Re: Getting better photos -how?
- From: "Roger N. Clark (change username to rnclark)" <username@xxxxxxxxx>
- Date: Mon, 03 Jul 2006 12:27:35 -0600
Wolfgang Weisselberg wrote:
Yep: the number is the 'size of the hole' (if it were at the
front element of the lens) in relation to the optical length of
the lens (often called f, "focal length"). Thus f/2.8 is a much larger hole in 200mm than in 50mm, but both
let through the _same amount_ of light[1], and thus they are a
very useful number to the photographer.
This is not correct. The larger the aperture, the more light
the lens collects. See:
The f/ratio Myth and Digital Cameras
http://www.clarkvision.com/photoinfo/f-ratio_myth
A 200 mm f/2.8 lens collects 16 times more light than a
50mm lens. But the 200 mm lens magnifies the image in
the focal plane more so that the light (number of photons)
is the same per unit area, thus giving the same exposure time
for the two lenses for a typical scene. If you were imaging
point sources, like stars, the 200 mm lens would record
fainter stars in the same exposure time (16 times fainter)
as the 50mm lens.
For the Original Poster:
Photography is a lesson in compromises.
Lenses are a built with many compromises. These include corrections
due to aberrations, total field of view, largest lens opening (aperture
or f/stop, also called f/ratio), all driven by cost. The f/ratio is the
lens diameter divided by focal length.
Most aberrations (e.g. spherical, coma, chromatic) decrease with
smaller apertures. Some aberrations may not
decrease with smaller apertures, like astigmatism. Other issues,
like field curvature (you want the focal plane to be flat, not curved),
do not change with aperture. One fundamental limit is diffraction,
which increases with decreasing aperture. So image sharpness
produced by a lens is a trade between minimizing the aberrations
that decrease with smaller apertures with increasing diffraction
as aperture size gets smaller. Many photographic lenses
reach that optimum in the f/8 to f/11 range.
Then sharpness can be affected by focus. Different distances
in the real scene come to a focus at different distances from
the lens. Decreasing aperture brings more depth into focus
but only to a degree, as diffraction increases, thus reducing
sharpness. Your settings become a compromise.
In general, slightly out-of-focus parts of an image are
distracting to the eye, and may even "hurt" the eye
(e.g. some people get watery eyes looking at a blurry
image). So there are generally two strategies in
photography (with the caveat that all guidelines can be broken):
1) Everything sharp, from near foreground to the most
distant background. Example:
http://www.clarkvision.com/galleries/gallery.large_format/web/c072099_L4_01a2-600b.html
In such an image, if portions of the image were slightly
out of focus, the "wow" factor would be lost and people's
eyes start to water at the blur. Everything sharp is balanced by
lens aperture, exposure time, and enlargement factor.
(The above example is with large format film and was shot
at f/64, but gives results similar to f/16 on 35mm).
2) The subject is in focus, but the background is very out of focus.
This is generally done a lot with wildlife images, such as
this one:
http://www.clarkvision.com/galleries/gallery.bird/web/great.blue.herons.the.kiss.JZ3F8149.f-700.html
The key in such photos is to have the subjects eyes be the main
focal point. This requires the widest apertures, but how wide
depends on how far back the background is. The above image was
at f/5.6 but the background are trees several times further away than
the birds.
In using a camera, I recommend only two modes: aperture priority
and manual. That way you control everything. Aperture controls sharpness,
and that is a key to the technical aspect of photography.
If you want fast shutter speed, open the aperture to its maximum.
If that is not fast enough, then increase the ISO value.
Camera meters are only an estimate. You need to learn to examine
the image data (e.g. view the image and look at the histogram), and
adjust exposure to compensate for the meter when it does not do
the best job (e.g. a small white bird in the scene may be
overexposed). Then use the exposure compensation dials on
the camera to adjust to get a better exposure, or change to manual mode.
Once you've learned the technical aspects, you are then open to learning
the creative aspects of photography, which if you are like most of us,
will take the rest of our lives ;-). I'm still learning.
Roger
Photos, other digital info at: http://www.clarkvision.com
.
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