[Leica] IR Filters; Rain and Sun

[henningw at archiphoto.com (Henning Wulff)]

At 4:56 PM -0800 3/2/07, Robert Rose wrote:
>If you read my (long) message about Yosemite, you will understand the
>following conclusions (your results may vary):
>
>1.  IR can be a problem ANYTIME.  Every shot I took in the snow that had a
>person, showed magenta jackets, etc.
>
>2.  Filters are your friends.  In the rain (which the M8 survived like a
>trooper) I was blotting the lens with a towel.  Very happy that it was not
>the front element.
>
>3.  The cut-off filters will need to be used with coded lenses and new
>firmware, for 35mm and wider.  The IR cut-off effect decreases at wider
>angles of view.  I have proof (unfortunately).

I think you will find that the center of the image is more magenta 
than the corners. This has been referred to as a 'cyan drift'. The 
central magenta look is actually correctly filtered but looks magenta 
because the auto WB averages things out, and the cyan corner 
conditions are the result of the oblique rays filtering not just IR 
but also visible magenta. This effect can be corrected in software, 
whereas the IR sensitivity can't.

There are methods available in Photoshop to correct this 'cyan drift' 
quite nicely for most wideangle lenses.

>4.  Once I get those puppies they are never coming off.
>
>
>Cheers,
>Bob Rose

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    *            Henning J. Wulff
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