Noise reduction

Image processing to reduce the noise produced by small sensors or high ISOs.

Noise is caused by random variations in the light captured by each photosite (pixel) on the sensor. Some cameras have sensors which apply 'on-chip' noise reduction, where electronic circuitry adjusts the signal values captured by the sensor's photosites. This is probably the best sort.

Otherwise, noise reduction is applied by the camera to the raw image data as it's processed to produce the photo. This reduces noise, but can result in smudging and loss of detail.

The same applies to noise reduction processes applied using software on a computer, though with more processing power available and more time to find the best settings, this may produce better results.

In any event, noise reduction is something you'd rather wasn't necessary at all. Using a small sensor and applying noise reduction is not as good as using a larger sensor which doesn't need it.

ISO | High ISOs are fine on SLRs, terrible on compacts

ISO is the measure of a sensor’s sensitivity to light. The numbers are the same as the ISO ratings given to traditional film. The difference is that a film only has one fixed ISO, while you can change the ISO of your sensor from one shot to the next. ISO values go in the following sequence (some cameras offer intermediate values too):

100 • 200 • 400 • 800 • 1600 • 3200 • 6400 • 12800

Each ISO value is twice the sensitivity of the one before, which makes exposure calculations more straightforward because this is the system used for shutter speeds and aperture settings too.

You can increase the ISO in poor light so that you don't have to use slow shutter speeds and risk camera shake. In full auto mode, digital cameras will adjust the ISO automatically with this in mind.

The disadvantages of higher ISOs on digital cameras are not unlike those of high-speed film. You get more noise (similar to film grain) and reduced definition.

The amount of noise you get is directly related to the size of the sensor and the number of megapixels. The small, high-resolution sensors in compact digital cameras produce much more noise at the same ISO than the larger sensors in digital SLRs.

It's like turning up the volume on an old audio cassette because the music is quiet. The music gets louder, but so does the background hiss, and the overall quality is pretty poor.

You can see that in this example where a small section has been blown up to show the image quality at ISO 100 (left) and ISO 1600 (right). Compact camera makers are constantly increasing the maximum ISO values on their cameras, but only at the cost of plummeting picture quality - no matter what the claims!

The two big problems are small sensors and high megapixels ratings. That's why digital SLRs are so much better at high ISOs than compacts. The sensors are far larger, yet the megapixel ratings are much the same.

Nikon's D3 and D700 are perfect examples. Both have full-frame sensors, but 'only' 12 million pixels. The results is truly astonishing high-ISO performance. The picture above was shot at ISO 6400, yet the enlargement shows that the picture quality is still excellent, with very little noise.

EV (Exposure Value) | How exposure works

EV, or Exposure Value, is a numerical measurement of the amount of light in a scene. We all tend to think of exposure in terms of shutter speeds and apertures, but in fact it's the exposure value that it all starts from.

When you use a handheld light meter like the Weston Master V below, it quotes an EV number which you then translate into shutter speed and aperture combinations using a rotating dial. When the camera measures the light level, it too starts with an EV number, which it then uses to decide shutter speeds and/or apertures, depending on the mode you're working in.

If you want to use manual exposure control, or take more control over what shutter speeds and apertures the camera uses, a basic knowledge of exposure values is really useful. In fact, you might not need an exposure meter at all, because it's possible to predict the EV values for different scenes surprisingly accurately. The table below gives some examples:

Once you know the EV number, either from a handheld light meter or using the table above, you can use it to work out what shutter speed and aperture you're going to use with the table below.

One thing to be aware of is that the EV values change at different ISOs. It's not something you need to worry about if you're using the camera's internal light meter because if you change the ISO the camera will allow for this automatically. But if you're using EV values from the table above, or transferring them from a handheld meter, you will need to match the EV to the ISO - you can do this with the table.

What you can see from this table is that for any one exposure value there are lots of different shutter speed and aperture combinations you can use. When you use the 'program shift' function on a camera, this is what it's doing.