Originally posted by rparmar ...
How can one calculate the optimum hood depth range for a given focal length? That is, the minimum length that will do any good or the maximum length that will work with no vignetting. Of course this will depend on sensor size as well....
Here's a design for a rectangular prism shaped hood. It is optimum in the sense that there is no other shape that lets in less light from outside the field of view for the same hood height. It also uses almost the minimum possible amount of material in its construction.
The fraction of light from outside the field of view is proportional to (1+Focal.Length/Hood.height)^2, so there is no optimum height (the higher it is, the less extra light fraction enters.)
Dave
EDIT: I'm not recommending that anyone build one of these (although it'd be easy to build and I'll post a cutting template if anyone asks); rather, I'm addressing the optimum hood question. It is likely that a conical hood just a bit deeper than the optimum rectangular hood can do as well (hmmm...I'll think about this...)
RE-EDIT: I've changed the drawing to remove an error involving masking of the lens.