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07-28-2014, 10:49 AM   #1
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what is the A setting for on camera lens?

I have a Vivitar Macro Focusing Zoom lens, and on the aperture dial after f/22 is the option "A" what exactly does this do?

07-28-2014, 10:54 AM   #2
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Auto aperture control within the camera.
07-28-2014, 10:58 AM   #3
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You push the little button on the aperture ring and set the lens to A, the you leave it like that. This means you can select the aperture on the camera body, but it also means the camera allows A and P and other modes, rather than just M mode with green button metering and A mode with full-open-aperture.
The only times you want to take it out of A mode is if you are using an old camera or bellows or extension tubes that don't allow auto aperture.
07-28-2014, 11:00 AM   #4
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When set on 'A' the camera can control the aperture using the e-dials. When set on a particular aperture the lens is controlling the aperture. This assumes your camera is capable of controlling the aperture, this would be all digital SLRs and the later film SLRs.

07-28-2014, 11:07 AM   #5
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I always thought if you set it on A, Mr T will show up and pummel photo bombers for you! "I pity the fool!"
07-28-2014, 11:27 AM - 1 Like   #6
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It helps to understand the history of Pentax lenses (and most mainstream lenses in general).

Originally, no lenses had an "A" setting.

Then, lenses started supporting an "A" setting, in addition to the manual aperture settings, for those cameras bodies which could control the aperture.

Today, most Pentax lenses don't have any manual aperture settings at all. The "A" is "built-in" and always enabled.

As suggested by others, you should just leave it on "A", unless you know you have a specific reason not to. In all other cases, there is no benefit to taking it off the A setting, and generally some disadvantages to doing so.
07-28-2014, 06:29 PM   #7
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Auto exposure was available in slr film cameras in 1965 so auto exposure was available fairly early given that 35mm didn't take over from medium format until the late 1960s and 35mm systems were only really available from 1959 on with the Nikon F system Certainly by the mid 1970s a full range of automatic functions were available in SLR cameras, Shutter Priority, Aperture Priority, Programmed Exposure, were three common modes fitted as standard on advanced film SLRs throughout the 1970s

On advanced professional cameras you would order the auto pentaprism head that gave you the mode you wish for, but the high end pro-am camera usually offered all modes.

My Canon AE1 Program of 1976 offered all modes including a very advanced (for the time) programmed exposure mode. But my later Canon F1n of 1981 had an optional shutter priority pentaprism head purchased as an extra. The basic camera was manual because professionals mostly worked in manual mode with auto exposure additions available.

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