5D mark III lens correction problem with dark shots

Question

Just found a new irritating problem with 5D mark III and Lightroom 4 (currently on 4.2). I shot quite a few shots in the dark over the weekend using high ISO (6400-16000), with EF 24-70 f/2.8L. If I enable the lens profile correction on the dark shots, the corners of the images all develop incredibly strong purple haze that renders the image unusable. Not enabling the correction allows me to use the images just fine. Also images that were shot with similar settings but were shot inside, with more light, don't have the purple fringing issue. Makes me wonder if this is some incompatibility between Canon's high iso noise reduction system and Lightroom's RAW processing when it does lens corrections.
Anyone else seen this?

Answer 1

Sulka Haro wrote:
Replicated the shot under a heavy blanket -> yes, the exposure looks the same. Whatever is causing the corner to be noisier than rest of the exposure doesn't seem to be caused by light.
At that very high of ISO you are actually seeing small differences in the sensor chip temperature due to heat from the electronics in the camera. The small amount of increased noise you're seeing in the corner at ISO 25600 is insignificant.
Sulka Haro wrote:
Interestingly enabling long exposure compensation doesn't seem to do very much to really improve the shots at these sensitivities. When enabled, the exaggerated corner noise does disappear, but also it looks like some areas in the middle of the exposure become more noisy with non-uniform noise blotches appearing instead of fairly uniform slightly higher noise pattern.
I haven't needed to use Long Exposure Noise Reduction with my 5D MKII, so have no firsthand experience. My guess is that at very high ISO settings (above 6400) a lot of what your are seeing is random noise or "shot noise," which changes from frame to frame. This causes the dark frame subtraction to "fail" in the way you are seeing. Just a guess.
There is an excellent 5D MKIII review that compares noise reduction with several other high-end camera models. The reviewer also states,
"The 5D Mark III's ISO 51200 and 102400 remain a complete mess even with strong noise reduction applied - You have to be desperate to use these settings. ISO 12800 and 25600 remain very marginal for my uses. I always shoot in the lowest standard ISO setting that will allow me to get my desired shot, but begin to cringe when settings above ISO 3200 must be employed. Your standards and applications may be different."