Noise cancellation

Hi everyone. I've read this on another thread (about taking photo's of lightning.) What does it mean? What are we talking about here?

"Exposure is variable. I generally use 30 seconds at a time. For the K10D and RAW, this works well without too much noise and without turning on the noise cancellation -- which requires a thirty second dark frame to be shot."

I've a K10D and have already taken a few night time shots with noise reduction on. Am I doing something wrong.
Ta. Chris

I think we are talking about the camera's own Noise Reduction.

There's nothing wrong with using it - except that it takes so long that you might miss a good shot in the meantime. The choice is your's whether you risk that, or don't bother with any NR, or do the NR later on your computer.

Isn't noise cancellation where you take a black picture and subtract that from a long exposure to remove noise/interference at the sensor to give you a cleaner picture?

Thanks for the explanation, but still none the wiser.

How do you subtract the black picture from the long exposure to reduce noise. I cant grasp this!!

Long exposures are prone to electronic noise, similar to excessive film grain in appearance. So the camera takes one exposure at, let's say, 30 seconds. It then takes a seconf exposure at 30 seconds, but without releasing the shutter so the image would be black. However, the noise pattern will be the same as in the real image.

The camera then electrobically subtracts the noise pattern from the main image, thus reducing excess noise.

The downside is the length of time that we can't take another exposure. It does work well though.

During a long exposure your camera's sensor generates a certain amount of electronic "noise" that will show in the photo. By having the long exposure NR switched on, your camera will take two shots; the first as normal and the second using the same parameters but with the shutter closed. The second shot will consist purely of the camera-generated noise and so this information is then subtracted from the first shot to leave a (theoretically) "clean" picture.

John's typing fingers are quicker than mine!

If you were using photoshop, simply create two layers. One with the photo, the other with he black frame. I believe you set the layer blend mode to "difference".

If the colors for each layer are the same the result is black.
So if you have a hot or dead pixel, its color will be the same for all shots with an exposure of a certain time.

Normally noise will cause the pixel to be a bit different color.
For any given tie, the pixel will be off by that amount consistently.

So take a blank black frame with he same exposure time of the photo in question. Subtract the pixel noise, and you have a fairly decent result approximating the pixel color without the noise.

A lot effects the quality of the results, but its is good enough that your camera has this functionality built in.
So you'll notice if you take a frame with a five second exposure, the camera will take about another five seconds before the image is saved.
It is during this time that the black subtraction frame is being taken.


So... to make a rather long story short...
For lightning you generally take very long exposures. (15-30 seconds or longer.).
If you allow the subtraction frame to be taken (as part of noise reduction), you need to wait 15-30 seconds or longer before you can think about taking your next photo.

The alternative is to turn off noise reduction and to always use a specific exposure time. (This will result in a more noisy image)
Then later take one black frame with the lens cap on for the same time.
You can then apply this same black frame to all of your images in Photoshop and probably save your self a lot of time.
(you'll only have a few good shots, shy waste 30 seconds on several hundred attempts?



OK, don't know if I made that any better or worse, but Google for noise reduction or subtraction frame and you should get some pretty good technical answers.

Right. I've grasped the theory now, but how do you do it in practise?
Do I set this up in the menu? Does the camera take a long exposure immediately followed by a second of the same length, and then produce the image afterwards.

Sorry about asking so many questions. Needs must you know. I want to get the best out of the camera.

The default setting for Noise Reduction is ON so you don't have to do anything. Just take the picture and the camera will take the exposure and then spend an equal time doing it's black exposure. When it's finished, you can take your next image.

Thanks chaps....

I've got the info I need from your replies. Now it's time to go and use the knowledge!
Ta, Chris.

if you try to do dark fram subtraction later in photo-shop, you may find the results are not what you'd like. It does work to an extent that way, but not as well as doing it in camera.

The reason for this is that it should really be done before the RAW data is converted to an RGB image.

The best way to do DF subtraction later on is to use a RAW converter like UFRAW which allows you to do it at the RAW stage before the conversion is carried out. I use UFRAW for all my conversions so I don't know how many if any of the other converters will do this, but it is by far the best way to do it.

Since UFRAW is free, if your converter of choice wont do proper dark frame subtraction then it's worth the download even if thats all you ever use it for. Make sure you get the most recent version, 0.13.