Back to Basics
Discover how your camera measures the light and comes up with an exposure
Discover what’s going on in the realm of in-camera metering
YOUR camera’s metering system measures the intensity of the light reflected by the scene that you’re photographing and determines the appropriate settings that will be required to record that light.
This is known as a ‘through the lens’ reflective light reading. You can also take an incident light reading using an external light meter, and set the suggested exposure manually on the camera. Incident light meter readings have the potential to be more accurate, but the process isn’t as convenient as using your camera’s meter. Lightly tapping the camera’s shutter release button activates the meter, and a suggested aperture and shutter speed combination – one that should give a ‘correct’ exposure, based on the lighting and the ISO sensitivity – is displayed in the viewfinder. There are a range of factors that can lead to the exposure not being quite as correct as it should be, which we’ll get to shortly.
The metering sensors found in today’s digital cameras typically do more than just register the intensity or luminance of the light coming through the lens. They can also take into account the colours in the scene or subject being photographed, and can prioritise the exposure for the parts of a picture that are in focus. These days, metering sensors are densely packed with pixels and are effectively more like low-resolution imaging sensors; in some high-end cameras, they’re being used in conjunction with the camera’s autofocus system to improve the detection and tracking of objects using the colour information, as well enabling face detection through the viewfinder.
Your digital camera might have a single metering sensor, but there are a range of different metering modes or patterns that
you can choose from. These determine how much of the scene is metered and whether the camera makes any adjustments to the meter reading. When the camera’s set to an advanced shooting mode, such as Aperture Priority, you can choose an appropriate metering mode.
The default mode is ’pattern’ metering, commonly referred to as Evaluative, Matrix or Multi-pattern. In this mode, the camera splits the image into a series of small zones from which it takes individual meter readings. These readings are then compared before the camera suggests an exposure setting that should give a balanced result. It effectively applies its own exposure compensation to take account of variations in brightness and where it determines the subject is.
The other metering modes that you’ll find on your camera don’t analyse the scene in quite the same way, and are best reserved for occasions when you have the time and experience to take advantage of them. Take spot metering, for example. This only takes around 1.5-3% of the picture into account, making it unbeatable for taking precise readings from small objects in a scene, or when you want to stop large areas of dark or light tone from
The metering sensors in today’s cameras typically do more than just register the intensity of the light through the lens
fooling the meter. The drawback is that the spot meter is configured to produce a mid-tone exposure value. Point it at a mid-tone subject, such as lush green leaf, and you should get an accurate exposure. But point it at an object that’s either brighter or darker than mid-tone and you’ll end up with an image that’s either underexposed or overexposed. (See ‘Metering problems’ opposite.)
Pattern metering doesn’t always get it right either. It can come unstuck in high-contrast conditions for instance, and it won’t take into account any creative effect that you’re trying to achieve. In both situations you can use exposure compensation to fine-tune the result. Shooting raw will enable you to tweak the exposure when you process the result, but you might as well try and get it close to perfect in-camera.
light meter An incident light meter can be used to measure the light falling on the subject manual exposure The exposure is then manually set on the camera INCIDENT
incident light The light falling on the subject is reflected towards the camera camera meter The camera measures the intensity of the light entering the lens REFLECTIVE