Originally posted by Sean Nelson Assuming we're talking about K-mount lenses, I gotta disagree with Steve. In K-mount terms:
"M" lenses (those without an "A" position on the aperture ring) are lenses where you set the aperture on the ring MANUALLY. Whatever you set (ie, f/5.6) is what's used for the shot. The diaphragm remains open until the moment of exposure, closes down briefly while the picture is taken, and then opens back up by itself, no "re-cocking" of the aperture is necessary.
"A" lenses (those WITH an "A" position on the aperture ring) operate identically to the above description if you choose a specific aperture (ie, f/5.6) using the ring. But if you select the "A" position on the ring then the camera chooses the aperture AUTOMATICALLY. In other words, if the camera determines that the exposure should be made at f/8, then it deflects the lens's aperture lever the appropriate amount to close the diaphragm down to that aperture. This lets you shoot pictures in varying lighting conditions without having to touch the aperture ring at all.
BUT - if we're talking about screw-mount lenses then what I just wrote does NOT apply.
Yes, there's a manual focus lens with an A designation on the aperture lens, and thus the lens aperture can be controlled in-camera.
But in older lenses..
Auto refers to an auto diaphragm lens, not necessarily an auto-aperture lens. As Stevebrot described. The auto-diaphragm lenses, once mounted on your SLR, the lens "stays" open via a lever which allows you to view and compose and focus, then when you press the shutter button, the lever releases and the lens diaphragm returns to the selected aperture setting to take the photo. In a (seems like) not too long ago period of time, this wasn't the case.... it's taken for granted now, but there was a time that this was a new novelty, and these new lenses were labeled as "auto" This designation also appears on some of the older Taks and m39 and m42 screwmount lenses. The auto-aperture lenses (designated as the A series) didn't appear until the early 80's.
take your lens off your camera and select a small closed down aperture, say f16, then flip the little lever and see the diaphram open up to max size. That's your auto-diaphragm and that's why when mounted on your camera body you can get as much light in to your viewfinder as your lens is fast.
Your SLR Camera's Automatic Diaphragm - The Facts You Need to Know Early Pentax Takumar Lenses
That's what the Depth of Field lever did on some film bodies was for (bypassed the auto lever and let you see with the lens stopped down, for checking DOF, etc. but the image would be darker)
On the K20D if you select "optical preview" as your preview button choice, that's also what you get. (you can also manually meter in this mode as well for older, more finicky lenses and it works quite well, also focusing a fast lens that suffers from a focus shift error)