Overview

Gain on a camera is defined as the pre-amplification of the light signal/photons before being converted to a digital value. Altering gain is a way of increasing or decreasing dynamic range. Increasing gain (for night time) generally decreases dynamic range to provide better visual representation of the data being received by the camera sensor.

Gain Terms

• Decibel or dB - Logarithmic unit of measurement used to express signal amplification
• ISO - Exponential unit of measurement in camera sensors to express signal amplification
• ADU - Analog-to-Digital unit - The conversion of the sensors/pixels analog reading to a digital value
• e⁻/ADU - Indicates the number of photo-electrons required to equal 1 ADU
• Unity Gain - The gain setting for a sensor where 1 photo-electron (e⁻/ADU) is converted to 1 ADU
• Analog Gain - Native hardware gain amplification
• Digital Gain - Software gain amplification

ISO vs decibels [dB]

• ISO is a more human friendly method of expressing sensitivity to light. If you want to double the brightness of a scene, you may either double the exposure or double the ISO. Halving the brightness can be achieved by halving the ISO. The following ISO changes will result in a doubling brightness:

  • ISO 100 -> ISO 200
  • ISO 200 -> ISO 400
  • ISO 400 -> ISO 800
  • etc

• decibels are a logarithmic expression of sensitivity to light. Doubling the brightness of a scene can be achieved by doubling the exposure or increasing the dB by 6.02. Halving the brightness can be achieved by reducing the dB by 6.02. The following dB changes will result in a doubling of brightness:

  • 0 db -> 6.02 dB
  • 5 db -> 11.02 dB
  • 14.5 dB -> 20.52 dB

Analog Gain Limits

Image sensors can natively amplify the light signal by applying gain. Analog gain is a hardware function to increase the signal levels while still controlling noise levels. It is possible to configure the sensor gain beyond the analog gain limits, but after a given gain level, only digital gain is applied. It is much preferred to stay within the limits of analog gain which provides true hardware signal amplification without amplifying [some] sources of noise. Digital gain amplifies both signal and noise at the same rate and has no benefit to the final data.

libcamera

Sensor	Max Analog Gain	libcamera Model	Value
IMX447	27.0 dB	RPi HQ Camera	22.26
IMX708	24.0 dB	Raspi Camera v3	16.0
IMX296	24.0 dB	Raspi Global Shutter	16.0
IMX462	29.4 dB	IMX462	29.51

Astronomy Cameras

Sensor	Max Analog Gain	ZWO Model	Value	ToupTek Model	Value
IMX676	30.0 dB	ASI676	300	AE676	3162
IMX678	30.3 dB	ASI678	303	G3M678	3273
IMX662	30.3 dB	ASI662	303	G3M662	3273
AR0130CS		ASI120
IMX224		ASI224		G3M224
IMX290	30.0 dB	ASI290	300	GPCMOS02000KPA	3162
IMX385	30.0 dB	ASI385	300		3162
IMX178	27.0 dB ??	ASI178	270		2238
IMX462	29.4 dB	ASI462	294	GPM462	2951
IMX715	30.3 dB	ASI715	303		3273
IMX485		ASI485
IMX585		ASI585
IMX183	27.0 dB	ASI183	270		2238
IMX533	36.0 dB	ASI533	360		6309
IMX294	27.0 dB	ASI294MC	270		2238
IMX492		ASI294MM
MN34230		ASI1600
IMX571	36.0 dB	ASI2600	360	ATR2600	6309

Vendor Reference

ZWO Gain

Most modern ZWO cameras configure gain in steps of 1/10th of a dB with a range of 0 to 600. A gain of 200 equals 20.0dB of gain.

$$ dB = \frac{Gain}{10} $$

Reverse

$$ Gain = 10 \cdot dB $$

ZWO ASI120

These are legacy cameras which do not necessarily follow the standard above. Gain is expressed in a value of 0 to 100. Actual unit correlation is unknown. NEEDS VERIFICATION

PlayerOne Astronomy

Same as ZWO ASI

libcamera gain

Solid information about gain for libcamera is hard to find. I believe the modern libcamera versions gain is expressed as 1/100th of ISO. The normal range of analog gain is 1 to 16 or 22.26. Gain 1.0 equals ISO 100.

Making the assumption that ISO 100 = 0dB of gain

$$ dB = 20 \cdot log_{10}\left(Gain\right) $$

Reverse

$$ Gain = 10^{\left(\frac{dB}{20}\right)} $$

libcamera legacy

There are old references to gain to ISO conversion, which I believe to no longer be the standard.

Algorithm: ISO = (AnalogGain * DigitalGain * 100) / ag_for_iso100

The information I found is that the baseline ISO 100 is a special value ag_for_iso100 which is unique to each sensor model.

ag_for_iso100 is

    1.84 for v2 camera
    2.317 for HQ camera

Digital Gain is almost always 1.0, therefore gain 2.317 is ISO 100 for the HQ Camera.

ToupTek gain

ToupTek expresses gain with a value of 100 to 10000 which is basically an ISO value. Gain can be converted to dB using the following formula:

$$ dB = 20 \cdot log_{10}\left(\frac{Gain}{100}\right) $$

Reverse

$$ Gain = 100 \cdot 10^{\left(\frac{dB}{20}\right)} $$

QHY gain

Some QHY cameras expresses gain directly in dB. A gain of 3.0 is equal to 3dB of gain.

Other QHY cameras express gain with a range of 0-4000. Unit correlation is unknown. NEEDS VERIFICATION

Gain calculations

Note: All gain values calculated with decibels dB

Maintain brightness when adjusting exposure

$$ NewGain = CurrentGain + \left(20 \cdot log_{10}\left(\frac{CurrentExposure}{NewExposure}\right)\right) $$

Alter gain instead of exposure

$$ NewGain = CurrentGain + \left(20 \cdot log_{10}\left(\frac{NewExposure}{CurrentExposure}\right)\right) $$

Maintain brightess when adjusting gain

$$ NewExposure = CurrentExposure \cdot 10^\left(\frac{CurrentGain - NewGain}{20}\right) $$

Alter exposure instead of gain

$$ NewExposure = CurrentExposure \cdot 10^\left(\frac{NewGain - CurrentGain}{20}\right) $$