Nikon D810 Dual Amplifier Read
Noise
Prepared 2014-12-20 by Bill Claff
Newer Nikon cameras take a new approach to amplification
(conversion gain); they use two amplifiers rather than one.
Apparently it's possible to use two amplifiers in series, each with a lower top
gain, and produce less noise than a single higher gain amplifier.
The approach is similar to DR-Pix (see white paper) except that the first
amplifier in the chain is variable rather than simply being either Low or High.
Although good news for photographers; but it complicates the
work of people, like myself, in trying to characterize sensors.
Because of variability of how the conversion gain is divided between the two
amplifiers, the analysis is not simple!
Read Noise Equation
Just a reminder that the key noise equation looks like this:
output_noise = sqrt((input_noise * conversion_gain)^2 +
conversion_noise^2) "the
equation"
And with two conversion gain stages we have to consider that the output_noise
of stage one is the input_noise of stage two.
The Nikon D810
I'll present my Nikon D810
findings.
For this article I'm using 14-bit values and I'm only showing one of the green
channels for simplicity.
The analog conversion gain range
is from ISO 60 up to and including ISO 1000.
From ISO 60 through ISO 250 the first amplifier remains steady (probably it
increases so slowly that I cannot detect the change).
From ISO 320 through ISO 1000 the gain of the first amplifier slowly increases
requiring less gain in the second stage.
The input noise at the pixel is 1.056e- and the conversion noise at the pixel
is 3.654e-.
When the first stage conversion gain is 1 and the pixel output noise is 3.804e-.
Above ISO 250 the conversion gain increases approximately 1/20EV for each 1/3EV
of ISO and the pixel output noise rises.
Conversion noise at the second amplifier is 0.696DN.
This table summarizes the
calculations for all of the analog ISO settings:
|
|
model |
measured |
||||||
|
|
pixel |
conversion |
conversion |
intermediate |
conversion |
conversion |
read |
read |
|
ISO |
noise |
gain |
noise |
noise |
gain |
noise |
noise |
noise |
|
|
e- |
e-/e- |
e- |
e- |
DN/e- |
DN |
DN |
DN |
|
62 |
1.056 |
1.000 |
3.654 |
3.804 |
0.208 |
0.696 |
1.055 |
1.050 |
|
79 |
1.056 |
1.000 |
3.654 |
3.804 |
0.262 |
0.696 |
1.217 |
1.215 |
|
100 |
1.056 |
1.000 |
3.654 |
3.804 |
0.331 |
0.696 |
1.438 |
1.458 |
|
125 |
1.056 |
1.000 |
3.654 |
3.804 |
0.417 |
0.696 |
1.731 |
1.743 |
|
158 |
1.056 |
1.000 |
3.654 |
3.804 |
0.525 |
0.696 |
2.114 |
2.135 |
|
200 |
1.056 |
1.000 |
3.654 |
3.804 |
0.661 |
0.696 |
2.610 |
2.596 |
|
251 |
1.056 |
1.000 |
3.654 |
3.804 |
0.833 |
0.696 |
3.245 |
3.200 |
|
317 |
1.056 |
1.035 |
3.654 |
3.814 |
1.014 |
0.696 |
3.930 |
3.927 |
|
400 |
1.056 |
1.072 |
3.654 |
3.825 |
1.234 |
0.696 |
4.772 |
4.796 |
|
503 |
1.056 |
1.110 |
3.654 |
3.837 |
1.502 |
0.696 |
5.805 |
5.842 |
|
634 |
1.056 |
1.149 |
3.654 |
3.850 |
1.828 |
0.696 |
7.072 |
7.103 |
|
800 |
1.056 |
1.189 |
3.654 |
3.864 |
2.225 |
0.696 |
8.623 |
8.606 |
|
1007 |
1.056 |
1.231 |
3.654 |
3.878 |
2.707 |
0.696 |
10.523 |
10.479 |
You can see by examining the
table or the following chart that the model read noise and the measured read
noise agree quite well.
(This is a log-log chart and the small discrepancies are generally well within
a 1/6EV tolerance.)
Conclusion
I have demonstrated that the
read noise of the Nikon D810 follows the model of two amplifiers chained
together.
This technique improves high ISO read noise but covers a much small range of
ISO settings than the DR-Pix technology applied to the Sony A7S.
The analog range for the Sony
A7S was from ISO 160 to ISO 25600, a factor of 160.
The analog range of the Nikon D810 was from ISO 60 to ISO 1000, a factor of
only 16.
It's not clear why Nikon took this approach in the D810. Perhaps we'll something more DR-Pix like in the future.