ISOless sensors, read noise and photography - many questions!

Started Oct 3, 2013 | Discussions thread
RussellInCincinnati
Senior MemberPosts: 3,197
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interesting that underexposed files can be smaller
In reply to John Sheehy, Oct 31, 2013

Thus the Nex C3 is a good deal of the way towards being Iliah Borg's "true ISO-less camera."
John Sheehy wrote:

Well, except for Canon DSLRs and some non-Sony sensors used in Nikons like the D4, most cameras with RAW are pretty "ISO-less" by ISO 400 or less. ...Shooting an exposure index of 6400 from the camera's ISO 800 gain not only gives you more headroom, but if that headroom is not put to use, lossless-compressed RAWs are smaller...high-order bits contain all or mostly zeros, which can be severely compressed.

Fascinating. Instead of letting the camera firmware throw meaningless/random lower-order bits into every data point, which is basically writing meaningful higher-order bits pointlessly padded on the right with random lower-order bits, you are just storing string-of-zero-prefixed "significant figures" in underexposed raw files. So that in some cases the raw files are smaller.

Upshot of all this is to make overexposing a low-light photo, or bothering to bracket it, is quaint history.

Well, I have Canons which aren't so-called "ISO-less" until 800 to 3200, but I have always balked at people's suggestions that "proper exposure is very important at high ISOs"; the fact is, the absolute exposure, not the relative exposure at a high ISO, is what determines noise first and foremost.

Sure. If the camera isn't usefully amplifying the analog signal (in some way that's better than digital manipulation) past some certain ISO, it's the greater or lesser quantity of photons in the image that are going to determine how much arithmetic multiplication you're going to be doing to the noise as you multiply the data. The fewer the photons hitting the sensor, the more you're going to be amplifying the inherent sensor noise on your way to making the real data show as full brightness.

It seems that many people have trouble distinguishing between relatives and absolutes, and seeing less noise at ISO 3200 with +2/3 EC, and more noise at ISO 3200 and -2/3 EC, they falsely assume that the relative exposure was the thing that made the former better, when, in fact, the only thing that made it better was the 1.33 stops extra absolute exposure, and -1/3 stop at ISO 1600 would give the same noise benefit, and 1 stop more highlight headroom.

It is weird to set a fixed, low ISO and watch the screen get arbitrarily dim as you reduce the shutter speed and/or close down the lens, in the field. Your instinct in the moment is the uncomfortable feeling that your image is falling off a cliff, because of the viewfinder-dimming-feedback you are getting (since your viewfinder doesn't know you're going to be raw post-processing). It takes some to get over that and realize that simply the less or more you expose the image, you're really only deciding to have more or less noise...with the final brightness always ending up fine no matter how deep you are diving.

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