D800E night sky shots

Started Mar 21, 2013 | Discussions thread
Jack Hogan
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Re: D800E night sky shots: Fabulous. ISO?
In reply to Astrophotographer 10, May 16, 2013

Astrophotographer 10 wrote:

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

Since I had DPR's Studio Scene Comparison  Raw files open to determine the histograms in the previous post, I also plotted the read noise from the D800e 'optical black' sensels against ISO.  These photosites are fully masked from light but are processed like all others, so the measured noise is unffected by llight (shot noise and PRNU), yielding 'true' read noise*.

Every time ISO is increased by a stop in these equally bright images, shutter speed is halved resulting in half the number of incident photons at the sensor and therefore half the signal (voltage) out of the photosite.  The camera needs therefore to amplify that signal by a factor of two in order to keep the Raw data (and consequently output brightness) constant.  It can do it analogically, boosting voltage before it is fed to the ADC, and/or digitally, multiplying the number output by the ADC.  We can see where the camera applies analog amplification, because it will also affect the (read) noise in the masked optical black sensels.  If it applies digital amplification, on the other hand, the noise will not change.

D800e Optical Black Read Noise in DN versus ISO

The graph clearly shows that the D800e starts amplifying the signal analogically after ISO 100 and stops before ISO 1600.

Jack

*and dark current shot noise, but we'll ignore that here as the longest exposure is 1/5 of a second at ISO 50 - is that why it looks a little noisier than ISO 100?

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