[Leica] George: 28mm Summicron IR problem

[henningw at archiphoto.com (Henning Wulff)]

>For color and panchromatic M8 work with the 28 'cron I use a Leica 
>IR/UV filter (not sure who manufactures their filters).
>For IR M8 work I use a B&W 093; which works just fine with the 35 
>lux asph and 50 lux asph; yet displays the hot spot on the 28 'cron.
>
>The fact that Len indicated the the phenomenal effect diminishes 
>with use of a "red" filter; would indicate that the phenomenon has 
>to do with the wave lengths processed by the optical system.
>
>Regards,
>George Lottermoser
>george@imagist.com
>
>

The effect is caused by reflections between the rear element(s) of 
the lens and the sensor/glass/filter pack. The coatings used on all 
of these are computed for the visible spectrum, and are unlikely to 
work correctly for IR spectra, as they are based on quarter wave 
cancellation, which is highly frequency dependent.

An earlier post stated that this has something to do with coatings 
not being IR transparent; this is not true. It has to do with the 
coatings just plain not doing what they are supposed to do, so that 
the IR radiation bounces back and forth between the sensor pack and 
the back of the lens a couple of times, and causes this flare in the 
middle. It's dependent on the lens element curvatures; if the 
curvature is fairly pronounced, the dispersion of the light is too 
great to notice flare. This 'flat', or low curvature surface can be a 
couple of surfaces in; it does not have to be the last surface, and 
it may not be actually flat as the light is bent by the last surface, 
it just has to appear flat through the back element.

Infrared film did not really show this, and that is due to the fact 
that film is not that reflective.

All lenses designed for general photography function as close to 
uncoated optics in IR applications. This is one of the reasons that 
IR digital photography results in generally fairly low contrast 
images. This effect was masked in film IR photography by the film 
characteristics, which tend to be high contrast.

My preferred IR lens on the M8 is the Tri-Elmar, on which I generally 
use the Heliopan 715 (similar response to an 88A), which is a bit 
darker than the 89b, B+W 092 or Hoya 720, but not as dark as the 87 
or B+W 093. Heliopan probably has the biggest range of readily 
available IR filters, with about 10 in their program. I also use the 
21/4 V/C. At times I use lighter or darker IR filters, but generally 
find that the 715 gives the best compromise between effect and speed 
that I like.

-- 
    *            Henning J. Wulff
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