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Re: [OM] Bracketing for snow

Subject: Re: [OM] Bracketing for snow
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Tue, 03 Feb 2009 14:31:19 -0500
The key to understanding your problem is likely to be found in
"particularly bright day".  I suspect that, despite exposing multiple 
images for an HDR blend, your exposure for the brightest highlights was 
probably still overexposed... but not totally.  Most cameras do not get 
blown out on all 3 color channels simultaneously.  I think most will 
give up the red channel first as do my Mynolta A1 and Canyon 5D.  I 
don't know which one goes next but I think the effect you are seeing is 
that the red channel (at least) is blown and maybe the green as well 
although maybe not as bad.  What you are left with is RGB highlights 
(snow) with no R and maybe no G and what's left is predominantly blue 
and blending into the sky.

Check the histograms on the brightest images and pay attention to the 
individual color channels and I think you'll find that I'm right.  Check 
those when you're shooting too and you may catch it in the act.  And, if 
you shoot raw you'll have some additional headroom for highlight 
recovery.  This is one place where different raw converters may march to 
different drummers.  The manufacturer's converters usually give up if 
they know that one channel is blown because they know the color accuracy 
is gone.  Other converters may try to guesstimate from surrounding 
pixels what the color should look like and fudge the numbers.  But I 
don't know if anyone attempts to handle two blown channels.

Chuck Norcutt


AS wrote:
> I attempted bracketing during a particularly bright day using the
> 420. I had a strange issue while trying to create a HDR using
> photomatix so I blended the images instead.
> 
> The snow came out blue matching the sky and I could not fix that.
> Therefore, I converted the image to BW to hide that blue snow.
> 
> http://www.flickr.com/photos/4love/3189868974/
> 
> 
> 
> 
> 
-- 
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