fd to nex adapter aperature control ring?

Started Aug 2, 2012 | Discussions thread
ProfHankD
Senior MemberPosts: 2,071Gear list
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EF lens aperture control via a rear iris... touchy at best
In reply to mikew5163, Sep 10, 2012

Note that EF has nothing in common with FD mount discussed earlier on this thread except they were both developed by Canon....

mikew5163 wrote:

I notice you say "there is a separate aperture iris in some adapters that doesn't work with most EF lenses". I just bought such an adapter and I find that with my EF 17-40 f4L and EF 100 f2.8 macro the aperture ring vignettes the image as soon as you start to close it down, so it's useless. My EF 50f1.4 has a bit of corner darkening as you stop down but it doesn't actually vignette like the others do.

Is this to do with the relative distance between the iris in the lens itself and the one in the adapter? Why would someone even make an adpater like this if most EF lenses will have this problem? I think I'll be sending mine straight back!

In order to act as an aperture stop, the iris behind the lens has to be a smaller opening than any other aperture stop in the optical system. In other words, there is a chance that it would stop vignetting once you've closed the rear iris to be smaller than the effective aperture inside the lens.

That said, the placement of the rear iris relative to the exit pupil of the lens is critical, and it is highly likely that it's in the wrong place for most lenses. Your 50mm f/1.4 probably has the happy combo of a better-placed exit pupil and a wider native aperture, which is why it sort-of works. Ironically, most lenses are much less picky about a stop located in front of the lens , so placing a Waterhouse Stop (dark filter with a hole in the middle) or an iris in front of your lenses has a better chance of working... if the front aperture is small enough.

In any case, what you really want is an adapter implementing electrical control of the iris inside the lens -- get one of the metabones adapters. They're not cheap, but it sounds like your EF glass is good enough to be worth the cost of a good adapter.

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