BetterPhoto Q&A
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Manual lens opening lever

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February 12, 2001

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February 13, 2001

While Jon's explanation about your lens is correct, the terminology used isn't. "Manual focus" SLR's have two types of lenses: "auto" and "manual stop-down." Whether the SLR body has some form of auto-exposure capability doesn't matter.

Very early SLR's have "manual stop-down" lenses. To use a "manual stop-down" lens, it's stop-down lever is initially set to hold the aperture wide open. The lens is then focused wide open (much, much easier to focus). The stop-down lever is then moved to stop the lens down to its aperture setting. The lens will stop down to the aperture shown on its aperture ring. If the camera has TTL (through the lens) metering, the aperture and/or shutter speed are set at this time. Even if the body has auto exposure capability, you would still adjust the aperture (for aperture priority systems) or shutter speed (for shutter priority systems) after the lens is stopped down. Then you trip the shutter. One must remember to stop the lens down after focusing and before metering TTL or firing the shutter!

SLR's were not in the marketplace very long before "auto" lenses became commonplace. The term "auto" does not mean "auto focus" or "auto exposure." It means the lens has a mechanical linkage to the body that automatically stops it down from wide open to its aperture setting just after the shutter release is pressed and just before the shutter opens. For camera systems that have TTL (through-the-lens) metering there is a second linkage on "auto" lenses that tells the camera body what aperture the lens is set to so the TTL metering can be carried out without having to stop the lens down. "Auto" lenses are much more convenient; the user doesn't have to remember to stop the lens down before making the photograph.

Even with SLR systems made for "auto" lenses, there are still a few lenses and macro accessories that may require manual stop-down. One of them is extension tubes. Some tubes are "auto" which means they have internal mechanical linkages to connect the lens aperture setting and stop-down levers to the camera body. Others are "manual" which means they do not have these linkages. Another is a shift lens. Nearly all of these are "manual stop-down." This is because the mechanical linkages required to make them "auto" are not feasible on a lens that can offset by up to 15mm in any direction from center on the lens mount. Shift lenses are almost always used with a tripod and their primary purpose is for architectural photography.

Make certain you do not confuse a "depth of field preview" lever or button with manual stop-down. This is a common feature found on "manual focus" lenses and sometimes bodies. This button allows temporary stop-down of the lens to its aperture setting to observe the depth of field the aperture setting will create in the image. If the lens has the mechanical linkages for the camera body to automatically stop it down when the shutter is fired, it is an "auto" lens.

-- John

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February 13, 2001