[Leica] IR sharpness issues

[hjwulff at gmail.com (Henning Wulff)]

As Nathan said, IR is more about the different tonality.

But.

I still do the focus compensation, which is, like I noted before, different 
for each lens, so you have to do tests before you can be comfortable with 
your procedure.

A lens that is excellent in the 'normal' 400-700nm region is not necessarily 
good in the 700+ region, and vice versa. Ya gotta test! Also some lenses 
'hotspot', in that internal reflections in the lens and reflections off the 
sensor create central 'hotspot's that are intrinsic to a lens in a certain 
IR wavelength range. Coatings are wavelength specific, and most lenses 
respond as 'uncoated' in most, if not all IR ranges. 

All has to be tested. As I mentioned, my favourite IR lens is the Tri-Elmar, 
and the fact that it is sort of like a 35-50-70 lens on the M8 just makes it 
all the more useful. The 21 Elmarit ASPH is useable, but not great while the 
WATE is better but more problematic because it takes 67mm filter with the 
adapter. I have a gel holder and some Kodak gels that I use on it. I'd 
better not damage them as they are becoming hard to replace. The 90 macro 
Elmar is a good longer lens, although I don't tend to use long focal lengths 
that much in IR.

Henning Wulff
hjwulff at gmail.com




On 2015-11-17, at 1:33 PM, 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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