Why is the 'A' setting on aperture rings usually next to the smallest aperture?
Asked 8/22/2017
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On several lens systems, the aperture ring’s Auto/'A' position is placed next to the smallest aperture (highest f-number), not near the widest aperture. That feels counter-intuitive because switching from Auto to common working apertures often means turning past the whole range, and it’s easy to accidentally land on a very small aperture.
Why is the 'A' position normally located beside the minimum aperture instead of near the maximum aperture?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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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.
Originally by user67820. Source · Licensed CC BY-SA 4.0
user67820
8y ago
0
Generated from our catalog & community — verify before relying on it.
Because on traditional mechanical auto-iris lenses, the aperture ring sets a physical stop that limits how far the iris can close. In manual aperture selection, the lens closes only as far as the stop you set. In Auto/'A' mode, the camera needs the freedom to close the iris anywhere in the full range, including all the way down to the minimum aperture.
Placing 'A' at the minimum-aperture end makes this mechanically simple: the stop is effectively at or beyond the smallest aperture position, so the camera can control the full range. If 'A' were placed near the widest aperture, the lens would need a more complex separate mechanism to move that stop out of the way for automatic control.
So the location is mainly a consequence of how mechanical aperture linkages are designed, not of ergonomics.
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