why do large format lenses enlarge image circle when you stop down ?

[brighamr]

hi all

why do large format lens enlarge image circle as you stop down where as 35mm lenses do not
is it because the iris is placed in the middle of the lens in the shutter

I recently was working with a 65mm movie camera in my day job and the 35mm lenses we had were supposed to cover the chip but the wide 18mm clearly vignetted
stopping down had no effect

best

robin

[Dan Fromm]

Two reasons.

Mechanical vignetting. Stopping down reduces the size of the exit pupil, increases the angle from which it is visible before the end of the barrel occludes it.

Off-axis aberrations. Some are sensitive to aperture and stopping down reduces them. This increases the circle of good definition.

What are the chip's dimensions? If its diagonal is > 43 mm there's no guarantee that a lens for 35 mm still will cover it. Also, with an 18mm lens the edge of a 43 mm circle will be 2.6 stops down from the center. If this is the problem, you need a center filter. No guarantee that there's one that will fit your lens.

[Mark Sawyer]

Pretty much what Dan said, but to look at it from another angle...

Look through the rear of a lens off-axis enough that the wide open aperture is partially blocked. At that aperture and angle, you're only getting partial illumination, so you're getting some fall-off, not full usable coverage. Now at the same angle, close the aperture until you see the full shape of the smaller aperture. That's where you're getting full illumination coverage.

Mind you, not all full illumination can be considered proper coverage in the traditional sense, as some lenses lose resolution at the edge of the image circle. But there the smaller aperture can save you too, as the small stops generally sharpen such things up appreciably.

BTW, unit-focusing 35mm lenses (those not shifting internal elements to focus) follow the same principles of LF lenses, they just don't have to allow for movements. If they cover at infinity, they cover period. Your 18mm lens simply didn't cover the movie camera's format.

[brighamr]

thanks Dan

the camera was a red Monstro 8k 46.31mm diagonal and the lens was a Canon K35
the job has been and gone and we just cropped in a little bit in post as that was still wider than the next lens up - a 24mm
i just found it interesting that stopping down had no effect as it can be seen to work with a short 5x4 lens like a 90 on a 10x8 ground glass
i guess that points to a mechanical vignette in the lens somewhere ?
the other K35s in the set all covered

best

robin