Desert Dweller <1.DeleteThis@invalidemail.org> wrote in news:YVini.38$fK1.17
@newsfe12.phx:
> I understand a faster ISO means the exposure will be created faster,
> thus requiring less light and enabling faster shutter speeds.
>
> What does the 50 in ISO50 mean? What does the 800 in ISO800 mean? etc.
>
> --
> DD
Without getting too detailed, the ISO number is a statement of the
amount of light sensitivity. It is linear (unlike f/stop) so ISO 200
means twice as much light will be recorded as ISO 100. ISO and f/stop
(aperture) and shutter speed combine to give you creative control over
the depth of field, stop action, etc.
You may have heard of the "sunny 16" exposure rule: On a sunny day, at
ISO 100, you have enough light for f/16 at 1/100 second exposure. It
follows that at ISO 50 with 1/100 second, you can only achieve f/11 -
half the light. If your camera (or film) is set to ISO 400 and you want
f/16, you need to make your shutter speed 4x faster.
More on Sunny 16 here:
> http://en.wikipedia.org/wiki/Sunny_16_rule
A general "Ohm's Law" of photographic exposure relates these three
variables: ISO speed, shutter speed, and aperture. If you fix any two
of these variables, the third is determined. Knowing which two to fix
is the creative aspect of photography.
From the tone of your questions to this NG, I would guess that you are
just starting out in creative control of photographs. You might want to
consider a course in photography at a local park or community college.
They can teach you all the basics in short order, faster than you can
learn from any book. With digital cameras, you can shoot all day long
and not worry about the cost of film!
have fun,
Ray
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