a7S high ISO claim - - -over hyped or not ?

Started Jul 3, 2014 | Discussions thread
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Rishi Sanyal
Rishi Sanyal dpreview Admin • Posts: 583
Re: a7S high ISO claim - - -over hyped or not ?

Well, I think the bigger issue here is that physics is physics, and once you've already gotten read noise down as low as many of these cameras have (through sophisticated techniques like correlated double sampling), the only way to drastically increase ISO performance is to capture more light. Capturing more light increases SNR by decreasing shot/photon noise contributions.

And the only way to do that is to (1) increase sensor size, or (2) increase sensor efficiency. Or, of course, use faster glass.

So Sony's investment in MF probably reflects these realities, to a certain extent. Increasing sensor efficiency is difficult, as far as I understand. Partly b/c sensors are already so good, and also b/c it's difficult to mitigate perhaps the largest source of light loss - the color filters. Short of making a 3-CCD/CMOS with 3 different light paths, that is.

By the way, the higher SNR due to lower shot noise I referred to above is also what gives medium format 'that medium format look' (aside from conversations about DOF and bokeh, of course). It's the same reason a shot taken at ISO 100 looks better than the same shot taken at ISO 400 on even the best performing camera. The ISO 100 shot received 4x as much light as the ISO 400 shot (assuming you matched exposures accordingly); since bright shot-noise limited tones have SNR = sqrt(signal), the ISO 100 image would have twice the SNR of the ISO 400 shot in well-exposed regions (so we're ignoring read noise and tones of the image affected by read noise).

Sony's new MF CMOS sensor has 1.68x the surface area of full-frame, so theoretically can potentially give you almost 2/3 EV ISO performance advantage. But that's assuming similar aggregate read noise performance (and remember, with 50 million pixels - that's twice as many pixels to be read as a 25 MP sensor), similar QE, and of course similar access to fast lenses (which is definitely *not* the case). And, by the way, before this MF CMOS sensor from Sony, previous MF digital sensors had really, really low performance (either low QE, high read noise, or both) that limited their ISO performance despite their theoretical advantage due to their size. But for well-exposed images (landscapes, studio lighting, etc.), you couldn't beat those MF shots (well, unless you used multi-imaging techniques). Not just because of their resolution, but b/c of the very high SNR due to lower photon/shot noise contributions.

And while we're on this subject - you can increase the SNR any camera is capable of by averaging multiple shots. For example, take two APS-C shots, align them, then average them together and you'll basically get pretty close to what a FF camera might have gotten given the same focal plane exposure. This is simply b/c averaging two shots is like collecting twice the amount of light as far as photon statistics is concerned - and a FF has ~2.25x the surface area of APS-C and so, generally, collects twice as much light given the same shutter speed/aperture. Neat, huh? It doesn't perfectly work out b/c the two APS-C shots together had twice as many pixels read as the FF would have (assuming equivalent resolutions). But, generally speaking, SNR increases as the square root of the # of images averaged.

Anyway, to answer your question: I think it is somewhat fair to say that sensors have gotten so good that we're running up against practical limitations that'll require quite some innovation to overcome. But look at how far we've come since the advent of digital cameras. Who knows what's on the horizon?

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“It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.” -- Sherlock Holmes

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