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Old Aug 17, 2006, 12:43 PM   #1
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I see references to stopping down a lens, but I am unsure how this works. Normally, the camera will pick the proper aperture, ISO, and shutter speed. Stopping down a lens sounds like manually overriding the aperture, so the camera thinks it is in the original aperture when it is really not, but I don't know the technique to do this.
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Old Aug 17, 2006, 12:52 PM   #2
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Stopping down is a term used to describe closing down the aperture (selecting a higher f/stop number which represents a smaller iris opening).

The auto exposure algorithms in a camera will generally select an aperture that insures that shutter speeds are 1/focal length or faster for most lighting conditions (if lighting permits for the largest available aperture of the lens at the ISO speed used).

But, you may want to select a different aperture than the camera's autoexposure would for a variety of reasons.

One reason is Depth of Field control. Selecting a smaller aperture (higher f/stop number) for greater depth of field; or a larger aperture (smaller f/stop number) for a shallower depth of field.

Another reason is more control of the shutter speed a camera is using.

You may want to use a larger aperture (smaller f/stop number) to get faster shutter speeds than the camera's autoexposure algorithms would give you in a given lighting conditon; or you may want to use a smaller aperture (higher f/stop number) to get slower shutter speeds than a camera's autoexposure algoirthms would normally give you.

In low light, most cameras will use the largest available aperture anyway, and that can lead to a shallower depth of field than you may want. So, you have to balance the conditions with what you're trying to accomplish.

Yet another reason you want to have control of the aperture is that most lenses are sharpest about 2 or 3 stops down from their largest available aperture. Most lenses are going to be softer at their aperture extremes (largest or smallest available apertures).

The best way to control aperture is to shoot in Av (Aperture Priority) mode. This is the mode I leave my camera in most often. In this mode, you select the aperture, and the camera selects the correct shutter speed for proper exposure.

With cameras that have Programmed Auto Mode (P Mode), you can usually spin a control wheel on the camera to step between available aperture/shutter speed choices for the same type of control. A number of aperture/shutter speed combinations will produce identical exposure for the same lighting and ISO speed.

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Old Aug 17, 2006, 8:42 PM   #3
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Thanks for explaining this. I've been leaving my Nikon in Aperture Priority mode, so based on your explanation, I have already been "stopping down" my own lens.
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