CCD and CMOS Dynamic Range Calculator
Enter the full well capacity and read noise from your camera's data sheet to see its dynamic range and whether the analogue to digital converter can hold all of it.
Sensor dynamic range
Dynamic range of a CCD or CMOS camera from full well and read noise.
Electrons a pixel holds before it saturates.
At the same gain setting as the full well figure.
Dynamic range
15.0 stops
33,333:1, or 90.5 dB.
- Ratio (full well / read noise)
- 33,333:1
- In stops
- 15.02
- In decibels
- 90.5 dB
- ADC bits needed
- 16-bit
- Gain at full scale
- 0.763 e⁻/ADU
- Read noise in ADU
- 1.97
- Usable range
- 15.0 stops
The sensor sets the limit. The 16-bit ADC has enough levels to cover all 15.0 stops the pixels can record.
Engineering dynamic range from data sheet figures. Real images lose some of it to sky glow, dark current and offset, and both full well and read noise change with gain.
How this is calculated
- 1
Ratio
Dynamic range is full well capacity divided by read noise. 50,000 e⁻ / 1.5 e⁻ gives 33,333:1.
- 2
Stops and decibels
Stops are log₂ of the ratio, so 33,333:1 is 15.0 stops. Decibels are 20 × log₁₀ of the ratio, 90.5 dB here.
- 3
ADC bits and gain
An ADC needs at least as many bits as there are stops, rounded up, so this sensor needs 16 bits. Spreading the full well over 2¹⁶ = 65,536 levels gives a gain of 0.76 e⁻ per ADU.
- 4
Which part limits
If the ADC has fewer bits than the sensor has stops, the converter limits the range and the faintest steps are lost to quantisation. Otherwise the sensor itself sets the limit.
Frequently asked questions
How do you calculate the dynamic range of a CCD?
Divide the full well capacity by the read noise, both in electrons. For stops take log₂ of the result, and for decibels multiply log₁₀ of the result by 20.
Is a 16-bit camera always better than a 12-bit one?
Not always. Extra bits only help if the sensor has more than 12 stops of range at the gain you use. Many modern CMOS sensors have very low read noise, so at moderate gain 12 or 14 bits is enough.
What is a good dynamic range for astrophotography?
About 12 stops is good, and 13 to 15 stops is excellent. More range lets you record faint nebulosity and bright stars in the same sub without the stars saturating.
What is unity gain?
Unity gain is the setting where one electron equals one ADU. Many CMOS astronomy cameras have it marked in their manuals, and it is often a good balance between read noise and dynamic range.