[Leica] A tripod, an M8, and two candles - do some science

[henningw at archiphoto.com (Henning Wulff)]

At 1:07 PM -0600 1/7/07, Jim Nichols wrote:
>Adam,
>
>I went back and reviewed the Noctilux data that Brian posted earlier 
>today. There does not appear to be a separate "R" index on the 
>Noctilux for use in infrared photography.  I recall being told some 
>time ago that the separate index was no longer needed on recent 
>Leica lenses because the color correction has improved to the point 
>that IR is rendered properly when using the normal focusing index. 
>If this is true, then one should be able to focus properly using 
>visible light, regardless of the nature of the light source.  Maybe 
>someone can elaborate on this.
>
>Jim Nichols
>Tullahoma, TN USA

Hmmm.

I thought it was because there was not definitive 'IR' focus index, 
as IR material differs significantly, and even the same material has 
a different optimum focus index depending on the filter used.

Anyway, that's what Leica should have said, if they didn't. :-)

At present I'm shooting with a modified DSLR that has had its IR hot 
filter replaced with an 87 filter. It now works great as an IR only 
camera, but certainly not equally with all lenses. It works very well 
with the standard zoom that I use, and with a number of single focal 
length lenses, including macro and wideangle. The focus has been 
adjusted so that manual screen focus (such as it is) and autofocus 
both allow accurate focus with these lenses. One longer zoom of 
standard type works quite well also, but another lens with a 
diffractive optic element, doesn't. It doesn't work because it has 
less longitudinal chromatic aberration than a standard lens, and the 
'correction' that was done to allow IR focus overcorrects this lens. 
On the other hand, lenses such as the 6.8 Telyts need more 
correction, and thus manual focus doesn't deliver the proper result 
either.

All this means is that the IR index seen on some lenses is an 
approximation, which might or might not do what you want. For IR 
photography, stopping down is really essential, and using the f/4 dof 
marker as a start is probably reasonable. But only as an 
approximation.

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