ISO rating for Film Speed
ISO stands for 'International Organization for Standardization'
and their film speed ratings are used to indicate the relative
amount of light necessary to give a proper exposure. A normal
film will be rated at ISO 100. A film rated at ISO 200 will give
a proper exposure with only half the amount of light compared
to the ISO 100 film, enabling you to shoot in lower light or with
a smaller aperture or faster shutter speed. The ISO 200 film would
be referred to as a 'faster' film. There are films available that
range in speed from ISO 25 to ISO 1600.
So why not use the faster films all the
time, what are the advantages of slower films?
The faster films have a more prominent grain structure the individual
grains clump together to form spots that are visible to the naked
eye, especially when you blow the photo up to A4 or larger from
a 35mm negative. In certain circumstances this effect can be used
creatively especially in black and white photography but mostly
it is undesirable.
How does all this effect digital cameras?
In the digital photography world the phenomenon is called 'noise'
not 'grain', the cause of the problem is slightly different. When
light levels are low, the sensor has trouble reading the scene
properly and pixels of random colour are thrown into the picture.
However to us photographers the end result is the same or very
similar.
The 'better' digital cameras have, usually hidden away among
the manual settings, a sort of simulation of the film speed effect.
My camera for instance, a Canon EOS 300D, has an ISO range from
100 to 1600. This feature is not available when you are in fully
auto mode but is available in all the other modes. I tend to keep
it set to ISO 200 most of the time as the grain structure is not
significantly worse than ISO 100 and it gives me that extra f-stop
to play with. When I am shooting fast action and I want to freeze
the action (not always the case - see shutter
speeds and apertures) then I'll select ISO 400 or 800.
If the light is very bad ie night time or indoors then
a shot at ISO 1600 is often better than a blurred shot caused
by using too slow a shutter speed or no shot at all.
Here are two images, you are looking at a small blowup from the
centre of each image, the left one was shot at ISO 100 and the
right one at ISO 1600. The difference is fairly pronounced at
this magnification but (this is at 100% zoom), at more normal
sizes the difference is harder to see. The grain becomes most
obvious in parts of the picture which are fairly plain, such as
the sky. These two shots, of course, were shot in daylight. The
effect will be even more obvious in pictures taken in lower light
levels.
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| ISO 100 |
ISO 1600 |
Some digital cameras have built-in noise reduction technology.
Next time you need to take pictures in low light try a few shots
with and without it and compare the results. Also have a look
at your photo editing software. Programmes such as Photoshop have
filters that can remove or at least reduce noise. Careful though
that this does not affect the overall sharpness of the picture
too much.
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Digital
Photo Frames
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