--- Robert Meier <[EMAIL PROTECTED]> wrote:
>
> --- Pawel Nabe <[EMAIL PROTECTED]> wrote:
> > 1) Why can you set 'film speed' on a digital
> camera?
> > What does this effect?
>
> If you set a higher film speed the camera will use a
> shorter exposure time. In order to get a correct
> exposure the values have to be multiplied by a
> certain
> factor. For example assume that each color value has
> a
> precision of 8 bits which is equal to 2^8=256. If
> you
> integrate (collect) photons (light) over a shorter
> time the output of your measurement will be
> considerable lower then 256. In order not to get a
> too
> dark picture you have to multiply the values (quite
> simplified). If before you has an absolute error of
> 10
> pixels (out of a range of 0..50) and you multiply by
> 5
> you get an error upto 50. On the other hand if you
> chose a longer exposure (lower ISO) the camera will
> add photons for a longer time and the output of your
> measurement gets close (or even higher) then the max
> value of 256. So you don't have to multiply the
> values. So if you have an error of 10 this absolute
> error will not change.
And the net result of this is more noise in the image
at higher ISO values.
=====
Bob Meyer
Life is uncertain. Eat dessert first.
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