In an optical system you want to minimize the vignetting, you don't want to loose any light.
This is not about the normal circular vignetting.
With this calculator you can get a feeling of the minimum diameter of your filter and the minimum distance from the prism in an off-axis adapter to the optical axis.
Anything that protrude into the light cone between the lens to the sensor will cause vignetting.
It start to calculate as soon you change or write new figures in the white or dark red boxes.
Do not exceed the maximum number of characters, delete characters if necessary.
You use the information on your own risk!
There can always be a mistake in my equations behind the calculations, check that the result is correct.
Let me know if you find something wrong and I try to correct it.
Some calculations are very simple done and not correct in the small details.