The common currency of light intensity at the film plane, the f-stop, is independent of lens view angle and independent of format.
It’s true that the intensity falls off as we move radially outward on the film plane from the central lens axis. But this is significant only for very wide angle lenses. For some (but not all) LF wide-angle lenses there are radially-graduated neutral density filters with maximum density at the center that falls off to match the light falloff characteristics of the lens.
The majority of the central image area is approximately evenly illuminated.
According to the Kodak Professional Photoguide, bellows compensation is only required when the lens-to-subject distance is less than 8f for a lens of focal length f. At lens-to-subject distance of 8f the light falloff is slightly more than 1/3 stop.
With respect to
“The illuminance at the film plane of a 210mm set at f16 is going to be a whole lot less (one-fourth) if the lens covers 8x10 as opposed to 4x5.”
If you mean the total amount of light falling on each area, that’s not how light INTENSITY is reckoned.
It is light intensity that matters in exposure of film. If we have a lens that covers the 8” x 10” format, the intensity of the projected light in the central area is the same whether the image falls on an 8” x 10” sheet or onto a much smaller film (of course the image will be considerably cropped at the same subject distance with the smaller film).
The need for bellows compensation is due to the greater distance the light must travel from the lens to the film when the lens is extended significantly forward of its infinity position for close focusing.
It makes no difference what format the lens is used on. If it’s extended forward 1 focal length so that the total extension is 2f from lens to film, then we must increase the exposure by 2 stops so the film is properly exposed.
That’s true for a lens that covers an 8” x 10” film even if we equip the camera with holder for a smaller film (like a 120 back).
Last edited by Ian C; 11-03-2011 at 12:39 PM. Click to view previous post history.