Why does Pentax long-exposure noise reduction sometimes take less time than the exposure?
Asked 6/24/2011
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On my Pentax K-r, long-exposure noise reduction after a shot is sometimes shorter than the original exposure. For example, after a 15-second exposure, the camera may only spend about 10 seconds processing noise reduction. I expected dark-frame subtraction to take the same length of time as the exposure. Why would it be shorter?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
6
The manual gives you your answer:
Determines conditions such as shutter speed, sensitivity, and internal temperature, and automatically applies Noise Reduction as necessary. (default setting)
This implies that the algorithm for noise reduction isn't a simple 1:1 match with the exposure time and, coupled with the quality of the K-r/K-5 sensor family, the amount it would normally have to apply would have been less than with the prior cameras.
For myself, I have it turned off for my K-5s. I would rather deal with the noise after the fact and not have the camera lose time repeatedly doing this action.
Originally by user472. Source · Licensed CC BY-SA 4.0
user472
15y ago
0
Generated from our catalog & community — verify before relying on it.
Pentax’s long-exposure noise reduction is not necessarily a simple full-length dark frame every time. According to the manual, the camera determines factors such as shutter speed, ISO, and internal temperature, then applies noise reduction as needed.
A true dark frame would often match the exposure length, but sensor noise also depends strongly on temperature. After a long exposure, the sensor is warmer, so the camera may be able to estimate or compensate for the dark current with a shorter reference exposure instead of a full 1:1 match.
Another possibility mentioned is that the camera may use a stored or partially modeled dark-frame reference rather than always capturing a full new one, though the varying times suggest it is adapting shot by shot.
So the short answer is: the K-r appears to use an adaptive long-exposure noise-reduction process based on exposure conditions, not a fixed “same-time dark frame” method.
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