I am certain you understand that pinhole cameras, to varying degrees, will vignette or darken the corners of the frame.
But have you thought any further than this?
Have you thought by how much this occurs & precisely where it occurs on the negative?
I have spent some time producing 'exposure maps' showing how exposure affects the negative with different camera designs.
My following comments refer to flat plane designs only.
When people state the specification of their camera, one detail given will be the f-number of that camera.
This is a convenient number which anyone can use to calculate exposures for that particular camera.
However, it is an incomplete specification. The stated f-number will be based around the 'Focal length' of the camera in question. But, the f-number actually changes for all pinhole cameras once we examine any other part of the negative beyond the centre of the negative (the point where the focal length is arrived at).
This is because light has further to travel at the corners of the negative, or put another way the focal length is longer at the corners.
This effect becomes progressively more pronounced the wider the camera (a product of a progressively shorter focal length).
So for my own 50mm camera, I always state that the camera (with a 0.010" pinhole) is f197.
To be completely accurate I should state that it is f197-f252. That is from centre to corner.
The following maps assume that EV0 is set at the centre of the frame & at the specified focal length of the camera.
Each ring marked is one f-stop apart. Obviously, the exposure is progressively less away from the centre point & the final print will show a gradual darkening.
If you look at the examples I have posted with each map, it is a surprise to note that the vignetting of the frame is not always as pronounced as one would expect. Conversely it is sometimes more pronounced!
Your thoughts are invited on this aspect, is the exposure affecting the corner shading? What about the brand of film? Or scanning of the negative? Does reciprocity failure affect vignetting (as you will have part of the negative correctly exposed & part of it under exposed).
Finally, if you have a short focal length design (let's take the first map below, a 4x5" camera, as an example), no doubt you are using the f-number at the centre of the frame to calculate your exposures. The resultant negative will be correctly exposed in the centre of the frame & progressively underexposed toward the corners.
But why not take the f-number of the camera to be one stop less? So now in theory you would have a ring on the negative which is correctly exposed, the centre of the frame would be one stop under & each ring outward would be as before, progressively one stop under.
The advantage would be that by the time you reach the corner of the frame you will be one stop under, rather than two stops.
Exposure latitude of the film would be allowed to deal with this +/-1 stop shift.
Is it worth the bother?
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