Why does RAW black level stay the same with long exposure noise reduction enabled?
Asked 3/9/2017
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I shot two RAW frames with the lens cap on at 10 seconds: one with Long Exposure Noise Reduction (LENR) on and one with it off. In both RAW files, the metadata reports the same black level: [200, 200, 200, 200] on a 12-bit scale.
I expected the LENR-on file to show a lower or changed black level, since the camera should subtract a dark frame from the original exposure. Why does the reported black level remain 200 after dark-frame subtraction?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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Noise reduction is concerned with maximizing the Signal-to-Noise Ratio (SNR) by differentiating between actual signal and signal created by noise. With a lens cap on the camera there is no signal in terms of electrons excited by photons belonging to light entering the camera that are striking the imaging sensor. It is all noise. How the camera acts under such conditions may not be indicative of how the camera acts under the conditions for which the LENR system was designed: reducing predictable noise from an image with actual signal in it.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
9y ago
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Generated from our catalog & community — verify before relying on it.
The RAW black level tag is usually a calibration offset, not a report of whether the image data has had all dark signal removed.
LENR is mainly meant to reduce predictable long-exposure noise, especially fixed-pattern noise, by subtracting a second dark frame. It does not eliminate all noise, and it does not necessarily change the metadata black level value.
With the lens cap on, there is no real image signal—only sensor/electronic noise. Some of that noise is repeatable and can be reduced by dark-frame subtraction, but other components, such as thermal noise and dark current variations, change from shot to shot and remain after subtraction.
So both things can be true at once:
- LENR can reduce fixed-pattern artifacts in the RAW image data.
- The RAW file can still report the same black level (200) because that value is a baseline calibration for the sensor/RAW processing, not proof that no subtraction occurred.
In short: LENR reduces certain noise patterns; it does not mean the RAW black level metadata must become zero or change at all.
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