculate what your "real" f stop is. That is, even if you set the lens on F8, the actual f stop is less if you focus at any distance less than infinity. Hope this makes sense. When focusing at 10 or 15 feet away the difference is negligible. But when focusing up close it is not. I posted a video on this subject some time ago. Here's a link.... https://www.youtube.com/watch?v=mpJWccGXe5E&t=1s
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