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12-19-2019, 09:38 AM   #31
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Does "peripheral illumination correction" affect this?

12-19-2019, 01:18 PM   #32
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I was first relieved when I checked (DNGs of) e.g. https://www.pentaxforums.com/forums/members/98441-jense/albums/13376-night-s...ture127114.jpg (ISO3200) and https://www.pentaxforums.com/forums/members/98441-jense/albums/13376-night-s...ture127116.jpg (ISO400), which show in identical fashion a very uniform noise coming out as I push exposure up, no color cast whatsoever. BUT those are long-term exposures with the respective automatic dark frame subtraction enabled, I think.

Looking through my pictures for examples, magenta mergins do come up unevenly as well when pushing 2-2.5 stops at ISO6400 or, rarely, pushing a bit more at ISO3200 - a 'hard' band on the bottom side in normal camera orientation, which means top side in the camera and, softer and less pronounced along the other long edge. I have at least one picture, where it is very disturbing at +1.4EV (ISO64000) already, so definitely would be in a practically relevant range. Anything other than at ISO6400, it don't find it in my pictures.

Lacking consistency, I tried a high-contrast scene with a smallish bright center, took an exposure series, a two minute video and another exposure series with the same manual settings. No difference, as far as i can tell. Doesn't seem to be an easily provoked sensor temperature issue. Camera was at 23C before and 24C afterwards.


Given that for me, based on my use so far, which involved quite a bit of happy high-ISO use, it is mainly an isolated ISO6400 problem and rarely affects any properly exposed pictures, I'll not bother. Attaching a JPEG of where I would have noticed it in practice - if the shot would have been sharp and not 1/2s handheld ;-) Cropped at the top, but not rotated to preserve the edge. Somehow it's very visible with a middle-grey background, but almost gone with the bright default in PF. Use a night theme or open it separately, my browser's 'view image' shows it on a dark background.
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Last edited by JensE; 12-19-2019 at 01:34 PM. Reason: attached picture defective
12-19-2019, 07:58 PM   #33
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I've got some anecdotal info on this issue. When I got my KP in 2018, I often used auto-ISO, which means I have plenty of photos taken from around 1600 to 6400. Reading this thread reminded my of only one photo I'd taken where I noticed a magenta band on the right side (vertical orientation, bottom of sensor), which was, as you may guess, ISO 6400. I'll say that was the only time I ever found it to be noticeable without bumping the exposure, though now experimenting with some other ISO 800-12800 photos I can really only find evidence of the amp glow between 800 and 6400, and it never seems to be as harsh as OPs examples.

Cut to a year later, this Spring, when I sent my KP to the repair shop for a strange, unrelated issue. Now, checking all my photos since the repair, I'm having more trouble finding the banding. These days, however, I tend to shoot with manual ISO and don't really set it higher than 1600, so I have fewer examples to test, but I think it's worth pointing out that my experience with the magenta banding seems to have diminished somewhat (6400 is now the only ISO where I have found it, and less so than before.) The repair I'd received was essentially a replacement of the main processor, though I don't know which part exactly, I assume that most of the internal circuitry is not modular and is replaced as a whole, which might explain why the image processing seems to be different. The sensor is probably unchanged though.

It's a shame that the issue exists at all, but at least it's not bad enough to affect my shooting. When you use that high of an ISO you really shouldn't be lifting the shadows much anyway, so I aim for proper exposure rather than underexposing in dark scenes, so I won't require any post-brightening. Though I read in the K1 thread that the banding exists on the original K1 as well, it still seems to be an issue with Pentax's processing pipeline that is only exacerbated in the defined range, while higher ISOs benefit from an adjusted processing method. So then, the cameras may not not be truly invariant throughout its whole ISO range, but ISO invariance is overrated anyway. It's simply not convenient to shoot at base ISO always just to avoid clipping highlights. The camera has the ability to process noise more efficiently and earlier in the pipeline than Raw developers, and while it may have some quirks that will hopefully be fixed in the future, I'd rather take advantage of the KP's noise reduction and expose correctly rather than jump through hoops for the absolute maximum potential image quality.

Something else I want to point out is the Photons to Photos dynamic range to ISO chart. Obviously we can see that ISO 636 is where the accelerator kicks in and provides greater dynamic range at these ISOs than Pentax's competitors, but directly after ISO 3200 you can see that the dynamic range starts to waver between full ISO-stops. I've heard that partial ISO stops usually add or reduce gain purely digitally, which can explain the jumps in ISO, but I wonder if the shift in stability between 3200 and 6400 has anything to do with the magenta banding. You can also see that ISO 1600-2011 is roughly where the accelerated DR begins to drop below the lowest accelerated DR (at ISO 503). Again, I don't know if this is relevant, but I'm interested in whatever connections there may be.
12-31-2019, 08:00 AM   #34
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Exposing the scene correctly is ALWAYS the best way to have better IQ and less noise. The Accelerator is for managing noise while boosting sensitivities.

01-01-2020, 07:40 AM   #35
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I've actually looked into this a bit and it seems even the digital 645 series gets or can get this effect (maybe not as badly) when you extreme shadow boost. I've noticed a definite magenta tint with some very extreme recovery, but I was easily able to handle it with chromatic aberration removal tools in lightroom. Your first shots however were completely out of line, I have never experienced such a prominent band of magenta on my unit.
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