Editors Note: Metering in Camera | From Lastolite School of Photography was originally written by Mark Cleghorn for www.lastoliteschoolofphotography.com and has been reproduced with permission.
Metering in Camera
On camera metering allows for very quick exposure readings to be taken as the light falling onto the camera sensor through the lens is measured. Then by adjusting either the ISO, the aperture, the shutter speed or a combination of any of the three an exposure is set.
Although it sounds simple, for accurate readings a little experience will go a long way. The different metering modes on a camera simply select more or less areas of the scene to analyse before setting a correct automatic exposure.
If you are using the manual exposure mode, the camera’s metering system will give you a guide in the view finder by showing you an under or over exposure reading, you then use these indicators as a guide when adjusting the aperture, shutter speed or ISO to gain a correct exposure.
Matrix or Evaluative Metering – This metering mode uses multiple areas of the scene to gather information in tone to generate an average exposure to the image using the most complicated on camera metering system. This is often referred to as ‘lazy metering’ as the camera is allowed to decide the exposure and 8 out of 10 times it’s pretty accurate.
Centre Weighted Metering – Is probably the most common metering method used when measuring an exposure with a DSLR as it meters for the scene but gives dominance to the centre of the image. CWM is most suited to people photography like portraits and weddings.
Spot Metering – Is the most accurate metering mode when used correctly as it allows you to pick a specific point in the scene using the TTL for the exposure. Spot metering can be as accurate as 1 degree with a hand held meter but cameras are around 3% of the area of the frame, which makes it perfect for complicated scenes especially landscape photography where different levels of contrast are almost guaranteed.
Partial Metering – This mode is ideal when the background is much brighter than the subject. Perfect for back-lit sunny wedding days as it will assist you in the metering, allowing you to keep your distance from the wedding party if you are shooting reportage candids.
Problems – The only real exposure errors occur when the on camera metering system is looking at either a very bright scene or a very dark scene, this is why cameras have the ability to set a plus or minus compensation to the exposure value being given.
For dark subjects – a – Minus setting should be applied in compensation as the metering will bias towards over exposing to bring out more detail, which will at the same time burn out the highlight details.
For light subjects – a + Plus setting should be applied as the camera’s metering will try and underexpose the image to make white a tone of grey. This results in a muddy and underexposed image.