3. Depth-of-Field Preview
Some SLR cameras have a depth-of-field preview, which lets you see in the finder
just how much depth of field you have. The automatic lens diaphragms in today's
lenses stay wide open for easy composing and manual focusing until you press
the shutter button to make the exposure. The depth-of-field preview stops the
lens down to the selected aperture. This, of course, makes the viewfinder image
darker. In dim light, you won't be able to see much of anything at smaller
apertures, so the depth-of-field preview is mainly a bright-light aid. But it
can be very handy when depth of field is a concern and the light level is sufficient.
SLR cameras feature a handy depth-of-field preview button, which
stops the lens down to the selected aperture so you can see in
the viewfinder just how much depth of field you have.
4. Hyperfocal Distance
You can maximize depth of field for any lens aperture by focusing at the lens'
hyperfocal distance for that aperture. This is easily done with lenses that
have a depth-of-field scale: Just set the focusing ring's infinity mark
opposite the selected aperture mark on the depth-of-field scale. Depth of field
will then run from half the focused distance to infinity.
Many manual-focus lenses, and some autofocus lenses, have a depth-of-field
scale. This gives you at a glance a good idea of how much depth
of field you'll have at a given aperture and focusing distance.
This lens is focused at infinity, with the aperture set at f/8;
depth of field at these settings runs from 10 meters (about 30
feet) to infinity.
You can maximize depth of field for a given aperture
by focusing the lens at its hyperfocal distance: Set the focusing
ring's infinity mark opposite the selected aperture mark
on the depth-of-field scale (f/8 in this example). Depth of field
will then run from half the focused distance to infinity (5 meters
to infinity here, with the lens set at f/8 and focused at 10 meters).