Re: [Openexr-devel] Newbie questions

[Florian Kainz]

Darrin Cardani wrote:
> 
> I have a few questions about using the OpenEXR source, and I was
> wondering if anyone here could point me in the right direction. I
> come from a purely digital background when it comes to image and
> video processing. As such, I don't have a lot of familiarity with
> things like f-stops, and such. Is there a good reference online that
> could explain these terms?
> 
> Thanks,
> Darrin
> --
> Darrin Cardani - address@hidden
> President, Buena Software, Inc.
> <http://www.buena.com/>
> Video, Image and Audio Processing Development
> 
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The term "f-stop", as used on the OpenEXR web site, is a way to express
relative amounts of light.  F-stops are the base-2 logarithm of the ratio
between two light intensities.

The term "f-stop" is derived from the lens aperture in photographic cameras.
For a lens with focal length L, and an iris with diameter D, the aperture,
f, is L/D.  Most photographic lenses have a an adjustable iris; the adjustment
ring usually has stops that can be felt when the ring is moved.  On the ring, 
each stop is marked with the corresponding value of f.  Customarily, some
subset of the following range of f values is marked: 1, 1.4, 2, 2.8, 4, 5.6, 
8, 11, 16, 22, 32, 45, 64.
The ratio between one value and it successor is close to the square root
of 2, which means that from one f-stop to the next, the area of the iris 
opening, and thus the amount of light passing through the iris, varies
by a factor of 2.

When we say that an image has a dynamic range of 10 f-stops, we mean that
the ratio between the brightness of the brightest and the darkest part of
the image is the same as the between the amounts of light passing through
a lens at two aperture settings 10 f-stops apart.  In other words, the
ratio between the brightest and darkest parts is two to the tenth power,
or 1024.

Florian