Photon Transfer Curve Example

Started Aug 24, 2015 | Discussions thread
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bclaff Forum Pro • Posts: 12,939
Photon Transfer Curve Example

I was working on a different post in which I plan to use a real Photon Transfer Curve (PTC) as an example and I got distracted by PTCs in general.

So here are two for the Nikon D7200.

The first is prepared from DxOMark derived data:

The light gray lines are constructs taken from Janesick.

The horizontal line is at the level of read noise.
The line with a slope of 1/2 is the shot noise line and it crosses y=0 at e-/DN.
The line with a slope of 1 is the Fixed Pattern Noise (FPN) line and it crosses y=0 at 1/PN where PN is the pattern noise.
The intersections of these lines divide the PRC into three regimes: read noise, shot noise, and FPN.
There is a fourth regime, full well, generally beyond FPN or shot.

Note that in this case, as if often true of modern sensors, the Analog to Digital Converter (ADC) reaches full well before the FPN regime.

We can contrast this PTC with one generated from actual measurements:

Note that although some values are slightly different this curve is pretty much as expected.
(This isn't always true. Some of the DxOMark data is pretty far off.)

The red and blue channels are above green because of Nikon white balance preconditioning.

The reason for the red and blue "bump" below a signal of 8 DN is not clear.
I've seen this consistently with multiple Nikon cameras. I haven't looked closely at other brands.
It's a curiousity but does not adversely affect my results.

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Bill (visit me at http://www.PhotonsToPhotos.net )

Nikon D7200
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