Why does purple edge fringing change with ISO and long-exposure noise reduction?
Asked 12/19/2015
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In a 30-second exposure on a Sony NEX-5R at ISO 3200 and f/2.8, I see a purple fringe along the left and bottom edges, strongest in the bottom-left corner. With the same exposure settings but long-exposure noise reduction enabled, the purple fringe mostly disappears. Lowering ISO to 400, 200, or 100 also reduces the purple color and brightness of the artifact.
Is this actually chromatic aberration, or is it some kind of sensor noise? Why would long-exposure noise reduction affect it if chromatic aberration is caused by the lens?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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There are two components of noise that most noise reduction algorithms reduce: luminance and chrominance. Luminance is the kind of noise that makes the shadows look grainy due to varying brightness from one pixel to the next. Chrominance is the kind of noise that causes spotty colors due to varying colors from one pixel to the next. Chrominance noise can be generated by long exposures as the sensor generates heat. Changing the ISO setting changes the amount of amplification applied to the analog information from the sensor as it is converted to digital data. The noise is also amplified along with the rest of the signal from the sensor.
The two examples in your question appear to exhibit a considerable amount of chrominance noise. The first seems to have it worse in the lower left corner, but the second has it worse in the middle of the light colored wall. This is what you seem to be mistaking for chromatic aberration.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
10y ago
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It’s probably not true chromatic aberration. CA is a lens effect and usually follows high-contrast details across the frame, not just the frame edges/corners in a way that changes a lot with ISO and long-exposure NR.
What you’re seeing is more consistent with chrominance noise and/or sensor glow from a long exposure. During a 30-second exposure the sensor warms up, which can produce colored noise, often stronger near edges or corners. Raising ISO increases amplification, so both the image signal and that unwanted colored noise become more visible.
Long-exposure noise reduction helps because it typically uses a dark-frame subtraction step: after the exposure, the camera records a second dark exposure and subtracts repeatable sensor-pattern noise and hot/glow artifacts. That can greatly reduce purple edge coloration caused by the sensor, but it would not “fix” real lens chromatic aberration.
So the relationship is:
- higher ISO = more amplification of noise/artifacts
- long exposure = more heat-related chroma noise
- long-exposure NR = reduces repeatable long-exposure sensor artifacts
The remaining white fringe may be a separate optical or highlight effect, but the purple component is most likely sensor-related rather than lens CA.
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