[Leica] IR sharpness issues

[Howard Ritter]

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 filter:
	http://gallery.leica-users.org/v/hlritter/For+Gallery/43+IR+tree+crop.jpg.html <http://gallery.leica-users.org/v/hlritter/For+Gallery/43+IR+tree+crop.jpg.html>

And without the filter:
	http://gallery.leica-users.org/v/hlritter/For+Gallery/47+IR+tree+crop.jpg.html <http://gallery.leica-users.org/v/hlritter/For+Gallery/47+IR+tree+crop.jpg.html>


 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.


—howard