Silly curiousity about rear elements...

This is a silly question - but I''m curious as to the differences in the rear elements of EF lenses. When I look a the rear of my 70-200 f/4, the black part where the contacts are is round, and much of the glass is visible. But when looking at the rear of my 24-105, the black extends over the glass and forms a sort of rectangle. On the 100L IS - round. On the 300 f4 IS - rectangle. Does anyone know why?

It's a baffle to stop flare.

melcat wrote in post #13822375
It's a baffle to stop flare.

Well, that makes sense. So why is it on some lenses and not others? I'm guessing it's something technical about the shape of the glass?

...dunno, but maybe you can find it in Canon Lens Works III. Should be able to satisfy a lot of curiosity in there:

http://www.canon-europe.com …n/ef_lens_work_​iii_en.asp

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Cool site - thanks!

Some lenses need it more than others, which generally depends on the design and maximum aperture, and with some lenses it is dealt with in a different way. F.e., the EF-S 60 Macro has two diaphragms, one being used as a baffle, and one as the actual aperture control.

Kind regards, Wim