[Leica] IR sharpness issues

[Nathan Wajsman]

Howard, a couple of things:

- George suggested a procedure for testing sharpness with the camera on tripod etc. to determine the focus compensation needed. If you really worry about sharpness, then do what he suggested.

But…

- for me, IR photography is less about sharpness and more about tonality and the special IR effect. In fact, I do not even bother with the focus compensation. When I shoot IR, I just focus normally. Most of the time, the subjects are anyway far enough away that precise focus does not matter that much.

And finally, do not bother shooting IR on a cloudy autumn day. You need plenty of sunshine for good IR. Although the pictures you posted earlier very nice.

Cheers,
Nathan

Nathan Wajsman
Alicante, Spain
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> On 17 Nov 2015, at 22:33, Howard Ritter <hlritter at bex.net> wrote:
> 
> 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
> 
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