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[OM] Re: #117

Subject: [OM] Re: #117
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Tue, 24 Jun 2008 16:04:12 -0400
Thanks to Dr. Exposition.  If he hits me over the head a few more times
with "look at the histogram" it may finally sink in.

Dr. Flash



Moose wrote:
> Chris Barker wrote:
>> I'm sure that that's a beautiful shot, Graham, but colour looks wrong on my 
>> monitor.  It seems always to be the case with poppies when I take photos: 
>> digital does not like that colour.
>>   
> Chris, and others who have replied in agreement,
> 
> This is a common problem with digital cameras and/or processing that is 
> especially common with intense reds. The cause is clipping on one colour 
> channel before others.
> 
> You can see the effect in the individual channel histograms in a small 
> section of the flower image here. 
> <http://www.moosemystic.net/Gallery/tech/Misc/RedClip.htm>
> 
> The original distribution of red in the subject was likely much like 
> that of green, a narrow, symmetrical  distribution. As colour neg film 
> has a great deal of overexposure latitude, red layer brightness in the 
> film probably pretty well matched the subject. Somewhere in scanning 
> and/or processing, the red channel was overexposed, and the right side 
> of the distribution all ended up with one value.
> 
> One visual result of this is a lack of subtle detail in the red 
> highlights. Another, as noted explicitly by Chuck and implicitly by 
> others, is a change in colour. In the brightest pixels, the red can't be 
> increased in proportion to the other channels, so color* casts appear. 
> In this case, as the blue channel is brighter than green, the color cast 
> is slightly blue. The reds in the central, shaded portions of the flower 
> image are clipped much less, so the color is more likely accurate.
> 
> The same thing can happen with film, but is less common and/or obvious 
> because of the exposure characteristics of film vs. digital sensors. The 
> softer shoulder of the film response curve doesn't sharply clip above a 
> certain brightness level. In theory, I suppose it's more likely to be 
> noticeable in slide film than color neg.
> 
> I also suppose the same effect could occur on a monitor set too bright, 
> so that a color channel clips where the others don't.
> 
> Moose
> 
> * Spelling of colo(u)r in this document is customized to the native 
> spelling of the poster(s) to whom each individual instance refers.
> 
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