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

Started Jul 3, 2014 | Discussions thread
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LSHorwitz1 OP Senior Member • Posts: 2,437
Re: Dynamic range, ISO-less sensors

ProfHankD wrote:

TThorne wrote:

Rishi Sanyal wrote:

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

This is devastating news to me. I am truly saddened to see this confirmed and thankful at the same time for the work you have put into it. I love everything about the A7S, but for practical issues on the job (for me at least) the A7 and A7R seem like the reigning champions, as I value low ISO DR above much else.

I don't know why people expect cameras with good very high ISO performance to do well at low ISOs -- that's really not how it works. The DR for the A7S at higher ISOs appears to be significantly better than any other camera DxO has tested, and 13+ Evs at low ISOs is pretty amazing for the same sensor. Makes one wonder where they put all that charge....

More specifically, there's been a lot of discussion in the past about sensors being ISO-less: underexposing at a low ISO and digitally adjusting the raw does as well as turning-up the analog gain (ISO setting). In fact, last year I had students do a little project that confirmed even some cheap Canon PowerShots using CHDK are essentially ISO-less. Most Sony sensors seem to be ISO-less, which is what you'd expect if you have good analog signal handling. Anyway, the point is that the A7S clearly is not ISO-less. Perhaps sensor DR really is that 15+ range Sony mentions (without citation) and the analog electronics just are not able to cover it all without a gain adjustment?

Personally, I miss ISO 25 because that's what I was most often shooting at with film (Panatomic-X in 1:100 Rodinal or Kodachrome 25). I know you can (usually) use an ND filter to get the same effect, but adding filters isn't ideal.... And yes: I would use ISO 25 more often than ISO 100K.

Hi everyone! I am the original poster, back from a busy day where I had no opportunity to reply earlier today as the wonderful replies to my original inquiry came in.

First, let me thank you all most sincerely for a rich, insightful, and very informative set of comments and remarks. I have learned a lot and, as is nearly always true, am just amazed at how much great knowledge and helpful people attend this and other DPReview forums.

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. The amount of noise, the spectral characteristics of the noise, and the changing characteristics of the noise spectrum over the dynamic range of the sensor make a complex, non linear, degredation which can affect shadow detail differently from highlights, etc. I am also concluding that a camera maker can and will, somewhat strategically, specify the top ISO for a specific body, understanding that a continuum of Image Quality versus ISO setting choices will be thus provided, leaving the photogapher to choose how much degredation they will tolerate.

Given the predictable gains in going from APS-C sensor size to full frame, and the additional gain coming from greater sensor efficiency arising from fewer, larger (12Mpixel) photosites (versus my NEX-7's 24Mpixel count), the remaining gain here is attributable to whatever sensor efficiency gains and noise reductions then can be squeezed out of very mature CMOS and DSP technologies. In this regard, the a7S now seems less of a breakthrough and more like a very nice low light optimazation, with 400,000+ ISO a bit fanciful, all things considered.

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. I am still very much a prospective owner of an a7S given the discussion of this and other recent threads, but frankly the evidence has yet to fully clinch the deal. The jury is still out, but the initial results seem to make a compelling case.

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

Thanks once again,


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