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

Started 5 months ago | Discussions thread
JonTafferOfPhotography
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Re: a7S high ISO claim - - -over hyped or not ?
In reply to LSHorwitz1, 5 months ago

LSHorwitz1 wrote:

As a long time satisfied NEX-7 owner contemplating the possible purchase of an a7S, I am trying to clarify just how much of an improvement in low light performance I might expect.

Using DxOMark measurements as a starting point, I see that my NEX-7 is rated at 1016 and that the a7S is measured at 3702.

One might conclude that this measured less than a 4-fold increase in ISO should result in less than a 2 stop benefit, since each stop is a doubling / halving.

The comparison of the published specifications of two camera bodies as described by Sony however suggests a much larger gain, since one camera, the NEX-7, was specified as having a max sensitivity of 16,000 ISO, while the new a7S is described as having a max sensitivity of 102,400, over a six-fold improvement.

Is this claim by Sony over-stated, and thus the promised 6+ fold ISO gain increase claimed only really measureses to less than a 4 fold increase?

I would be disappointed to go from my current NEX-7 and all my associated glass to the a7S at considerable expsnse, only to find that the DxO measured improvement of less than 2 stops was all that I gained.

To avoid the confusion which may arise from Sony's offering extended ISO beyond 102,400 to 409,000, I want to ask anyone who replies to please leave this extended claim out of the discussion. I do realize that further claims by Sony to extend the a7S to yet higher extended ISO of 409,000 makes the a7S appear to have yet additional sensitivity, but I would judge this to be no more than manually "push processing" the sensor further, no different from manually setting my NEX-7 to expose a couple more stops underexposed. I am trying to compare both bodies based entirely on their measured sensitivities for their normal claimed range of ISO.

Thanks for any insights here,

Larry

based on the simple dimensions of a full frame sensor, you will gain more than a stop in SNR performance holding everything constant.

then if you factor in just pixel pitch, there is roughly another stop there, if you are comparing at 100%.

finally, if you consider 2 years of sensor and more importantly image processing advancements, I'd estimate another stop if not 1.5 stops based on image samples I've seen compared to my d4

id say you can expect something like a 3 stop improvement over what you have in the nex-7 in terms of SNR and probably more gradual DR and CD fall off as well.

this method above is better for thinking about practical ISO performance gains, rather than comparing max ISO which is, by in large, misleading and manipulated.

also remember dxo ISO ratings are normalized, so a high MP body will benefit due to downsampling. so in reality the ISO performance of the nex-7 is actually far worse at 100% than you would think based just on dxo's ISO rating. the d800 folks can't seem to wrap their heads around that, but it is true. it becomes really obvious once you start cropping.

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