Originally posted by rparmar you're talking about a conical hood and I'm talking about a cylindrical hood.
Just ignore the sides of the "cone". It's only the diameter of the hood and the distance from the lens face that counts in a practical sense.
What makes MY head hurt is calculating stuff when I can accomplish the same goal through practical trial-and-error - mostly correcting the error!
Remove both ends of a soup can, round oat meal box, whatever suits the situation. Heck, even a square or rectangular box will suffice. Center it fairly well around the lens in question and play with it til you see vignetting. Take your dimensions from there. '
Duck' tape helps.
Also, remember that the solution for a one-time project doesn't have to be gold plated nor attach to the lens in a conventional fashion either. Take a tip from the cylindrical, built-in hoods on some lenses (which usually get bonus points in reviews, incidentally) and fabricate a make shift hood from rolled black paper and a rubber band. Adjust length as necessary. You can even trim the end like the flower hoods - the trim
shape is dependent on the sensor proportions not the FL. The only important dimension is that the hood length be long enough to prevent unwanted light from reaching the front optics with no, or acceptable, vignetting.
H2
It's fun to watch the faces (sneers?) of guys with $,$$$-worth of equipment as they watch you use 'Scotch' tape and a small brown paper bag to fabricate a field-expedient hood. An' I'm still lookin' for an excuse to cut an 'X' in the bottom of a paper cup and slip it over a lens but it really doesn't make much sense unless you can make the inside of the cup black with something scrounged as well.