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Stop Down Metering

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February 12, 2001

First, stop-down metering assumes you are metering TTL with a meter built in to the camera body. It makes no sense if you are using a hand-held meter.

Second, stop-down metering is most often found with very old, early SLR's that use "manual stop-down" lenses. On these cameras focusing is done first with the lens wide open because it is much easier with more light pouring through the viewfinder. Metering is performed after focusing. To meter TTL, you had to stop the lens down to the aperture setting you planned to use. Then you tripped the shutter release. To refocus, you have to stop the lens wide open again to get enough light coming through to focus with.

Nearly all SLR bodies made in the past 25-30 years use "auto" lenses with two linkages between the body and lens. (This is _not_ auto focus, but auto diaphragm operation.) One linkage automatically stops the lens down to its aperture setting after you press the shutter release and just before the shutter travels. After shutter travel, it stops the lens wide open again. The other linkage moves with the lens aperture ring to bias the TTL metering. It tells the camera body (and its metering) what the lens aperture setting is. This is because the lens is held wide open until just before shutter travel. Without this biasing lever, the metering would act as if you had set the lens wide open.

Even with camera systems geared for "auto stop-down" lenses, there are sometimes special accessories or lenses that must be manually stopped down and require "stop down metering" when you use them.

One is manual extension tubes for macro photography. Some tubes are "auto" which have the mechanical linkages to connect the lens to the body through them. Other tubes are "manual" without these linkages inside them. One must use "stop down metering" with a manual tube even if the lens (and body) are "auto." With a manual tube between the lens and body, there's nothing hooking the linkages together.

Another is a perspective correcting shift lens. This is a specialized lens most often used for architectural photography and nearly always used on a tripod to get it dead level. A shift lens allows offsetting the axis of the lens in any direction about 10-15mm from the center of the lens mount. Because the entire lens (and aperture diaphragm) is free to shift off-center in all directions, it makes the mechanical linkages an "auto" lens has impractical. To try to design this into one of these lenses would be a mechanical nightmare. Because of this, nearly all of them are manual stop-down lenses without the linkages. Just like manual extension tubes, one must use stop-down metering with them. My shift lens has a lever on the side to manually stop the lens down from wide open to the aperture setting on its aperture ring.

Hope this helps explain what it is. Be glad for "auto diaphragm" lenses! With macro photography and when using a shift lens for architectural work, manual stop-down is not too bad. It's just one more step among many in a very deliberate process to make the photograph. However, it's not something you would want to go through trying to shoot candids at a wedding reception!

-- John

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February 14, 2001