Quote:
Originally Posted by luka
Absolutely correct!!!
(and you can thank Humayun straight away  )
When you subtract your darks from your lights, the final image will contain the noise from the light and from the dark added together.
In other words, the ICNR reduction will double the noise.
Luka
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Absolutely
incorrect, I am afraid.
Noise is a random signal, and it is pretty much irrelevant if it is subtracted or added in averaging process (the AC part of course). The result of combining noisy frames on top of each other is not multiplying the noise level, but averaging (smoothing, due to th enature of noisy signal). Only permanent features (like hot pixels and bias), however, can be removed (by subtracting) or multiplied (by adding dark frame by mistake).
The ONLY important thing is that the darks are taken at same temperature as light frames (because those permanent features are VERY dependent on temperature of the chip), and that the ALL other settings are identical.
So, INCR per se WILL NOT double the noise, provided the other parameters were the same. What could go wrong here is if you have Internal long exposure compensation ON, and if you apply darks during stacking later, then the whole math done by stacker will not be done correctly and the final image will still contain the hot pixels and other permanent nasties (inversed, though)
Also, the problem will be there if the temperature was different when taking lights and darks , or any other parameter was not of the same value (ISO, or exposure time). That is why I take darks at the same time as exposures (by having long comp ON), and I do not worry about them later.
As far as INCR is concerned, I will have to have a look at 450D manual to clarify the definition.. But I am pretty sure it is the same as what I meant by "Long exposure Compensation". Canon (and others) usually do not change those things from model to model without a lot of announcements, so I am pretty sure 400d and 450D are very similar/same in this respect.
That means, when this feature is ON, camera takes exposure, then takes dark (same "exposure" time) and subtract them from light (to remove permanent features from final frame). The only drawback is double time needed for this.. a problem when taking a long exposures, and/or plenty of them.
However, if INCR in 450D world is something else (like some sort of low pass filter), it is better then to set it to OFF..