Quote: Thank you very much for these pics. I have no technical expertise, only some real-life shooting with the K20 at higher ISO. But I do not shoot @ 3200 & 6400 often.
Note* it is immensely helpful, in a careful examination like this, to use a picture viewer which allows you to immediately scroll from one pic to the next, with EXIF data and histograms displayed for immediate comparison & evaluation.
That said, when I have shot high ISOs I learned how vital it is to expose to the right as far as possible. When I underexpose, at any ISO for that matter, noise begins to rear its ugly head. It is clear, using the histogram tool, that every shot you present here is underexposed. Note that img9 is your most exposed to the right shot you have.
Compare img8 to img9 for a sec, both of which were shot @ ISO 6400. Img8 has the horrible blue cast to which you refer, but img9 is mostly absent of the cast. What is different about them? Look @ their respective histograms for a sec and you'll see img 9 is closer to the right (though could/should be more so) than img8. Not a lot closer is img9 than img 8 to the right--which demonstrates how even a small difference in exposing can make all the difference when at extreme ISO limits.
Unfortunately, you do not provide a similar composition for img7, at a different exposure, which is also shot @ ISO 6400. However, img7, particularly when you see how horribly underexposed it is with a histogram tool, is a very good shot for ISO 6400. I am willing to bet, if you took the same shot, exposed to the right, this blue cast would be gone. You are shooting at the camera's most extreme limitations, and underexposing to boot. I am pretty sure I could produce similar casting problems with my K20 if I too shot in such low light, with such underexposed images. I could go try it, but it has been a long day/night. :ugh:
Img3, while still underexposed @ 6400, is more to the right than all but img9 and it shows; the image is not as bad as those where the underexposure is more severe.
I could go into more depth, but hope to have made my point. There is here, what social scientists refer to as a significant, negative correlation ie., the lesser the exposure, the greater the problem. I honestly believe this, but am surely open to the explanations of others. I could produce a nice ISO 3200 or 6400 shot with my K20, but I assure you accompanying those nice shots will be histograms which are
exposed to the right!
Img4 is a horrible example I think because you have the extremes of darkness on one hand, with bright lights on the other. You exposed for the lights, and so the dark areas are tremendously underexposed. You simply cannot get away with underexposure at a camera's limits. Bear in mind that ISO 6400, on the K20, is only there for emergencies--by all accounts.
Finally, to sign off, look at your img5. It is shot @ ISO 100, ISO 100! And yet it has way too much noise in it. Why, well look at the tell-tale histogram--it is underexposed, by a lot--it is only exceeded in underexposure by img6, which you shot @ ISO 1600. It is a wonder img6 looks as good as it does--the noise, given the EXIF data, is not that bad.
Show me a great high ISO shot and I'll show you a great exposure--particularly indoors, in low light.