Common Imaging Problems: Part 1 |
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Noise Any part of a digital image that was not present in the original scene is referred to as 'noise'. It may appear as specks in the photograph, washed out areas of white, or blotchy patches where there should be an even gradation of shade. Traditional photographs also have imperfections which have come to be an accepted part of the photographic medium. Film grain is a pattern of noise which is created by the film emulsion and is often used for artistic effect. Digital photography is expected to be much 'cleaner' than film, however, so noise levels have become an important measure of the quality of digital images. There are several different sources of noise in digital photography. It is essentially a byproduct of the process of converting light into digital numbers. The individual photosites which make up an image sensor function by converting photons into electrons, and because of the somewhat random fashion that photons arrive at the photosites, the conversion to electrons can produce unwanted artifacts. Noise can also be produced during the amplification of the electrical signal produced by each photosite. Because the electrical charge is so small it must be amplified to a usable level. The noise produced during amplification can be thought of as 'static' similar to that found in radio broadcasts, although in this case the static produces visual artifacts. Reducing Noise There are several things you can do to keep noise to a minimum. It is most noticeable in shots with extended exposure times or high ISO settings. ISO settings are related to film 'speed' -- high-speed film can capture images at very short exposure times. This is ideal for action shots such as athletic events. High ISO settings in digital cameras, however, produce more noise than low ISO settings. Similarly, long exposure times also introduce excess noise. Long exposure settings can be used to take pictures in low light conditions such as dimly lit interiors. Keeping ISO settings low and exposure times relatively short will minimize noise as you take photographs. Removing Noise Noise can also be dealt with after the photograph has been taken. There are several software packages which are specifically designed to remove noise artifacts. They work by applying 'camera profiles' to reduce noise typically caused by specific camera models. Noise reducing software is easy to use and relatively inexpensive. The problem with camera profiles, however, is that they will never exactly match the noise produced by your specific camera. In order to get the best noise reduction possible, a very simple technique can be used. After taking a photograph which you expect will be affected by noise, take a second photograph with the same camera settings. This second photograph must be completely black -- the camera shutter must be closed, the viewfinder covered, and ideally the camera will be placed in a bag to prevent any light from entering the camera body. This second photograph can be used as a 'noise profile' that will almost exactly match the noise of the first photograph. The noise profile is used in photo editing software such as Photoshop to subtract the noise from the first photograph. The best results will be obtained with RAW files, but the technique can also be used for JPEG images. Discover
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