Hi,
Last night, while I was loading my holders with some 8x10 film, I noticed that a t the center, the film was bulging quite a bit. I estimate the bulge to be 1-2 m illimeters.
So today I did some calculations to see how it would affect the focus, and I was astounded by the results.
For example I calculated that a 2mm bulge would cause the focus to shift from 10 feet to 9.31 feet on a 300mm lens. On a 110mm lens the focus would shift from 1 0 feet all the way to 6.57 feet!
A 1mm bulge would cause the focus to shift from 10 feet to 7.93 feet on a 110mm lens.
Then I calculated what the lens need to be stopped down so that the depth of fie ld could "cancel" this shift and the answer was f45 or smaller! Even at 1mm!
Does this sound right? Or are my calculations ways off? Is my estimate of the bu lge too pessimistic?
What is the average bulge for 8x10 film? And what about 4x5?
If this indeed is the case, then film flatness is VITAL. And anything but a perf ectly flat film will TOTALLY waste the capabilities of a fine modern lens.
Comments please...
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