Noise Performance in an ISOless System

Started Mar 26, 2013 | Discussions thread
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dosdan
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Noise Performance in an ISOless System
Mar 26, 2013

Conceptually, as far as noise is concerned, a DSLR signal path can be considered as:

Sensor -> PGA -> ADC

Each of these stages contributes noise. Beside the photonic noise inherent in light itself ("shot" noise), the most common noise is read noise. The contributions of read noise from the 3 stages above can be lumped together by combining the 3 noise components in quadrature (root-mean-square) into a value called the Total Read Noise.

However since some of these noise components are affected by ISO gain, the total read noise changes as gain is applied. It is important to understand this and its effect on overall system noise performance.

The PGA (Programmable Gain Amplifier) is used to apply stepped amounts of analogue gain (increasing the ISO sensitivity) when the signal from the sensor is weak i.e. the exposure level is low. In this discussion, I'll generally omit its noise contribution. You would hope that an amp used to reduce the total noise level was of a low-noise design itself. Some of the PGA noise will come from the input stage of the PGA, and therefore increase as you boost the analogue gain. Another part of the PGA noise will come from the output stage, so it will be of a constant level and not affected by analogue gain changes.

The sensor read noise will be boosted as the analogue gain is increased.

The ADC read noise, coming after the PGA stage, is constant in level, regardless of the amount of analogue gain.

So the simplified signal path becomes:

Sensor (analogue gain influenced) -> ADC

Since noise is coming from different parts of the signal path, the noise is either "input referenced" (photo-electrons or e-) or "output referenced" (DN - Digital Numbers, or ADU - Analogue to Digital Units).

I'll be using www.sensorgen.info data, which is input referenced. This means that the noise is considered as if it was another signal coming from the sensor. So the sensor read noise level (I'll call it just "sensor") might be 3e-. Since the ADC noise is after the PGA, we need to reverse the effect of ISO boost, if we are to think of it as another sensor signal. For example, an ADC has a read noise level of 16e-. At base ISO100 we'll consider the PGA gain to be 1x (it will probably be different from this, but we're interested in relative gain changes in this discussion). So at ISO1600 the PGA gain is 16x, and our ADC noise needs to be divided by 16. Therefore, with input-referencing, the ADC noise is seen at ISO100 as an 16e- signal from the sensor, and at ISO1600, as if it was a 1e- sensor signal. So the the input-referenced total read noise value tends to fall as ISO increases.

The formula for the simplified total read noise we'll be discussing here is:

Total = sqrt(Sensor^2 + ADC^2)

The shape of the graph of this equation vs ISO is very important. We'll be considering the ISOs of ISO100 & ISO1600. Generally, with modern sensors, we're still within the ISO range were analogue gain is applied and any in-camera raw NR is not yet enabled.

Let's look at the K-5 total read noise within this range (Manufacturers ISO used):

http://www.sensorgen.info/PentaxK-5.html

ISO Total e-
100 3.5
200 3.1
400 2.6
800 2.4
1600 1.9

The "16x" ratio (ISO1600/ISO100) of the change in the total read noise as ISO increases is 1.4:1, or just 1.4x. If you play around in a spreadsheet you can roughly get the corresponding values that produce this curve. The values are: sensor=2e- & ADC=2.8e-.

So the K-5 values are:
Sensor: 2e-
ADC: 2.8e-
16x: 1.4x

Other APS-C:

60D:
Sensor: 3e-
ADC: 13e-
16x: 4.3x

550D
Sensor: 2.7e-
ADC: 11.6e-
16x: 4.3x

7D
Sensor: 2.8e-
ADC: 8e-
16x: 2.9x

A77
Sensor: 2.7e-
ADC: 3.5e-
16x: 1.3x

D5200
Sensor: 2e-
ADC: 3e-
16x: 1.5x

FF sensors:

D600
Sensor: 3.7-
ADC: 6.3-
16x: 1.7x

D800
Sensor: 2.9e-
ADC: 3.6e-
16x: 1.2x

D4
Sensor: 2.4e-
ADC: 18.5e-
16x: 7.7x

5D III
Sensor: 4.2e-
ADC: 33e-
16x: 7.9x

1D X
Sensor: 2.6e-
ADC: 38e-
16x: 14.6x

The sensel read noise value will be affected by its size. Here we're not interested in its absolute value, but the way it contributes to the total read noise value.

The "16x" figure shows how effective the use of analogue gain is in improving the total read noise value, as the sensor output level decreases (i.e. at lower exposure levels). Sensors where this value is <2 are approaching ISOless operation. You can also get some idea how bad an idea it would be to operate a camera like the 1D X at base ISO and rely on much boosting of the rendering brightness afterwards in PP. Operating in a suitable ISO range is much better for this camera.

Once the amplified sensor read noise is much bigger then the ADC read noise, there is little reason to apply further analogue gain, as sensor read noise dominates the total read noise, and the total read noise now increases at the same rate as the amplified signal increases. So the SNR due to the read noise remains the same. Therefore for high-ISO operation, cameras switch to digital gain.

With some cameras, particularly the D600 & D800, there are difficulties in getting reasonable curve fits using just Sensor & ADC read noise values. Since the sensorgen info is derived by solving a curve fit of the DxOMark Full SNR graph, and since DxoMark is known to smooth the curves, this is possibly the reason. It could also be that PGA noise does need to be considered in some cases. Or it could be that Nikon is doing something unusual.

The 7D is unusual in having a relatively low 16x ratio compared to the other Canon cameras. It looks like the ADC read noise level is unusually low for a Canon camera.

Some camera have such low 16x values that it seems no change of analogue gain is being used at all: D7000; RX100. Perhaps the unusual results from the D800 should also be considered this way.

Note: just because a camera is ISOless, does not means it's a low-noise design. Some P&S cameras used only digital gain to change ISO. This may have been done just for simplicity.

Why worry about ISOless operation?  Boosting ISO can be done in either an analogue or digital manner. Doing so digitally only improves the rendered image brightness. Using analogue gain at low ISO helps with reducing the contribution of ADC noise to the total read noise, as well as increasing the brightness of a rendered image. However both forms of gain have a negative effect: they decrease the DR (Dynamic Range).

The max. level is determined by the saturation level, either the sensel become full of photo-electrons i.e. reaching FWC (Full-Well Capacity) or reaching FS (Full Scale) in the ADC output level (running out of bits). FWC is only a consideration at base ISO. Once you need to apply extra gain (i.e. due to a low exposure level), FS clipping is the main consideration. Since FS is fixed, as you further amplify the signal, the distance between the noise floor and max. level decreases. If total read noise was constant with ISO, DR would decrease 1 stop for every doubling of gain/ISO. However, since total read noise usually changes in systems using analogue gain, DR decrease steps tend to be smaller at low ISO and increase in size, up to a max. of 1 stop per doubling of gain, at mid ISO. At high-ISO, where digital gain is used, the DR decrease should be exactly 1 step. However two thing can cause the DR value to be different.

1. High ISO involve small sensor signals do it becomes more difficult to get accurate measurements.

2. Cameras may apply raw-level NR.

The problem with decreasing DR as ISO increases, is that it also increases the likelihood of a highlight or specular reflection being clipped. If you're shooting raw, and the total read noise is relatively low, it becomes feasible to shoot at either base ISO (ISOless), or at a relatively low max. ISO, e.g. ISO400 (semi-ISOless), and then increase the rendered image brightness afterwards in PP.

So instead of, in a low-exposure situation, shooting at ISO1600, you instead shoot at ISO100 and afterwards apply 4-stops boost (16x) in PP, or in an auto-ISO setup, shoot at ISO400 (your set max. ISO), and later apply 2 stops boost in PP.

This means that relatively low levels have been recorded and it is unlikely that any highlights will be blown in the capturing phase. Of course, if you just applied the same boost in PP to reach the same rendered brightness level, you run the same chance of blowing a highlight. But since the highlight, as captured, is not yet blown, you still have an opportunity to fiddle with the tone curve to successfully render the image within the limited DR of a print or display screen. So this form of operation becomes like a superior form of highlight protection/recovery.

The disadvantage for the raw shooter is that the review image is darkish. But if cameras & raw file formats were set up to accommodate this type of operation, the review image need not be darkish at all. As the relative contribution of ADC read noise to the total read noise keeps decreasing, it is rather frustrating to see that no manufacturer has yet offered this mode.

Dan.

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