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: 607
Re: Dynamic range, ISO-less sensors

LSHorwitz1 wrote:

My consclusions from all of this are that DxOMark ISO sensitivity only tells a very partial story about how the sensor behaves, and DxOMark.s choice of a specific reference SNR and their measurement approach for sensitivity does not characterize the low light behavior in absolute terms but rather only relative to others which they have measured.

Well, DxO does provide full SNR curves at all ISOs. So, technically, you can compare performance for almost any tone a sensor can capture. It's just hard to do so across cameras b/c they don't allow you to compare full SNR curves across different cameras (admittedly, the graphs could get quite complex). Essentially, you have to enter the data manually into a graphing software.

For example, using their full SNR data, you can tell that the A7S at ISO 409k has pixel-level SNR well above ISO 102k on the 5D Mark III for dark tones; but for brighter shot-noise limited tones, the 5DIII, naturally, wins:

DxOMark's full SNR curves for the Sony A7S and the Canon 5D Mark III at very high ISOs. Not normalized.

Normalizing to equivalent resolution will of course move the 5D Mark III curves to the left, which means that the A7S ISO 409k advantage will be limited to only the very darkest of tones. But, still, the fact that there's any advantage over ISO 102k is impressive.

I am very much looking forward to learning about how the -4 EV autofocus world behaves, and whether the a7S makes focusing decisions which greatly improve upon the a7 and a7R, both of which frightened me off as a buyer given their lackluster performances.

The a7S focuses demonstrably better in low light than the a7R. That said, in my own subjective low light tests of a face in very, very low light, the center AF point of my 5D Mark III + f/1.8 prime focused better than that of the a7S + f/1.8 prime (which, in this scenario, kept hunting). I think part of this is up to the type of detail these systems are attempting to detect; i.e. horizontal, vertical, diagonal line sensitivity. The more of these types the merrier. Last I checked the Sony's were only vertical line-sensitive (admittedly, this may have changed; I don't know). It's my opinion that some of these contrast-detection systems still have a ways to go before catching up to dedicated PDAF systems (despite the accuracy advantage of CDAF systems).

I especially appreciated the wonderful first offerings from DPReview along with the superb comments from Rishi Sanyal and others here.

Thanks Larry.


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