Maus' Photography Tutorials VII: Defeating the enemies of clarity II: Noise

This is the second of a two part guide on diagnosing and treating the factors that prevent you achieving clear, sharp photographs. The first covers blur and can be found here.

NOISE

Noise is a complex, technical field and it is easy to get bogged down in detail here. In this, I will attempt to simplify it to a usable level, at the risk of misrepresenting it.

The image on your camera’s sensor is produced by particles of light (“photons”) hitting it in a patterned manner. This is the “signal”, or the useful information that your sensor is collecting to make a photographic image. The photons trigger the light-sensitive elements on the sensor, called the photocells, and this information is passed on to the processing part of the camera to piece together the image, like building a mosaic of light.

However, as well as being hit by all the useful light coming in, the photocells can also be triggered randomly, such as by the stray photons that are inevitably flying around. This produces unwanted points of light in your image. This is called “noise”.

Diagnosis

Noise is light, sometimes coloured, specks on the image. It can also show as bands of light or colours. It is most commonly seen in dark areas of the image.



This is a classic example of the effects of under exposure combined with a high ISO. The detail shows what the image looks like when brightened to approximate a normal exposure. The noise is so marked that it drowns out the useable information in the image.

Cause(s)

High ISO

Increasing the ISO of your camera increases the sensitivity of the sensor to light. This is useful when there is not much light, enabling you activate enough photocells to get a shot. However, it has its own problems. Increasing the ISO not only increases the sensor’s sensitivity to useful light, it increases its sensitivity to those pesky, stray photons. This, then, leads to an increase in noise.

Underexposure

Underexposure is when there was too little light activating the photocells on the sensor, due to a fast shutter speed, a small aperture or a low ISO. The resulting image is therefore too dark. Underexposure in itself doesn’t cause noise. However, it does lead to any noise that might be present to become a problem.

As stated above, the useful information in an image is produced by patterned light hitting the sensor. In the bright areas of an image, the amount of light hitting the sensor is great. There is therefore a lot of useful information. In the dark areas, the amount of useful light is little. There is therefore little useful information. As noise remains constant across all areas, the proportion of noise in comparison to actual, useful information in a dark area is substantially more than in a light one. Noise therefore becomes disproportionately represented in shadows. This is why the best way to look for noise problems is to peek in the shadows.

With most images, this is not a substantial issue. There is enough useful information in the bright areas to make the image and a small amount of noise in the shadows isn’t going to lose anyone any sleep. However, it does become a problem when an underexposed image is lightened in post processing. When you brighten the image you are not only lightening the useful information, you are lightening the noise as well. As the ratio of signal to noise is low in these exposures, the brightened noise will become hideously noticeable. If you combine underexposure with a high ISO, you make the problem even worse. The result will be a speckled mess with no useful detail.

Long exposure

A long exposure allows more light to hit the sensor. In low light conditions, this is a useful alternative to increasing the ISO and thereby sensor’s responsiveness to random photons. However, it can itself be a cause of increased noise. This is due, in part, to the fact that, the longer the sensor is exposed, the more chance there is of random photons hitting coming in and producing noise.

Remedies

Increase the light

The more light you have, the lower the ISO and shorter the exposure you will need to get a good shot. This therefore minimises the noise caused by these, two elements. Ways to increase light were covered in the guide on blur, so won’t be expanded on here.

Expose correctly

The best solution to the troublesome effects of underexposure is to “expose to the right”. This means setting your exposure as high as you can without blowing out the lightest areas of the image into glowing, white glare. This will give you maximum, useful signal, minimising the impact of any noise. You can do this by using the highlight alert on your camera, if it has one. This shows blown out highlights as flashing patches of colour when you view the image on the preview screen. Take a test shot and increase or reduce the exposure to the point just before things start to flash. This is easy to do if you are using manual exposure, by just popping the shutter speed up or down. Alternatively, you can use exposure compensation if you are using aperture or shutter priority exposure modes (check your camera manual for how to use this very useful facility).

Watch your ISO

Keep your ISO at or below its maximum, workable level. This involves getting to know the capacities and limitations of your own, particular camera. Most modern DSLRs can cope fine with ISOs of 400-800, without producing enough noise to be troublesome. When you know your camera’s maximum, useable ISO, you can set to this and work your shutter speed around it (your aperture size will depend on what depth of field you need for your particular shot). If yours is a popular model of camera, someone might already have done some tests on this, so Google around. If not, take some test shots at ISOs 100, 200, 400, 800 and determine the point at which the amount of noise becomes a nuisance. When you have identified it, don’t go beyond it unless there really is no option.

Reduce noise in post processing

Noise reduction software smoothes out the speckles produced by noise. It can be done in some cameras, but the most effective way to do it is in post-processing, by using a noise reduction filter. However, it is not possible for software to distinguish between random noise and fine detail, such as hair. Smoothing the noise therefore also diminishes the detail. When overdone, surfaces and edges can also look gain a distinctive “plastic” look. It is therefore a good idea to minimise the noise as you take the photo and to use any post processing noise reduction in moderation.

The final word

The amount of acceptable noise or blur is relative. If you are blowing your photo up to poster size or looking at it with a magnifying glass at three inches, any imperfections will be obvious and you would want to reduce noise and blur to an absolute minimum. However, a 600 x 400 image for the web will be quite forgiving. So, be realistic in your expectations as some of the solutions produce their own, intrinsic problems. Also, stressing too much over the technicalities of a shot takes your attention away from what should be a fun, creative process. Sometimes good enough really is good enough.


Photo credits: Many thanks to feaydrak, yiuu, Jatzu, Balljointed Adventure, and Zoe for the use of their example photographs.

 

Sorry, TomB. Not sure I get you. Is this with reference to the expose to the right recommendation?

What I think you are saying is that your camera will automatically adjust shutter speed if you adjust aperture, and vice versa, keeping the overall exposure value constant. Yes, it will if you are shooting in aperture or shutter priority. If you are using one of those, you can increase or decrease exposure by using exposure compensation (which enables you to reduce or increase whichever of shutter speed or aperture that your camera is setting automatically). My example relates to shooting in manual, but the same would apply if shooting in aperture priority (which is the most useful mode if not in manual), just by using exposure compensation. However, it sounds as though this is not clear, and I can see that it could be confusing. I didn't want to get too bogged down with too much how-to-use-your-camera basics in this, but I will make that bit more explicit in the text. Thanks for the feedback.

 

No problem. I did add to the explanation to make it clearer.

It is very difficult to produce a guide that will be at once comprehensive and at the same some time accessible to serious beginner photographers, which is my aim. I am bound to get it wrong at times. I shoot and think in manual (because it is FAR easier than you could possibly imagine, if you have never used it) and need reminding that not everyone does. I do appreciate the input.

 

Thanks for the input, Adam.

As I said in the guide, I would urge people to get to know the optimal ISO for their own cameras and to set that themselves. I only use auto ISO when I am outdoors, shooting fast and in manual. In our studio setting, shutter speed is the obvious one to leave on auto (so, shooting in manual or aperture priority, with a known ISO) as, with a tripod, it is the one that will cause least damage.

For the highlight warning, I personally prefer not to clip at all, even when shooting in raw. This is because, if you let it go and judge it wrong, you have blown out detail that you will never be able to retrieve. So, better to play it safe. Also, there is no need. There will be plenty of information available if you expose properly to the right without clipping highlights. Having said that, it is always advisable to shoot in raw anyway, as it does enable you to save a near-miss, should you produce one. I include shooting in raw in my long (website) version and talk about compression artefacts and noise. I decided to take it out of here as I had to prioritise to the points that people would be most likely to be able to use. Beginners generally have a mistrust of raw, which I can completely understand. There are various other, more technical causes and recommendations in the long version that, if people are sufficiently technically-minded to consider, they can.

With the in-camera trick, do you mean multi shot noise reduction? I don't have that facility on my 60D, so can't offer any personal recommendations on it. Does it work better than using a good noise filter in post processing? I will look out some reviews. If it is relatively available and does a good job, I will add it to the long list.

Thanks again for your input.

 

OK. We are getting into the nitty gritties of exposure now, which is going slightly off piste for the noise and blur guide. But given that it refers to some of my recommendations, I will spend some time on it.

In your high dynamic range situation of dark foreground and bright sky, exposing for the sky will indeed lead to an underexposed figure and the risk of the underexposure noise I discuss in the guide. However, exposing for the figure will lead to a blown out sky, with no detail. So, neither situation is ideal (I would personally use exposure stacking in a situation such as this) and I don't think it reduces the value of recommending exposing to the right being the generally best way to avoid underexposure noise.

Yes, most cameras will shoot raw, but most beginners still won't use it. In these guides, it is therefore best to go with what people will actually use. As I have already said, I do discuss it in the long version, so it is there for the more technically confident or for when that beginner is ready to move onto the next step. There are other, less daunting solutions that can be tried before raw becomes necessary.

Interesting on the multi shot noise reduction. I will certainly consider that as an additional to the long guide.

 

I need to be realistic, and I am aware that a serious beginner is more likely to buy a tripod than shoot in raw. It involves additional processing and software that many beginner photographers are not yet ready to learn. And, if you take your shot carefully, JPEG does a perfectly adequate job. I would place raw in the same category as mirror lock up, for those who have reached the point of proficiency in photography that they are aspiring to pin sharp. For that reason, it is included in the long guide.

 

I am too but, for the reasons given a few times up there (hence why I stopped. No point saying the same thing over and over, ad infinitum) I didn't include it in this, basic version of the tut. It is included in the full tut on my website.