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

Started Jul 3, 2014 | Discussions thread
Rishi Sanyal
Rishi Sanyal dpreview Admin • Posts: 809
Re: a7S high ISO claim - - -over hyped or not ?

Pixel Pooper wrote:

With Sony sensors there is no difference between raising ISO and push processing apart from the lower DR at higher ISO. There is only one sensitivity, and every setting above base ISO is push processed.

While generally true for Sony, this is actually not the case with the A7S. There appears to be some downstream read noise that limits shadow performance at lower ISOs, and this is what caps the low ISO pixel-level dynamic range below that of the A7R.

Also, to the OP: this article we recently published may give you some idea of real-world ISO performance. If you make it through to the 2nd page, you'll note that ISO performance can't simply be talked of in terms of 'this many stops better', because different tones in the image are affected differently by the various sources of noise. For example, brighter tones (higher signals) will really only improve if the sensor gained efficiency (sometimes referred to as 'effective quantum efficiency'), while shadow performance will increase both by this and by lower read noise. We've only seen a tiny increase in actual efficiency, so we believe most of the benefit comes from optimizations in representing very low signals (lower aggregate read noise due to less pixels, lower upstream read noise, lower quantization error, or any combination of these & other factors). That means most of the benefits of the A7S will only be seen in deep shadows of high ISO shots, or in overall performance above extremely high ISOs (circa ISO 51,200 & above). But have a look yourself - different folks have different cutoffs for what they consider 'significant improvement'

That said - and someone already mentioned this earlier - the increase in sensor size alone will give you at least a stop better ISO performance than the NEX-7. Simply b/c a full-frame sensor literally can capture ~2.25x as much light as a similar APS-C sensor. Factor in other optimizations, and you do really have a 'low light king'. But, again, as we say in our conclusions of that article, the benefit of the A7S over, say, the A7R can only really be appreciated at very high ISOs. And more appreciated in the shadows of these high ISOs.

There's also a dynamic range benefit at higher ISOs. At base ISO, however, there's a dynamic range cost compared to the A7R.

Hope this helps,


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