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

Started 7 months ago | Discussions thread
Rishi Sanyal
dpreview AdminPosts: 49Gear list
Re: Dynamic range, ISO-less sensors
In reply to ProfHankD, 7 months ago

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....

I think TThorne's point is that he takes issue with Sony's claim of 15.3 stops sensor DR. That's where the 'expectation' came from. I'm inclined to agree. As you say later on, perhaps the sensor DR really is 15.3 EV, but why make that claim if you can never see it? Even Canon DSLRs have fairly high sensor-level DR due to their low upstream read noise, but we never see it after all the downstream read noise added somewhere between the analog signals read off the sensor & the digital files we work with (presumably, at least partly, due to off-imaging-chip ADCs).

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?

This is the million dollar question.

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