Is ISO 400 stretched in PP the same or better DR than ISO6400 not stretched?

Started Aug 10, 2013 | Discussions thread
Per Baekgaard
Contributing MemberPosts: 991Gear list
Like?
Re: Is ISO 400 stretched in PP the same or better DR than ISO6400 not stretched?
In reply to Astrophotographer 10, Aug 10, 2013

Greg,

As far as I remember from previous analysis and discussions, the D800 starts to use digital gain somewhere around ISO1000 - ISO1600. At least the histogram at ISO1600 starts to show small amounts of discrete gaps in the green channels too and it gets worse above that (one example: http://blog.kasson.com/?p=2932). See also http://home.comcast.net/~NikonD70/GeneralTopics/Sensors_&_Raw/D800_Read_Noise.htm.

If that holds, I think you will be able to capture more faint objects (at the expense of loosing dynamic range) up to ISO1000. Above that, you will just loose dynamic range but not really gain any significant resolution at the darkest end.

So for astro use, somewhere around ISO1000-1200-1600 should be "optimal". Above that, you will loose dynamic range as highlights clip more easily even though you will not be able to see more dark objects, as the gain is "just" digital.

At lower ISO's, such as ISO400, you will obviously have more dynamic range, but there are dark objects that are not represented after the ADC stage due to lack of (analog) gain.

Between the two (ISO400 and ISO6400) I would expect ISO6400 to be slightly worse than an amplitude stretched ISO400: You loose two 2+ stops of dynamic range vs ISO1000 -- even though at ISO400 you still miss out some 1+ stop of dark objects. But it depends on what you're looking for.

There are other factors involved, of course. I'm not sure how the inherent photon noise impacts this and thus whether the (lack of) additional analog amplification above ISO1600 is a practical problem or "just" a theoretical one. And I'm also not sure about other "trick" Nikon may possibly be using, such as changing the ADC reference at high ISOs. But surely clipping of highlights IS a problem, though.

I've experimented mainly around ISO1600 so far, as that appears to be the "best" compromise. But obviously if you can increase the exposure time, an ISO400 shot will look better

YMMV...

 Per Baekgaard's gear list:Per Baekgaard's gear list
Nikon D800 Nikon AF-S Nikkor 70-200mm f/2.8G ED VR Nikon AF-S Nikkor 14-24mm f/2.8G ED Nikon AF-S Nikkor 24-70mm f/2.8G ED Nikon AF-S Micro-Nikkor 105mm f/2.8G IF-ED VR +15 more
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