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This is something that has puzzled me for a while and it always seems counter-intuitive. When a lens has an aperture ring with an auto position (A), it is always (at least on 3 systems that I have with such lenses) next to the smallest aperture as seen here:

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Why is the A position next the smallest aperture rather than near the largest?

What this means to be is that I have to move the aperture ring a lot more than it would be necessary if it were next to F/2 rather than F/22. Also some lenses it is easy to accidentally move the ring from A to a very tiny aperture which invariably causes unexpectedly long shutter-speeds and blurry images, either from the slow speed or from motion. Personally I also never use the diffraction limited apertures so I must skip over a pretty much useless range before reaching the aperture I am looking for.

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The normal case of affairs for an auto-iris lens aperture (that's auto-stop-down, whether or not you have AE going) is that the aperture ring is adjusting the position of a physical stop that determines how far the iris can close. The difference between manual mode and AE/A is that the camera determines how far to move the linkage in AE/A, while in manual mode it just "lets go" and lets the iris hit the stop position you set.

Since the iris may have to close down to the minimum in AE/A, the "A" position will either be at the minimum (highest f-number) or beyond the minimum. Doing it any other way with a mechanical linkage would be vastly more complicated, requiring a separate mechanism that moves the physical stop out of the way. If you find an iris-by-wire system that works the same way, that would be by convention rather than out of strict necessity or economy.

  • That seems reasonable for historical mounts but why does something completely new like GF-mount lenses do it too? Just to keep consistent with the past? – Itai Aug 30 '17 at 18:20

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