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Exposure
after the mirror flips up, you might be confused at this point. You turn the aperture ring on the lens and the image
remains just as bright in the viewfinder. Moreover, out of focus objects don't get any sharper as you stop down.
You're using a lens with an automatic diaphragm, introduced in the 1960s. The lens will be stopped down by the
camera an instant before exposure, just as the mirror is flipping up. If you're just viewing and composing
pictures, the lens is kept wide open for maximum brightness. To see what the film will see, you press the depth of
field preview button. This lets you visualize in the viewfinder the focus effects of stopping down the aperture but
it takes some practice to adjust to the extreme dimming that occurs by f/11 or f/16.
Shutter Speed
For a given amount of exposure on the film, the shutter speed can be determined by the
aperture that you set for aesthetic purposes. If you are taking a portrait and want to
throw the background out of focus, choose a wide-open f/2.8 aperture. Suppose that
implies a shutter speed of 1/125th of a second. If you change your mind and want to
ensure that the background is sharp, stop down to f/22, 6 f-stops less light. The film
will need to be exposed for 2^6 times as long. Two raised to the 6th power is 64 so
you'll need a shutter speed of 1/2 second to achieve the same density of exposure on
film.
A camera with built-in meter can do this calculation for you. Professional
photographers most typically use an exposure mode called "aperture-priority
autoexposure". The photographer picks the aperture and the camera picks the shutter
speed. Does it matter what shutter speed the camera picks? Not as long as neither the
camera nor subject is moving. If they are standing up, most subjects won't be able to hold acceptably still for the
1/2 second exposure mentioned above. The photographer will be advised to open the aperture until the shutter
speed is 1/15th second or faster. If the photographer is handholding the camera, i.e., not using a tripod, the 1/15th
of a second exposure will very likely result in an unacceptable amount of camera shake being recorded on film.
When using a normal lens, the general rule is to use shutter speeds of 1/60th or faster. Longer lenses magnify the
subject but they also magnify camera shake. The traditional rule for handheld photography is to use shutter
speeds of at least 1/focal-length. So if you've got a 250mm lens you'd use shutter speeds of 1/250th or faster.
You'll be well advised to use faster speeds if you intend to make big enlargements from your originals. You can
get away with slower shutter speeds if you either (1) brace yourself against a solid object, (2) rest the camera/lens
on a solid object, or (3) use a lens with electronic image stabilization, as explained in the photo.net review of the
Canon 600/4 IS lens.
There are sometimes aesthetic reasons to use different shutter speeds. If you are taking a picture of something
moving and want to show the motion, you'll need a slowish shutter speed. If you're taking a picture of something
moving and want to freeze the motion, you'll need a fastish shutter speed, the exact speed depending on the
velocity with which your subject is moving and whether the direction of moving is towards the camera or
sideways across the frame (note: the best way to freeze motion is with an electronic flash, which is actually a
kind of strobe light; a cheap on-camera flash may have a duration as short as 1/30,000th of a second).
http://www.photo.net/making-photographs/exposure (5 of 13)7/3/2005 2:22:14 AM