noise reduction in 20D

[Jonathan Wienke]

It affects RAW, basically it is a dark frame subtraction done in-camera. I always leave that feature on.

[Tim Gray]

I suspect you could test this.  Try taking 2 dark frames, do the subtraction and if the frames were identical it might work. I would expect you would want to take a reference dark frame perhaps a couple of times during an night session since variables such as temperature would likely change the noise profile.

[TeddyLoves]

by "temperature" did you mean "environment temperature", not "color temperature"?

there's a fireworks event this weekend and i really want to try this function. from the fireworks event last week, i suspect that, most of my shots are about 2 seconds at f/8 and ISO100. since i dont have a 1 series body, and i dont want to wait for 2 seconds every shot, should i do this instead: i set the mode to Manual, f/8 2 seconds and ISO100, shoot several shots, then at every break, i cover the lens, and make a dark frame.

--> the point is, since this is taken at the time of shooting, there shouldnt be too much difference in temperature, right? i think this is similar to the 1 series in some way: whenever it has time, it does the dark frame pattern. the only difference is that, it does it in the camera body, while i do it manually in PS

[TeddyLoves]

recently ive realized that the Canon 20D has a custom function to reduce noise (C.Fn2: Noise Reduction). the manual indicates that this function is effective for exposures of 1 second or longer. and when it is turned on, the camera will need the exact time of the exposure to do the noise reduction process.

i'm very interested in this because i shoot infrared a lot (basically long exposure and high ISO, which means a lot of noise)

my question is, does this function affect RAW images or does it just work for JPEG images?
and, is this function offered only in the 20D body or in other bodies as well (like, 1Ds mk II)

[TeddyLoves]

thanks. now i know that it is dark frame subtraction. so, basically it takes a second shot with the exact same time (shutter speed) without exposing the sensor to get the noise pattern, then use this noise pattern to "subtract" to that in the image, virtually decreasing image's noise.

since this is a powerful feature, it may cause some inconvenience, shooting fireworks for example. i learn that, if we shoot with the lens cap put on, for the exactly same amount of time, we'll get the noise pattern. then by putting this noise pattern on top of the image in photoshop, and changing its blending mode to "difference", we can "subtract" the noise from the image.

my question is, is it a good idea to just turn this function off to save shooting time and avoid missing precious moments, and make the "noise pattern" later (with that camera) and do the dark frame subtraction manually in photoshop?

[Jonathan Wienke]

With the 1-series bodies, you can abort the dark-frame exposure and shoot additional frames by pressing the shutter button. The images won't be cleared from the buffer until the camera has a chance to catch up doing dark frames and completing the processing, but you don't have to wait for the dark frame after every single shot. I recommend doing this in-camera, as the noise pattern changes with sensor temperature and exposure time and ISO setting, and the odds are quite good that if you shoot a dark frame very long after the original exposure that one of these factors will change enoug to reduce the effectiveness of the process.