Circular IR gradation?

[3D-Kraft.com]

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I am in the phase of learning from my newly acquired M8.2 dinosaur.
When shooting with an Voigtlander Ultra Wide-Heliar 12/5.6 without IR-cut filter, I get this (from DNG):

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Of course, due to the extreme wide angle, there is a light fall of towards the edges. But it also seems like the IR-sensivity degrades towards the edges when using an ultra wide angle, because the greens grade from yellow-green towards blue-green.

Have others had similar experiences?

Even more confusing is, when looking at the out-of-camera JPEG:

Here you do not see the grading. The white balance seems ok and similar to the development from DNG as you can see in the gray tones of the trailer.

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[jaapv]

Yes, Cyan vignetting. Pretty much standard on the M8 with wider than 35 mm lenses. Use Flat Field Correction in postprocessing.

[3D-Kraft.com]

Thanks. That's why I decided to give the M8.2 a try. Modern digital cameras are just too perfect and this perfection makes them boring. I like the challenge and the look of imperfection 😉

[dpitt]

Before the M8, Leica claimed that a digital M was impossible because the short flange distance causes very steep angles at the edges. They managed to develop a sensor that works for most existing M lenses.

The 12mm Heliar is extreme, even on film M's, so it is no surprise that it does not work perfectly on the M8 (or any FF ). The same goes for the 15mm Heliar which I have, and sometimes use on my cropped MFT or APS-C sensors. I am surprised how good the M8.2 copes with the 12mm!
Did you code the lens?
If you code it to the most similar Leica lens, it will probably work even better.

[3D-Kraft.com]

Thanks. Yes, I also was surprised from the result with the 12mm UW-Heliar but fortunately the area, where things get really worse are cropped by the APS-H sensor anyway.

I did'nt code it - as I do not plan to use it further at the moment. What you get, is simply too far away from what you see in the rangefinder...

Edited May 5, 2024 by 3D-Kraft.com

[dpitt]

20 hours ago, 3D-Kraft.com said:

What you get, is simply too far away from what you see in the rangefinder..

I can recommend finding a 15 mm Heliar with Bessa L combination. They should have the 15mm voigtlander VF included. And with some luck you can find them for a few hundred Euro's. Not much for an extra lens and viewfinder. The body is just a bonus if you want to shoot film 🙂
I use it for all my super wides. Although on the M8 with the 15mm Heliar the 21  mm VF should be closer.

Edited May 6, 2024 by dpitt

[3D-Kraft.com]

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Just an additional question, although it get's a little bit off-topic:

When I want to use a wide open lens (at f/1 or so) at bright daylight, this requires a ND filter (in that case a B&W ND 0.9 taking away three stops of light). Will this also have a positive side effect on IR, so that I do not need to cascade two additional glas planes in front of the lens... (IR/UV-cut + ND)?

[adan]

On 5/5/2024 at 2:03 AM, 3D-Kraft.com said:

I am in the phase of learning from my newly acquired M8.2 dinosaur.
When shooting with an Voigtlander Ultra Wide-Heliar 12/5.6 without IR-cut filter, I get this:

The M8 does have an IR filter built-in (just a somewhat weak one).

If you open the shutter with no lens mounted, you will notice how green the sensor's filter/cover-glass is (and the Bayer filters beneath, since they 2 green pixels per 1 red and 1 blue - but that's another story).

With a really-wide lens like the 12mm, covering 121° diagonall7 (103° horizontal, 61° to each corner /\) - the image is passing through that built-in greenish IR filter at a large angle at the corners, rather than straight through.

Making the filter effectively thicker and denser (sines and cosines and all that) approaching the corners (i.e. "more" green-cyan).

It starts eating into the visible-red wavelengths, and overcorreecting for the IR. Thus the picture gets greener towards the corners. As well as lsightly darker (lost visible red light).

That is the reason Leica originally introduced 6-bit coding for focal length - to eliminate the GREEN corners the M8's own IR filter would produce, progressively, with any lens wider than ~50mm. And even more so with an external IR filter on the front of the lens as well.

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Edited May 12, 2024 by adan

[3D-Kraft.com]

Thanks @adan for your detailed explanation. I could follow your explanation, when the filter is colored through the whole glass thickness.

I thought, IR-cut filters work by generating an interference for certain wave lengths, where the waves extinguish each other at certain frequencies. This requires only a coating on the glass surface. But may be, Leica has choosen another type of filter (and we all know about the downsides of that decision).

[farnz]

8 hours ago, 3D-Kraft.com said:

Thanks @adan for your detailed explanation. I could follow your explanation, when the filter is colored through the whole glass thickness.

I thought, IR-cut filters work by generating an interference for certain wave lengths, where the waves extinguish each other at certain frequencies. This requires only a coating on the glass surface. But may be, Leica has choosen another type of filter (and we all know about the downsides of that decision).

Yes, extremely well for those of us who enjoy using the M8 to photograph subjects in infra-red hand-held.  

(As a friend of mine who had worked at a rubbish tip was fond of saying: "One man's trash is another man's treasure.")

Pete.

[jaapv]

On 5/11/2024 at 2:21 PM, 3D-Kraft.com said:

Just an additional question, although it get's a little bit off-topic:

When I want to use a wide open lens (at f/1 or so) at bright daylight, this requires a ND filter (in that case a B&W ND 0.9 taking away three stops of light). Will this also have a positive side effect on IR, so that I do not need to cascade two additional glas planes in front of the lens... (IR/UV-cut + ND)?

No it has a negative effect as it blocks more visible light than  IR light. The same with polarizers.

[jaapv]

Super 35 is 24.89 x 18.66 mm  which is effectively the same as APS-C. The difference is in the use for Video, which is not the subject of this topic.

[tommonego@gmail.com]

It also depends on how you are processing your DNG files. Just checking Photoshop ACR there is correction for the Voitlander 12mm, and 15mm, you may want to use the later as I find over correction with M8 files. This will probably hold true with Lightroom. I use an 18mm Zeiss and it needs correction, just ordered a template so I can use the M8 firmware correction.