Centre-weighted metering

This is one of the various light metering patterns offered on most digital cameras. It’s a relatively crude system which averages the light across the whole scene but gives special emphasis to the centre. It’s less reliable for for novices shooting in a wide variety of conditions, but its simple response to scenes actually makes it easier for more experienced photographers to interpret the results.

Bulb (B) exposure

Usually, the camera’s exposure time is set by the shutter speed you’ve selected, so that the exposure ends automatically. But in Bulb mode the shutter stays open for as long as the shutter button is held down, so it’s used a lot for night photography, where exposures can range from 30 seconds to 30 minutes (in moonlight). In the old days you’d use a cable release with a locking screw; these days you’d use a remote release with a bulb mode (using a remote release means you don’t risk moving the camera by keeping your finger on the button).


Taking the same shot at a series of different exposures with the intention of choosing the best one later or merging them together to create an HDR image. Most cameras offer an auto exposure bracketing option. You choose the bracketing interval (the difference between the exposures, typically 1EV) and the number of frames (usually 3, sometimes 5 or even 7). Some cameras offer other types of bracketing, e.g. white balance bracketing or even focus bracketing.

Average metering

This is a very simple type of exposure reading where the camera’s light meter just measures the total amount of light in the whole scene. It often leads to underexposure because bright areas in the scene have a disproportionate effect. Today’s digital cameras offer a range of more sophisticated exposure metering patterns and only a few still over averaged metering amongst these – some photographers still like it because although it’s a crude way of measuring the light, it’s quite predictable and easy to interpret.

Auto exposure

This is where the camera measures the light levels in the scene using its in-built light meter, works out the exposure value and then sets a shutter speed and lens aperture to give the correct exposure. Practically all cameras have auto exposure systems and its only the more advanced models which offer manual exposure.

Aperture (lens)

This is the adjustable hole in the lens diaphragm that controls how much light passes through the lens and is used to adjust the exposure. Aperture setting values are the same across all cameras and lenses, and here’s a part of the series: f/2.8, f/4, f/5.6, f/8, f/11 – though the theoretical aperture range is much wider than any single lens can manage. The maximum aperture – how wide the lens opening can go – is a big selling point because wider apertures let more light through. The lens aperture also has an effect of depth of field, or the near-to-far sharpness in the picture, and the number of aperture blades is a selling point because it affects the way the lens’s ‘bokeh’.


This stands for AE (auto-exposure) and AF (autofocus) lock. Often it’s useful to fix the exposure settings and focus point ahead of taking a picture and on most cameras you can do this by half-pressing the shutter button, holding it in position, then reframing the picture. By default, the AE-L/AF-L button does the same, locking both the exposure and the focus, but you can also configure this button to lock either the exposure or the focus, not both, so it’s more versatile than simply half-pressing the shutter button.

Active D-Lighting (Nikon)

An exposure mode on some Nikon digital cameras which balances up the exposure in high-contrast scenes. The camera reduces the exposure to make sure it captures bright highlight detail and then processes the image to brighten up dark shadows. It can be applied in different strength settings.