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Exposure settings: Digital noise

Film, grain and digital noise

Photographic film is made up of an emulsion coated on a flexible base. The emulsion contains silver halide grains. These grains are sensitive to light. There are a couple of ways to alter the sensitivity of the film to light, and so alter its ISO rating. First, you can add chemical sensitisers to the emulsion. Second, you can increase the size of the silver halide grains. Mostly, a combination of both is used.

Grain size is important, because of the way they are changed by exposure to light. They 'flip' from a non-latent state to a latent state. In their latent state they can be chemically converted (developed) to impure silver (which is black), creating the familiar black-and-white negative image.

It takes a certain amount of light to flip the grain to a latent state. If we think of light not as a continuous stream, but as individual elements (photons), we can imagine that it takes, say, five direct hits by photons to flip the grain. If the photos are evenly spread over an area, larger grains are more likely to be hit more often than smaller grains. In photographic terms, this makes the larger grains more sensitive to light.

The downside is that the larger grains become more visible in a photographic image. It is a classic compromise situation. Do you want a low-sensitivity film which can only be used in bright light, but which has very fine grains giving beautifully smooth gradations, or do you want a high-sensitivity film which can be used in low light, but which has large grains giving rough tonal texture?

For most photographers, the answer is to use the lowest-sensitivity film suitable for the situation, and accept 'grainy' images when the alternative is no images at all. For some subjects, of course, you can use a slow film in low light with the camera on a tripod and long exposures.

You can't change the size of the light-gathering pixels in a sensor once it is installed in your digital camera. Sensitivity has to be increased in another way. In fact, the sensitivity of the sensor doesn't change at all. Instead, the electrical signal generated by each pixel is amplified. To increase the effective sensitivity of the sensor, the amplification is increased.

However, all electrical circuits emit a degree of 'white noise'. If you turn the speaker volume up really loud on your hi-fi, you can hear a familiar hiss even when there is nothing playing. Or if a television is not perfectly tuned to a station, the 'white noise' creates the 'snow' that appears on the screen. When a television is correctly tuned, the signal strength overpowers the background noise.

Similarly, digital cameras produce a varying degree of background noise. In bright light, the pixels produce a strong signal and less amplification is needed. The signal overpowers the background noise. In technical terms, there is a strong signal-to-noise ratio.

In low light, the signal is weak. If you increase the amplification by setting a higher ISO rating, you increase both the signal and the background noise, and the noise becomes significant. There is a weak signal-to-noise ratio. The noise shows through, degrading the smooth tones of the image.

It is exactly the same dilemma you have with film. As the ISO rating increases, the image quality decreases.

You have to compromise between sensitivity and quality. However, at least with a digital camera you can change the ISO rating much more easily than film users can change film, so the chances are you will achieve a better compromise.

Image noise and pixel size

Increasing the signal-to-noise ratio reduces image noise. Larger sensors have larger pixels, which trap more incident light, resulting in higher signal levels and an improved signal-to-noise ratio. This is why the EOS 5D and EOS 1Ds Mark II cameras have such low noise levels – the larger full-frame sensors are able to trap more light in a given time period.

To keep noise to a minimum, cameras with smaller sensors and smaller pixels use micro-lenses to focus the light into the pixels so as much light as possible is trapped and little is wasted by falling between the pixels.

Long exposure noise reduction

Many EOS digital cameras feature a long exposure noise reduction function, which aims to remove some, or all, of the fixed pattern noise that is sometimes present in images taken with exposures longer than a few seconds.

Temperature plays an important part in the conductivity of electrical circuits. As the temperature increases, the conductivity decreases, causing more background noise. When you take a picture using a long exposure, the sensor is active and it heats up, making it more susceptible to fixed pattern noise.

The long exposure noise reduction performs a dark field subtraction – in other words, it calculates the fixed pattern noise and then removes it from the image you have just shot. In practice, it's not worth turning on unless you are planning on taking images with an exposure time of more than 2 to 3 minutes.

Long exposure noise reduction is a menu item on the professional EOS-1D and 1Ds series cameras as well as on the EOS 5D, and EOS 40D.

A 25-minute night exposure with the EOS 5D using long exposure noise reduction.