ISOless sensors, read noise and photography - many questions!

Started Oct 3, 2013 | Discussions thread
bobn2 Forum Pro • Posts: 59,699
Re: ISOless sensors, read noise and photography - many questions!

boardsy wrote:

Accepting the standard definition of exposure as the amount of light registered by the sensor per unit area, and dependent solely on aperture size and shutter speed (not on ISO selection, even if this is used to influence aperture size and shutter speed!), a few questions arise in my mind:

- in order to maximise exposure and DR and minimise noise, do you really need to know your sensor's read noise and ISO DR curves?

Bobn tells us here that the flat read noise characteristic of the Nikon D7000 makes it effectively ISOless:

- if the sensor is effectively ISOless then is base ISO always optimal (providing that you can see enough of the image on your camera to compose and focus in a sub-optimal exposure scenario), or are you as well off raising it in camera (providing highlights are preserved) rather than in RAW/pp?

There are other factors to take into account, in no particular order:

- on those graphs the 'base ISO' is not always too accurate, because the sensor may have been run into non-linearity.

- there are qualitative factors as well as the measure of read noise to take into account.

- is it possible or likely that the camera's ISO implementation is better tuned to its sensor and RAW data than your RAW software? Aside from highlight preservation on exposure, are there other benefits to "doing ISO" in post?

It's possible. Nikon, for instance, does some subtle gain adjustments with its cameras. Presumably they wouldn't be doing them unless the engineers thought that they brought some advantage. BTW, I think it's worthwhile distinguishing between 'processing' and 'post processing'. I think the brightness setting is best done in processing (raw conversion), not post-processing.

- for a given sensor may it be more effective to raise ISO to some minimum/optimal level vis-a-vis its read noise?

Yes, I think that's true. The D7000 above is better at 200 for instance.

- how do us non-scientists interpret these sensor graphs? E.g. for the Sony NEX C3 (probably the same sensor as my F3, maybe, lol), read noise actually drops down to ISO800 ...but so does DR! Is ISO400 the best compromise here, or should best DR (at ISO200) trump lower read noise (at ISO800)? And could this non-base ISO be valid for all shooting, or just low-light where optimal exposure becomes problematic?

DR trumps lower read noise if you need it, which is one of those judgements based on experience. A rule of thumb which covers most cameras pretty well is go with ISO but stop about 2 stops faster than base (3 for an APS-C Canon, 4 for a FF one).


- finally, could all this be solved for a given sensor by simply shooting a set of fixed aperture/shutter shots at each ISO level, normalising the resultant RAW brightnesses (with the, er, "exposure" slider :-|) and comparing noise (in the shadows, I guess?) by eye?

I think personal tests to one's own requirements are always the best - part of getting to know your camera. Just to add, the same goes for you raw processing tools.

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