Again, this post is intended as an introduction in the hope others make useful additions to what is certainly not intended as a ‘definitive’ post. (BTW, this is a rehash of my previous post here, which had 318 visits and no replies…)

(Non-technical) Definition

In film, we had grain; with digital, it’s noise. Basically, it means “coloured dots” in colour images (it still looks like grain in B&Ws, which is what they translate into when converting to monochrome). Your camera basically “invents” pixels when it doesn’t see enough of them. This is particularly the case with compacts, which have very small sensors. It happens with all cameras when their sensitivity (ISO setting) is raised. A (poor) rule of thumb is that noise becomes a nuisance when the ISO setting exceeds 400 with compacts (sometimes less), 800 on entry-level DSLRs and 1600 with “enthusiast” level cameras. I’m tempted to say the purchase price of your camera in $ or € is generally the maximum ISO setting at which your camera can perform…

Noise can also be added when you (or your camera) over-sharpen an image (“artefacts” – see below).

Note: in small prints (up to A4), noise shouldn’t be a problem, except in special circumstances (night-time shots).

It’s worth mentioning that noise can sometimes produce a pleasing effect. Let it be when *you* want it, not the camera.

When does it happen?

Whenever the lighting is difficult. If the camera thinks it has to invent pixels, it will. Typical situations are low-light ones (night time, dark areas), but also high-contrast ones. Any captures involving long exposures (> 1 second) need special attention.

In-camera workarounds

Some cameras (compacts, entry-level DSLRs) are just noisy above base ISO settings. There are, however, some basic rules:

1. If you think noise is going to be an issue, shoot in RAW (if you can).

2. If you have to use flash, set the ISO to 100/200. Don’t make a bad situation worse…

3. Shoot at a wider aperture, although this affects the depth of view, vignetting and image softness. You can juggle aperture, speed and ISO to the highest acceptable values given the exposure. Easy? No – but that’s why they teach photography at university… It does come with practice, however, and every success is a pleasure to savour.

4. There are noise reduction mechanisms in your camera. Hunt around the menus to activate the noise-reduction algorithm. Be wary of the ‘strong’ setting. This often makes images too sharp (and adds artefacts, which is the even-more evil twin of noise). Remember to deactivate afterwards or your next shot might be ruined.

Post-processing workarounds

Firstly, always check your image at 100% magnification. You can miss it otherwise.

Secondly, reducing noise = softening the image. There ain’t no such thing as a free lunch. Be careful not to over-sharpen afterwards.

Thirdly, the Texas Chainsaw Massacre solution is to use a levels adjust layer to set absolute blacks (shadows) and whites (highlights). Texas chainsaw? Yes, because this results in massive clipping (you lose detail). But sometimes it does the job.

There are also professional-grade (and expensive) applications available, such as Noise Ninja, which tackle this specific issue. Other applications, including the ones shipped with your camera, also include noise-reduction tools.

Remember: reduce noise, sharpen the image afterwards.

Below is a quick and nasty tool I learnt for use in Photoshop.
1. Click on the Channels tab next to the Layers tab
2. Click on the second channel (red)
3. Go to Filters > Noise > Despeckle
4. Press ctrl/cmd + F to repeat this process once
5. Click on the next channel (green) and press ctrl/cmd + F twice
6. Click on the final channel (blue) and ctrl/cmd + F four times (blue is the prevalent colour in most shots and needs more work)
7. Click on the composite RBG channel to restore your colours and continue editing.

Please help me and others learn noise reduction techniques by adding useful tips you've learnt.

Well I haven't a lot to add, but I didn't wan't you to think that no-one cared. Thanks very much for all your efforts Stéfan. I hope everyone else finds them as informative as I do. A while ago you were talking about running a photography course, what happened with that, is this part of your material?

One tip from me: Where possible use a tripod. Having a longer exposure means you can turn the ISO setting down. Beware though - long exposures in themselves can be a source of noise so don't overdo it.

Thanks, I have just tried your channels tip and it works really well, brighter, seemingly bigger and better shots. A great airound helpful tip, thanks again

Thanks a lot for ur tips... will sure try them out the next time i feel lots of noise is entering my image. I had another doubt though.. I own a Sony DSC H10... (maybe you can call it a compact... but i do take pride in it.. and its surely not compact enought to be carried in my pocket... and its my first cam...lol)...
The maximum ISO setting it has provided me is 3200. I tried using the ISO setting in dark situations >800 and have found high noise levels. One reason was me not using a tripod... Will using a tripod help in that scenario??? If i cannot use higher ISO settings why does the manufacturers provide it???

Why do cheap car speedometers go up to 220 kmh? ;-)

A good tripod will stabilise your camera, not the subject, during long exposures, but has no effect on the noise in your image. Perhaps I should have been more technical in the definition. The sensor in your digital camera is analog. When you amplify its sensitivity, the signal to noise ratio worsens in proportion. If that doesn't help, use the poor rule-of-thumb: $400 camera - 400 ISO.

lol.... cannot think of a better example... u sure did hit the nail..
Signal to Noise should is pretty much understandable... i'm an electronics engineer by profession.. So we'll not dwell much in the poor thumb rule... lol..

My two cents regarding the noise reduction software:

Photoshop has a filter called "Reduce Noise", well, don't use it - one of the worst filters I ever tried.

Of all noise reduction programs I've used, probably the best one is Noiseware Pro, can be used as a Photoshop plug-in as well.

Best quality / price ratio I'd say goes to Neat Image - it's free for non-commercial use

Thanks for the info. I'd forgotten the PS filter, which is pretty dire. I'm off to try the Neat Image one - this is the kind of contribution I was hoping for!