D800E night sky shots

Started Mar 21, 2013 | Discussions thread
Jack Hogan
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ISO and Astrophotography
In reply to Astrophotographer 10, May 16, 2013

Hey Greg,

I had a beautifully thought out answer to your post, but DPR saw fit to eat it when I hit post.  DAMN IT GUYS!  Here is the abridged version.

Astrophotographer 10 wrote:

Is that ISO1600 the point where gain kicks in referenced somewhere?

D800e Raw Green Channel Histogram at various ISO: DPR sample comaprison images

As you can see the ISO 800 histogram of the green channel is fully populated, the ISO 1600 is showing a small amount of digital amplification, the 3200 and 6400 are simply the ISO 1600ish ADC integer output multiplied digitally by 2 and 4 - with every second and every fourth value present respectively.  There is no difference between doing the multiplication in-camera or in the raw converter through +EC correction, except that in-camera you may clip desirable highlights which you may regret later.  The Raw converter needs to be well behaved for extreme (>3 stops?) recoveries to work as expected (LR not being one of them).

It also tends to vary depending on the lens as well. I have some tracked images that turned out very nicely at lower ISO as you suggest. ISO1600 is indeed a good ISO if you can track but too dim for 30 second shots.

First you max out Exposure by choosing the longest shutter speed and widest aperture allowed by your technical/artistic constraints, thereby gathering the largest number of photons possible.  Then you choose ISO as a compromise between minimizing shadow SNR and reducing DR.  Bill Claff has worked out where increasing ISO no longer helps shadow SNR for the D800e : unsurprisingly it stops improving at an in-camera ISO setting of 1600 (ignore the blips further up, probably introduced by in-camera noise reduction best performed in PP).  In fact if you needed more DR, you could stop at ISO 400 - that would result in  less than a 1/6 of a stop penalty in shadow SNR for an additional 2 stops of DR.

Cheers,
Jack

PS Of course when one uses such maximum IQ strategies, decoupling Exposure from ISO in manual mode, one needs to correct for pleasing brigthness and tonality during raw conversion - with no penalty in IQ of course.

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