[Leica] George: M8 IR Different filters

[Paul Roark]

The relevant M8 data sheet seems to be this:
http://www.datasheets360.com/pdf/4613689109339751409
(Thank you Marty for sending the URL off list.)
At page 28 it shows the graph of the cover glass spectral transmission.
Interestingly, the graph does not look all that different than what
Kolarivision shows for various OEM cover glasses.  See
http://kolarivision.com/articles/two-spectrum-conversion/
However, the M8 cover glass appears to have, as you said, some IR getting
through it way out into the longer wave lengths.  The little chart on the
Datasheet shows this, whereas none of the graphs show enough information to
see this with either the M8 or the other OEM filters.  I just have to
assume that the thicker glasses reduced this IR "leakage" to nil.

Paul
www.PaulRoark.com

On Mon, Nov 30, 2015 at 8:12 AM, Howard Ritter <hlritter at bex.net> wrote:

> Since you mentioned it, Paul, I have looked at the graph of the M8’s
> KAF-10500-CX sensor’s spectral response with a 0.5 mm BS-7 coverglass. I
> don’t think I can attach it to this post, but I can tell you that it shows
> that both the green and the red photosites have a non-zero, and
> non-trivial, response out well into the IR. By inspection, it suggests that
> the red sites’ response even at 1100nm is maybe 5% of, or only about 4
> stops down from, its peak sensitivity at about 610nm, with only a slight
> falloff in efficiency from 800 to 1100. The green sites’ efficiency is
> maybe half of the reds’. Unfortunately, the graph extends no further. And I
> don’t know whether this coverglass is the same as is used in the M8.
>
> Since the 093’s passband begins just to the long side of 800nm, there’s
> plenty of overlap between the M8’s sensitivity and the 093’s passband. But
> there’s a wide difference between the shortest and the longest wavelengths
> detected by the M8/093 combination, and even greater in the M8/092 combo,
> meaning that unless the lens being used is just fortuitously achromatic
> across this part of the IR spectrum, it may be impossible to get an image
> that shows well-focused fine details. (If there were a sharp peak in the IR
> part of the response curve, that would mean that when that wavelength,
> contributing most of the light to the image, is in focus, the rest might
> contribute little enough that there’s not much blur. But this isn’t the
> case; the curve is very flat in the IR.) And, since the lens is clearly not
> achromatic from visible thru IR (thus the need for the focus shift), I see
> no reason to think that it would suddenly become achromatic again in the IR.
>
> The idea that the blur in the fine details in the IR images that I’ve made
> so far (see
> 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 the next image to compare IR at best focus vs. visible) is due to
> achromatic error across the IR spectrum is bolstered by my observation
> that, although focus for IR is clearly shifted from the visible focus
> point, there’s no appreciable difference in the focus of IR images whether
> I use the f/4, f/5.6, or f/8 DoF marks as the focus index—just as would be
> expected if there’s no “best-focus” part of the IR spectrum with a lens
> that is far from achromatic in the IR.
>
>
> Link to graph: http://www.gamutvision.com/docs/camera_scanner.html <
> http://www.gamutvision.com/docs/camera_scanner.html>
>
>
> —howard
>
>
>
>
>
>
>
> > On Nov 29, 2015, at 10:15 PM, Paul Roark <roark.paul at gmail.com> wrote:
> >
> > Interestingly, George, I'm interested in replicating the M8's spectral
> > response with the Sony a7r by using the Kolori astrophotography cover
> > glass.  It's a close match to my favorite B&W film -- Technical Pan.
> >
> > Did you ever have occasion to see a graph of the M8's spectral response?
> >
> > Paul
> > www.PaulRoark.com
> >
> > On Sun, Nov 29, 2015 at 7:00 PM, George Lottermoser <
> > george.imagist at icloud.com> wrote:
> >
> >>
> >> On Nov 29, 2015, at 3:47 PM, Frank Verizon wrote:
> >>
> >>> Kolari will replace your existing glass cover plate... On a Nikon or
> >> Canon, it costs about $350. I have asked them the effective loss of
> light
> >> and was told about a stop.  No filter required.
> >>
> >> I enjoy the M8 and its hyper IR sensitivity just as it is.
> >> It allows me to have Both:
> >> 1) two flavors of IR sensitivity with 092 and 093
> >> as well as
> >> 2) a full spectrum back up camera to the M
> >>
> >> ain't broke
> >> so
> >> ain't "fix'n" it
> >>
> >> Regards,
> >> George Lottermoser
> >>
> >> http://www.imagist.com
> >> http://www.imagist.com/blog
> >> http://www.linkedin.com/in/imagist
> >>
> >>
> >> _______________________________________________
> >> Leica Users Group.
> >> See http://leica-users.org/mailman/listinfo/lug for more information
> >>
> >
> > _______________________________________________
> > Leica Users Group.
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>
>
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