Noise Reduction set to off. Por Que?

[learjet035]

I was going through my XT and saw the noise reduction on long exposure and was wondering why it defaults to off? What does it hurt leaving it on? I figured I would ask before I switched it. Thanks

[Jon]

Because it works by taking a second shot (without opening the shutter, so all it captures is the noise) of the exact same duration to get the info needed to cancel out the noise without losing photo data. So your 15 sec. exposure will take 30 sec.

[learjet035]

Wow, thats cool. What length of exposure does it kick in? Do most people leave it on, or does it really matter? Is the only downside storage space? Thanks

[Jon]

It's not storage space. It XORs the "noise" image against the "real" image to cancel the noise out and just writes the result. It's the extra time. On the 20D it kicks in at 1 sec.

[learjet035]

Got it, thanks a lot.

[lmelendez]

Because it works by taking a second shot (without opening the shutter, so all it captures is the noise) of the exact same duration to get the info needed to cancel out the noise without losing photo data. So your 15 sec. exposure will take 30 sec.
I didn't know that.... I love this forum, every day I learn something new
Thanks Jon

Leo.

[tim]

Great info Jon, I wondered what it was doing when it took twice as long!

[Jon]

The manual doesn't explicitly state it, but it can be reasonably safely inferred from the statement "the time required for noise-reduction processing will be the same as the exposure time". I saw this referenced somewhere else, as well but I can't remember where. It's a fairly standard manual noise-reduction technique as well (where you actually do capture that second, blank, exposure). There are several algorithms that could be used to achieve this result in-camera, but they all essentially require that a second "exposure" be taken.

[tcaran]

Excellent info. Knowing how it works is half the battle of getting it to do what you want.

[Wazza]

Wow, interesting.

I just thought the small chip inside was doing it's own guess work, and removing bad pixels, and anything odd. Just thought it must be Windows operated, as it was so slow. :p

[Tom W]

Dark frame subtraction. Astrophotographers do it often, but it used to be a manual function.