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

Started 5 months ago | Discussions thread
JimKasson
Contributing MemberPosts: 892Gear list
Like?
Re: Dynamic range, ISO-less sensors
In reply to Rishi Sanyal, 5 months ago

Rishi Sanyal wrote:

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.

It may be that Sony is talking about inter-scene dynamic range: the range of light that the camera can respond to throughout the range of the ISO control. The most generous way to compute this would be to divide the full well capacity by the read noise measured in electrons at the highest ISO, but this is really like measuring automobile top speed downhill with a tailwind.

When measured that way, the a7S has an inter-scene DR of slightly better than 18 stops in single shot mode.

http://blog.kasson.com/?p=6188

Most of us think of intra-scene DR, the range that can be captured in one shot. The most generous way to compute this would be to divide the full well capacity by the read noise measured in electrons at the base ISO, but this is still like measuring automobile top speed downhill with a tailwind.

When measured that way, the a7S has an intra-scene DR of slightly worse than 14 stops in single-shot mode, and slightly worse than 13 stops with the drive mode set to continuous. Silent shutter mode also takes its toll.

http://blog.kasson.com/?p=6172

Then there's what I call photographic dynamic range, which is the full-well exposure in photon-seconds divided by the exposure in photon-seconds that gives a signal to noise ratio of three or four stops. That DR is not much affected by read noise, and is mostly controlled by photon noise. With its 150,000-plus electron FWC, the a7S should do well there, but I haven't measured it yet.

Jim

-- hide signature --
Reply   Reply with quote   Complain
Post (hide subjects)Posted by
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark post MMy threads
Color scheme? Blue / Yellow