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Depth-of-field
Left Image: Canon EOS D30, Tamron 70-210mm
f-2.8, Kenko extension tube,
1/45sec at f-8,
Bogen tripod and ball head. Multi-segment metering, on aperture priority and
manual focus.
Depth-of-field is the zone of sharpness
both in front and behind the main subject. When you focus on your subject, you can choose
to blur the background and foreground, or have everything in sharp focus. You can control
depth-of-field by changing the lens aperture. Larger apertures (smaller numbers) give
shallower depth-of-field which soften the background and the foreground (above picture).
Smaller apertures (larger numbers) give greater sharpness. Image size also has an effect
on depth-of-field, as well as lens selection. Depth-of-field decreases as the image size
increases at any given f-stop. If you switch from a 200mm lens to a 100mm lens, and shoot
from exact same spot and f-stop, you get more depth-of-field with a 100mm lens. What you
have done by using a shorter lens is decreased the image size, and when you decrease
magnification, you gain more depth-of-field. Regardless of the lens focal length, all
lenses produce same depth-of-field at the same f-stop, if you keep image size the same.
For instance, if you use a 100mm lens set at f-11 and then back off twice as far and
switch to a 200mm lens set at f-11 and shoot same image size, depth-of-field will be the
same with both lenses. The background will look different in the two pictures because of
the different angles of view, but depth-of-field will be the same. You might have heard
that wide-angle lenses have greater depth-of-field than telephoto lenses. This is not
exactly true. Only if you shoot from the same spot wide-angle lenses give more
depth-of-field.
You can preview depth-of-field if your camera has a depth-of-field
preview. First select an f-stop and then press depth-of-field preview button. The lens
closes down to the selected f-stop, so you can check the zone of sharpness in the
viewfinder. The image will look darker when you use depth-of-field preview since you are
no longer viewing with the lens wide open.
Depth-of-field preview
button
Some people have difficulty using the
depth-of-field preview. They only see a darker image but can't see increased
depth-of-field. The best way to use depth-of-field is by closing the lens down 1 or 2
stops at a time so your eyes have enough time to adjust to the darker viewfinder. If you
go from f-2.8 to f-22 you'll have difficulty seeing through the viewfinder. You can also
use depth-of-field scale on your lenses to estimate depth-of-field. First focus on your
subject, and then check depth-of-field scale for f-stop that you think gives the result
you wish to obtain. Distances are shown for different f-stops for both the near and far
limits of the zone of sharpness. Not all lenses have depth-of-field scale, especially
zooms, but almost all fixed focal lenses include depth-of-field scale. Use the f-stop that
gives you the depth-of-field you need. If f-11 gives you enough depth-of-field, don't use
f-22. You loose two stops of light which results in a slower shutter speed that might
cause blurred images if your subject is moving. Very small apertures also reduce overall
sharpness due to diffraction.
Canon EOS D30, Tamron 70-210mm
f-2.8,
1/125 sec at f-8,
Bogen tripod and ball head. Multi-segment metering, on aperture priority and
manual focus.
At times you will notice that no matter how much you stop
the lens down, you still won't get everything in sharp focus. Large format users have a
great advantage over other formats. They use camera movements known as tilt and shift to
alter the plane of focus. The plane of focus is usually parallel to the film. If your
plane of focus is not parallel to the film plane you might have difficulty bringing
everything into sharp focus. Large format cameras allow you to alter the plane of focus
and move it instead of having it parallel to the film plane. This gives much better
depth-of-field. Some 35mm and medium format systems offer tilt and shift lenses which
allow lens movements similar to view cameras. These lenses can be used for increasing
depth-of-field.
When you want to buy a new lens, always
consider its largest and smallest apertures. Many photographers use fast lenses not just
because they offer a brighter viewing or faster shutter speeds, but for more control over
depth-of-field. Portrait photographers for example, carry 85mm f-1.4 and 135mm f-2 lenses.
They can use the wide open apertures to create a soft background to remove any
distractions. Wildlife and sports photographers use 300mm and 400mm f-2.8 telephoto lenses
to get the fastest shutter speeds possible, while fashion photographers use these same
lenses wide open to photograph models from a distance and create a soft out of focus
background. Understanding depth-of-field is as important as learning exposure and
composition if you want to produce best possible pictures.
Recommended Lens
Sigma 70-200mm F2.8 EX HSM Lens
Sigma's has been known the world over for their innovative design, ease of
use and for the excellent results that their zoom lenses produce - this
offering is no exception. The new 70-200mm f2.8 EX APO IF HSM lens features
an apochromatic optical design and four elements composed of Special Low
Dispersion (SLD) glass for high image contrast, resolution and color
saturation. For optimum performance in just about any photographic
situation, an internal focusing system means that the front barrel does not
rotate during focusing. For stability and the use of specialty filters, This
internal focusing system is a must. The internal zooming mechanism allows
the lens length to remain constant during zooming. Sigma's new Hyper Sonic
Motors (H.S.M.) provide silent, responsive autofocus action with both Canon
and Nikon AF SLR cameras. The H.S.M. feature also permits manual adjustment
of focus without the necessity of switching off the autofocus function.. The
Focus Free mechanism keeps the manual focus ring from spinning when the
camera is auto-focusing. The 70-200mm f2.8 EX APO IF HSM is fully compatible
with both the Sigma AF 1.4X and 2X APO EX Tele-converters. The new dedicated
1.4x APO Tele-converter features a high performance 5 element/3 group
optical design, and it is optimized for use with long telephoto and tele-zoom lenses.
For Canon
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