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If a lens is specified as (EF 50mm f/1.8), where exactly is the 50mm distance if I were to show it in a ray diagram? Considering that a lens is made up of multiple elements, I could not find a satisfactory answer anywhere.

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up vote 5 down vote accepted

You cannot find answer because there is not one. It depends on the exact formula and shape of lens elements and their position at the set focus distance. Note that most lenses are specified for focus at infinity, so the 50mmm F/1.8 may not be 50mm at other distances.

From your question, I guess you already know that it would be easy if your lens was a single-element lens. With a complex 50mm lens, there is may not be an exact 50mm distance. You could check where it forms the image manually by shining a light through the lens elements and finding where is is most focused.

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Thanks. Any pointers to resources on ray diagrams for a compound lens will be greatly appreciated. –  VijayS Aug 28 '12 at 6:12
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