Archived posting to the Leica Users Group, 2015/11/17

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Subject: [Leica] IR sharpness issues
From: hlritter at (Howard Ritter)
Date: Tue, 17 Nov 2015 16:33:25 -0500

More experimenting.

Today I took some photos with the M8 and Elmarit-M 24/2.8 both with and 
without the IR filter because I was unhappy with what appeared to be very 
soft focus in the IR photos from yesterday.

The first thing I found is that with the B+W 093 (830 nm) filter, an 
exposure compensation of about 5 stops is needed. When the subject is 
predominantly cloudy sky, it?s more like 8 stops,  clouds, like blue sky, 
apparently being relatively IR ?cool".

The second thing I found is that the sharpness of the images taken through 
the IR filter really is dismal. Here?s a small crop of the frame with the IR 

And without the filter:

 Aperture was f/2.8 for both. Focus was compensated (f/5.6 mark) for the IR.

Any thoughts on the dramatic difference in sharpness between the two? Given 
that the filter is a genuine B+W from B&H, I?m thinking it?s not because of 
lousy glass. The only other thing I can think of is that the lens might not 
be achromatic for the full range of IR wavelengths passed by the filter and 
detected by the sensor, so that in wavelengths progressively further from 
the one that?s exactly focused, their contribution to the image is 
progressively further out of focus.


Replies: Reply from george.imagist at (George Lottermoser) ([Leica] IR sharpness issues)
Reply from hjwulff at (Henning Wulff) ([Leica] IR sharpness issues)
Reply from photo at (Nathan Wajsman) ([Leica] IR sharpness issues)