Do you always use the Auto setting on your digital camera? Many of us do, but it isn’t always the best option available to us.

When you press the button to take a picture the last thing you expect is that the image captured will come out too bright or too dark. Digital cameras have come a long way in a short time and many offer such advanced metering and exposure systems that it can come as a bit of a shock to find they aren't completely foolproof. Sometimes when things go wrong the reason is quite straightforward: it doesn’t matter how good the metering system on your camera is, the camera doesn’t know what the subject of your picture is. This means the camera has to make assumptions with regard to metering and exposure and it can’t always get it right.

Most modern cameras don’t look at the whole image when they predict exposure metering, but use multi-segment metering that measures the brightness of as many as 256 different parts of the frame. The results are compared with a database of thousands of typical scenes stored in the processor and the camera selects what it considers the most appropriate settings. There will be times when the image you desire is non-standard; you may want a slightly brighter or darker overall image for instance, or wish to highlight a face that is back-lit perhaps.

We can help our camera to get things right by making use of a technique called AE Compensation (AE stands for Automatic Exposure). To do this we need to adjust the EV setting to make the resulting image either brighter or darker.

Yes I know, AE, EV; how many other abbreviations am I going to throw at you? Well maybe one or two, but let’s try and kill the jargon now.

AE = Automatic Exposure and is the setting your camera will predict for any given scene.
EV = Exposure Value, where a null (zero) EV is the AE setting, +1EV is double the exposure and -1EV is half the exposure). The amount you can alter the exposure by is variable, usually in steps of 0.3 EV.

Now forget all about Exposure Values and just remember this:
A positive (+) AE-C makes the picture brighter
A negative (-) AE-C makes the picture darker

Roughly speaking a +1.0 EV setting will make the image twice as bright, a +2.0 EV four times as bright (the reverse for minus EV settings). So you can think of Automatic Exposure Compensation (AE-C) as a variable 'brighter/darker' feature, + for brighter, - for darker. Simple really isn't it?

How do we turn AE-C on? Look for a button on your camera that has a small '+/-' logo, or explore the camera’s record menu.

Here’s what happens to an image when we use AE-C by adjusting the EV settings:

Chillies in a bowl - correctly exposed Chillies in a bowl - correctly exposed.

Chillies in a bowl: EV-0.33 Chillies in a bowl: EV-0.33

Chillies in a bowl: EV-0.67 Chillies in a bowl: EV-0.67

Chillies in a bowl: EV-1 Chillies in a bowl: EV-1

Chillies in a bowl: EV+0.33 Chillies in a bowl: EV+0.33

Chillies in a bowl: EV+0.67 Chillies in a bowl: EV+0.67

Chillies in a bowl: EV+1 Chillies in a bowl: EV+1

Clicking any of the above images will open a larger version in a pop-up window.

When might we use AE-C?

Here are a few typical situations where AE-C will help you take the kind of pictures you want:

Backlighting: Where your subject (say a person) is fairly dark against a much brighter background. The camera exposes for the background and your main subject comes out too dark. Solution: plus (+) exposure compensation.

Small subject: A very similar problem to backlighting. If the main subject is quite small in the frame, and is much brighter or much darker than the rest of the scene, there is a risk of under or overexposure. Solution: So if you're shooting a white cat on a black rug you may need a plus (+) compensation, and if you're shooting a black cat on a white rug you'll need a minus (-) setting.

Large expanse of sky: If the sky is very bright and fills more than half the frame the camera can again be fooled into underexposing, making anything else in the frame come out too dark. This happens more with hazy, overcast skies than deep blue skies. Solution: plus (+) exposure compensation.

High key scenes: Cameras generally presume all scenes have a fairly wide range of tones from dark to bright, but if the scene is dominated by pale tones the auto exposure system may produce an underexposed result (with whites looking grey). Solution: plus (+) exposure compensation.

Low key scene: If your scene contains mostly darker tones (shadows) the camera can be fooled into overexposing, producing a result that looks slightly washed out. Solution: minus (-) exposure compensation.

Quick Tips

[1] Generally underexposure (too dark) is better than overexposure (too bright) – you cannot fix overexposure in post processing, whereas you can usually go some way towards rescuing dark shots.
[2] If your camera has a spot metering option you can use it to avoid the problems of small subjects or backlighting by only measuring the brightness of the main subject, not the rest of the frame. This does take practice though!
[3] Some problems can also be avoided by the use of AE lock. We'll look at AE lock next time.
[4] Backlighting problems can also be overcome by the use of fill flash.