[Leica] First IR photos with M8

[hlritter at bex.net (Howard Ritter)]

So the filter came from B&H today, a B+W 093 filter with a lower cutoff at 
830 nm. Wow, it?s really black! And I got the problem with modern SD cards 
sorted and now the M8 is humming along with a 32GB card.

Late autumn does not present the best landscapes for IR, with a paucity of 
green foliage, but here in NC there are still enough leaves on the trees to 
allow for testing the gear. I stepped out of the house and found enough to 
make for a good picture, with blue skies and some picturesque clouds.

Here are three: http://gallery.leica-users.org/v/hlritter/For+Gallery/ 
<http://gallery.leica-users.org/v/hlritter/For+Gallery/>

All were taken at ISO 640, hand held, with an Elmarit-M 24/2.8 lens wide 
open and set with the infinity symbol at the f/5.6 mark. Exposures were in 
the 1/20 range or longer. The files were stretched so the histograms filled 
the entire range rather than the original ~60%, improving the brightness and 
contrast. All were converted to greyscale.

At full screen, all three seem to lack the expected sharpness. I can think 
of three possible reasons, the obvious ones being uncertainty about the best 
focus point (I tried several focus marks, but I?ll have to experiment 
further) and hand holding with a slow shutter. A third reason occurs to me: 
The lens is achromatic across the visible part of the spectrum, but 
obviously not for the part that extends into the IR, or there would be no 
focus shift to compensate for. Can we assume that it IS achromatic across 
the full range of IR wavelengths that the sensor is sensitive to? I don?t 
think so. If it isn?t, and there?s enough energy in parts of the IR spectrum 
that are far enough away from the notional focusing wavelength, then there 
will be progressive blurring of images formed by wavelengths further and 
further from the wavelength that is exactly focused.

In fact, this latter effect MUST be present with IR filters whose passbands 
extend into the visible deep red: When the lens is focused on the mark, the 
achromatic property of the lens puts the visible red in focus, but not the 
dominant IR, or there?d be no need for focus compensation. So there must be 
some degree of blurring with such filters; why isn?t it an issue, visually 
speaking? That would be an argument for using the smallest practicable 
aperture, while I was using the lens wide open for shortest shutter speed. 
Also an argument for using ?tighter? filters, with longer cutoff 
wavelengths, to limit the range of wavelengths. Which however call for 
longer exposures or faster shutters. But with ?looser" filters, the 
additional light that enables shorter exposures is less in focus? Oy, my 
head hurts.

Tomorrow, experimenting with a range of apertures. On a tripod.

Any thoughts?

?howard