@RobertTheFat wrote:
@TTMartin wrote:
@ebiggs1 wrote:
The only one that affects a RAW file is Long Exposure NR, it takes a dark frame after the exposure and subtracts that from the RAW file before it is written to the CF card. Not reversable!
This is not to say that Long Exposure NR is a bad thing to do even when shooting RAW.
From Long Exposures: Capturing the Passage of Time
'So, if you make a 30 second exposure, Long Exposure Noise Reduction will take another 30 seconds of processing time, during which time you will be unable to take another photo with your camera.
What's happening is this: your camera makes its exposure for the image. Long Exposure Noise Reduction then kicks in, and charges the camera's imaging sensor for the same amount of time, making a "dark frame." The camera then compares the noise in the dark frame with the noise generated in the image, and removes any noise that is the same. This method is known as dark frame subtraction.'
The interesting thing about this process is that it should make no difference when or where you create the dark frame. All you're doing is establishing a point on a graph of your sensor's noisiness as a function of exposure time. (To the actual mathematicians reading this: Yes, it's a many-valued function, so the result is an ordered set, not a point. But that's just an implementation detail.) So in principle you could create the dark frame any time after the exposure; all you need is the exposure time that was used for the original. Then you'd apply it in post-processing, with the option of ignoring it if it didn't give the desired effect (i.e., reduce the apparent noise). It might even be more accurate than the way it's done now, which practically guarantees that the sensor will be hotter for the dark exposure than it was for the original, thus exaggerating the amount of noise to be removed.
Or am I, in my profound ignorance of a feature I've never used, missing something significant?
Yes, you are missing something significant.
The temperature of the sensor impacts the amount of noise it produces, by taking the dark frame right after the long exposure it ensures the sensor produces the same amount of noise. For example if Long Exposure noise reduction is set to Auto and you are taking a 30 second photo in the winter, the camera may not even do a dark frame, because it is not needed. However, a 1 second exposure on a hot summers day the camera may do a dark frame because it is needed.
This is the example of one factor that I know of that impacts the sensor's noise that varies from the time the shot is taken.
There may be other factors* that vary the noise, but, the sensor temperature is one that I am sure of.
* humidity, electronic interference, ???
As far as 'exagerating' the amount of noise removed being good, even dark frame subtraction can reduce detail. So as always the least amount of noise is best.
