Size of a Lens Hood (length)?

I was wondering...anyone know what determines the length of a lens hood?

I'm just going to throw a number out here...but if a lens hood is 6 inches long,
why not 5 or 7 inches?

Doug

http://toothwalker.org/optics/lenshood.html

This has good descriptions and illustrations on the factors involved. Though it doesn't provide any of the formulas used, I suspect our science and mathematics guru, will not disappoint us :)

Pretty awesome...Thanks Chris.

Quote:

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Originally Posted by Chris Ober

http://toothwalker.org/optics/lenshood.html

This has good descriptions and illustrations on the factors involved. Though it doesn't provide any of the formulas used, I suspect our science and mathematics guru, will not disappoint us :)

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That's a pretty good web site. I can't improve on that. Basically, the light going to the edge of the lens that is destined to the edges and corners of the frame dictate the maximum length of the hood for a given diameter if no image-forming light is to be blocked. Not that crop sensors can have a longer hood without blocking light. To figure what might or might not blocked, one needs to do a full ray tracing of the optical system.

Roger

Quote:

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Originally Posted by Roger Clark

That's a pretty good web site. I can't improve on that. Basically, the light going to the edge of the lens that is destined to the edges and corners of the frame dictate the maximum length of the hood for a given diameter if no image-forming light is to be blocked. Not that crop sensors can have a longer hood without blocking light. To figure what might or might not blocked, one needs to do a full ray tracing of the optical system.

Roger

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If you want a simple equation, for simple telephoto lenses the hood diameter, H, for a given hood length, L, from the front element would be:

H = 2 * L * tan(a/2) + focal_length / f-ratio

where a = the angle from image center to the corner = arctan((center to edge distance in the focal plane / focal length).

This falls apart for more complex and wide angle lenses. Someone should check my math--it was a quick derivation.

Roger