High ISO Noise Trick

Hi - first post to this group.

I read somewhere recently about a trick to eliminate noise on a high ISO shot. Basically the technique involved taking the high ISO shot, then taking another shot with the same settings but with the lens cap on.

The article (post? I'll be damned if I can find the original link) then went on to suggest that the second shot is used to create a mask to hide the noise in the first image.

Has anyone else heard of this (or did I dream it?) and if so, could you give me a clue as to how it's done?

Thanks

i don't know what the point would be. if the trick needed a black layer for combination purposes, then you could just create a black layer in photoshop

No - this was more than just creating a black layer. It had the noise 'pattern' on the second exposure.

Cheers! Martin

I believe this is what you are looking for:

http://www.photo.net/l​earn/dark_noise/

About halfway down the page on this link is a workflow.

It is called "Dark Frame Subtraction" and ONLY works with raw images, not jpg because the interpolation of a jpg is different for every shot.. The dark frame must be shot at the same ISO as the original shot. Preferably at the same temperature. HERE is a primer on how/why it is done.

Rad

OK looks like I was beaten to the link by seconds.

Yep, just about to post that exact same link! Anyway, second layer (dark frame) in photoshop needs to be set to "Difference".

Wicked!

Thanks folks. I'll give this a try tomorrow.

Cheers! Martin.

Did you know that newer Canon cameras (20D, 30D, 5D, 350D) can already do this automatically?

It's used when you make very slow shutter speed exposures. In the Custom Functions you can switch it on or off.

I didn't know that - thanks for the info. Sadly won't help me with my ancient 300D.

Are the noise patterns for each ISO always the same?

What function is that? I'm assuming the 10D doesnt have it, as that is what camera the write-up is based on...

Yes... but if you turn it on, you have to wait approximately the same amount of time for the noise reduction as the actual shot. (i.e. if you have a 10 second exposure, you have to wait an additional 10 seconds for the file to write) This will render the camera busy without the ability to take another shot until it's done. Most cases this is ok... but one area where that could be bothersome is shooting long exposures on fireworks. Unless you don't mind missing a few bursts between shots.

No - the noise pattern will vary depending on the ambient temperature when the shot was taken.

Rad

So which function is it...I guess I could go read the manual. LOL

CF #2 on the 20D.