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     áòèé÷ :: Filmscanners
Filmscanners mailing list archive (filmscanners@halftone.co.uk)

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RE: filmscanners: This Gamma Thing...?



On Fri, 02 Feb 2001 22:16:33 -0800  Tom Christiansen (tomchr@softhome.net) 
wrote:

> Thanks for the explanation. Now I just need to get some kind of idea of a 
> good value for gamma. Windows defaults to 2.2. My scanner software defaults 
> to 1.4. If I change the scanner software to gamma=2.2 images look WAY too 
> bright... Why the difference?

Probably your monitor adjustment and calibration.

A 'good value' is a difficult question which has occupied vast amounts of 
argument. Unity gives equal precision to all values, which might seem a good 
idea from the data's POV (and Timo Autiokari's) but our perceptual needs are 
more idiosyncratic - we are very sensitive to brightness variations in the 
midtones and shadows, less so to highlight differences so precision in the 
former is usually more important. The optimum value of gamma tends to vary 
depending on the type of image and what is important to the photographer. Any 
value is a trade-off.

To be more practical, Macs use a system gamma of 1.8 and are historically the 
de facto standard in imaging. Therefore 1.8 is a 'good value' if you are 
working with designers who will be using Macs. However Windows machines are 
much more numerous. If you are aiming at cross-platform medium such as the WWW, 
2.2 is probably a safer assumption.  

There's some stuff on my site about all this, and it's well worth reading Prof 
Charles Poynton's FAQ about Gamma (link at my site). But don't be too taken in 
by it - he is talking only about video systems and optimal presentation. We 
tend to have to worry about data precision as well, through iterations of 
editing operations.

Regards 

Tony Sleep
http://www.halftone.co.uk - Online portfolio & exhibit; + film scanner info & 
comparisons




 




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