Our cameras' light meters are impressively accurate instruments most of the time, helping us to judge the exposure of scenes and produce gorgeous photos. However, they're not infallible and sometimes we need to use our better judgement to over-ride the automatic meter reading and achieve our ends. When you're using aperture priority, shutter priority, or programme mode the feature that you'll need to do this is exposure compensation.
Why do our cameras mis-judge the exposure?
For all that our cameras are capable of rendering the world in glorious Technicolor, their light meters are remarkably simple: they can only 'see' in shades of grey. When a light meter attempts to judge the correct exposure for a scene, it does so under the assumption that the scene's average brightness is middle grey (18% grey), which is exactly half-way between absolute black and bright white.
Most of the time this seemingly crude operation is pleasingly accurate. Aiming for the mid-point between black and white is a safe bet for correct exposure, but if the overall brightness of the scene varies too far from the mid-point, you'll find yourself with a too light or too dark photo.
What is exposure compensation?
When you encounter a lighting situation that fools your camera's light meter and renders a photo too light or too dark, you will need to correct for the mis-reading by deliberately over- or under-exposing photo, contrary to what the meter recommends. This is easy enough in manual mode: use a larger or smaller aperture; decrease or increase the shutter speed; or bump up or take down the ISO accordingly.
If you're using aperture priority, shutter priority, or programme mode you'll need to wrest control from the dictatorial clutches of the exposure meter and assert some authority using the exposure compensation feature. It's commonly referred to as 'EV'. With this button (usually it's a button, but not always) you can tell the camera that it needs to over- or under-expose the scene by anything up to five stops (depending on camera model) in third-of-a-stop increments from what the meter suggests. It's an exposure over-ride button, really.
When do you need to use exposure compensation?
Have you tried to capture a crisp, snow-white scene unsullied by human foot or automotive tyre-tread only to find it rendered in uninspired shades of grey? Or perhaps your attempts at a night-time scene looked washed-out, rather than shadowy and moody? If you're trying to photograph a scene where the overall brightness or darkness is way off of middle grey you'll find it under- or over-exposed respectively. The light meter can't compute that a scene really can be that bright or that dark, which will require you to take action.
The sorts of scenes and environments when you might need to use exposure compensation will include, but are in no way limited to:
- Birds or aircraft (or any subject, really) against the sky
- Sunrise or sunset
- Forests and woods
- Low-key scenes
- Night scenes
Scenes that are 'too bright' for the light meter's assumptions will be under-exposed without exposure compensation. To correct for this and deliberately over-expose the shot you'll need to dial in positive exposure compensation.
If you're trying to photograph a dark scene, the meter will over-expose it. This will require negative exposure compensation to render it properly dark.
Whether or not you need positive or negative exposure compensation, how much you need will depend on what you're photographing and how you want your photo to look. Try one stop and see how you go from there.
Unless you've selected a particular metering mode, for example spot or centre-weighted, your camera's default setting is to take a light reading from across the entire scene. Like metering for mid-grey, this is largely accurate, but every now and again can produce an anomalous result, for example if you're photographing a back-lit subject. In this case, your subject will come out at least under-exposed and maybe even in silhouette. By dialling in some exposure compensation you should be able to expose the subject properly, although the background will probably come out too bright.
How do you use it?
When your camera is set to aperture priority, shutter priority, or programme mode, adjusting the exposure isn't quite as simple as altering your aperture, shutter speed, or ISO. In these modes, you're choosing one variable and the camera is selecting the others to give an exposure based on the light meter reading; you can't tell it to over- or under-expose by adjusting one of the variables because it will automatically adjust the others to adhere to the light meter's suggestion. Instead, you need to use a specific function—exposure compensation—to dictate to the camera that you want an extra half-stop of exposure, or that it must under-expose by a stop.
How, precisely, you dial in exposure compensation will vary from camera to camera, so do read the manual! As a general rule for entry-level and mid-range cameras, whether they're Canon, Nikon, Pentax, or Sony, you hold down the EV button, which is usually marked with a +/- symbol, and turn the control dial until you've added or subtracted the right number of stops or fractions of stops.
When you look at the exposure meter, the marker indicating 'ideal exposure', which is usually on zero, will move towards the left for negative exposure compensation and to the right for positive exposure compensation. Go left if what you're photographing is dark and you want to keep it that way. Head right if you've a bright scene and you want to over-expose it. Bingo! Correctly exposed images are yours!
Remember to take it off again when your lighting conditions have changed, or you'll be over- or under-exposing your photos when you don't mean to.