Aperture vs. Shutter Speed
By: Larry Huckabee
 Photography's
root comes from the Greek roots "photo" which means
light, and "graph" which means to write or scribe.
Photography is all about light and how to interact
with it. Two basic functions of an SLR camera
accomplish this through the aperture and shutter
speed.
Shutter speed refers to the amount of time
that the film, or in the case of a digital SLR, how
long the image sensor is exposed to light. Depending
on the camera, shutter speeds can vary from 30
seconds to 1/8000 of a second. Some cameras have a
'bulb' mode that allows the user to open and close
the shutter manually using a switch.
The aperture setting of a camera refers to
the amount of light that is let into the camera body
by the lens. An iris contracts or expands depending
on this setting on the camera, thus controlling the
light that hits the film. Aperture terminology may
be confusing as the "higher" the aperture, the lower
its number. Lenses are rated by their aperture and
focal length. For instance, Canon might have an
F/2.8 28-75mm lens. The F/ prefix is the highest
aperture that the lens can produce. In this case, an
aperture of 2.8 is the maximum amount of light that
the lens can let in. A lens that has a high aperture
is considered a "fast" lens.
Shutter speed and aperture values are at a constant
tug-of-war concerning light. Suppose someone was a
handed a camera and was told that its aperture and
shutter settings were set to ensure proper exposure
using a shutter speed of 1/60th and an aperture of
F/8. However, suppose that they wanted a faster
shutter speed to capture a moving object, so they
set the shutter speed to 1/250 and took a picture,
they would get an underexposed photo because they
didn't allow the film to be exposed for the 1/60th
of a second it needed. Therefore, they would need to
change the aperture to allow more light in, perhaps
by changing it to F/1.8, or whatever the maximum
aperture of the lens is. It is always good once you
have changed your settings to check them with a
light meter and see if you are giving enough light
for proper exposures.
 More
on Shutters
Cameras traditionally have two shutters, or
curtains, that cover the film ready to be exposed --
one shutter sits over the film and one rests off to
the side. Once the shutter button has been pressed,
the first curtain is moved away so that the film is
exposed to the light coming through the lens. Once
the film has been exposed for the length of the set
shutter speed, the second curtain moves into the
position that the first curtain initially sat. Why
can't there be just one shutter? For two reasons.
First, if the shutter speed is set to 1/1000 of a
second, the shutter would have to move away from the
film over a short period of time. Once the shutter
has completely moved away from the film, the edge of
the film that was last uncovered by the shutter has
been exposed less than the edge of film that was
first uncovered. This means that if a single shutter
were to come back the way it left across the piece
of film, it would again underexpose the already
underexposed edge of the film, leaving the more
exposed edge exposed longer still. Secondly, 1/1000
of a second is not even enough time for the first
shutter to completely uncover the film; hence, a
second shutter comes behind the first shutter and
begins covering the film the first shutter has
started to reveal. This has the effect of computer
scanner moving across a piece of paper. This is why
you cannot flash-fire a single burst of light for a
photo with a shutter speed greater than 1/250th,
you'll have one part of the photo more exposed than
the other.
Larry Huckabee has been a photography enthusiast for
over 25 years. He has worked professionally with two
large clothing retailers for modeling catalogs and
takes his passion for photography all over the world
for his beautiful calendars. Check out more articles
by Larry at
http://www.autoabout.com.
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