Vignetting at long exposures

[Johnclare369]

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Morning all

I'm an experienced Leica user, having an M10, M9 (converted to IR) and a Q2. On both the M10 and Q2, vignetting has suddenly appeared. It appears to only happen at longer exposure, eg 10secs. I've used both cameras all over the world and never come across this. See attached images, straight out of camera. Both were 10-15 second exposures at f11/f13. I was using Lee Filters 10 stop grad with the relevant holders for the lenses.  I've used exactly the same kit many times and never had this problem. The same shot at 'typical' exposures eg 1/125 etc, does not produce the vignetting.

Since this happened (yesterday) I have updated the firmware on both cameras.

Advice welcome!

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[jaapv]

If it suddenly happened to both cameras I think that your processing software is not applying the lens corrections any more. 

[Johnclare369]

Thanks jaapv. In fact have checked that and I can improve it by manually moving the vignette slider to 100 and the midpoint to 0 in LR. However, the vignetting appears in camera, so I can see it on playback. I will update my software to be 100 per cent sure.

Regards

JC

[adan]

Let me toss out this tidbit and see if it makes sense.

Leica M wide-angle lenses have a tendency to vignette on digital sensors. This is because sensors are like a grid of 3D city streets in the low angle of a winter sun - the microscopic 3D structure of the pixels blocks more and more light as one moves away from the center of the image.

And especially so with wide-angle lenses (the sample images appear to be ~24mm (M10) and 28mm (Q2)).

Leica's M firmware corrects for this un-film-like vignetting with camera processing of the images before they are saved to the SD card.

Back at the time of the M9 intro (first full-frame M sensor) I recall Leica mentioning that the firmware would intentionally reduce the amount of vignetting correction that is applied to pictures, in cases where there was the chance of significant image noise. Thus leaving the corners darker.

This is because the correction (electronic brightening or amplifying of the dark vignetted corners) would also amplify any noisy pixels. It takes effectively a 2-stop "push" of the sensor signal  in the corners (on a smooth gradient, of course) to remove the vignetting.

At the time, not being a long-exposure user, I thought of this only with reference to high ISO settings.

However, since long exposures also often produce hot/noisy pixels, it is possible the same dialing back of vignetting correction also kicks in when the camera is set to exposures longer than 1 second (or thereabouts).

Basic equation - a 15 second exposure increases the odds of a random non-image photon (for example, an infrared photon from a warm part of the camera electronics) being recorded as noise 1875 times over a 1/125th second exposure.

I don't know that Leica has kept such code in iterations of M firmware across 13 years, and at least 3 new iterations of cameras and new sensor types. Or whether is is applied to long-exposure-related noise as well as ISO-related noise. but it seems like a possibility.

Edited January 11, 2023 by adan