Front Variable Diopter
How to choose FVDs:
When building the anamorphic lens, you can always reuse the anamorphic lens, but you have to change the front FVD based on the focal length of your taking lens to get the best performance. Otherwise, the astigmatism will blur your image, forcing you to stop down to get a sharp image.
Recommend pairing 35-80mm taking lens with FVD-8A, FVD-16A and FVD-35A. Main difference among these three FVDs is the FOV coverage. Of course, they are quite different in shape, size & weight too.
Recommend pairing 85-135mm taking lens with FVD-16B.
Check the Max coverage and FOV test of each scope to avoid vignetting.