Digital SLR photography — Understanding exposure compensation
Modern digital cameras do a great job at getting the exposure right — most of the time. Here’s how to get great shots the rest of the time.
Exposure compensation allowed me to avoid the white plumage being blown-out (badly overexposed) in this photo of a Cattle Egret.
Why your camera sometimes gets it wrong
Imagine all the scenery surrounding you was made of paint. Scoop up all those colours and stir them up, and the resulting gooey mix would most likely end up a boring mid-toned grey.
That’s sort of how your camera thinks. It samples the amount of light and dark it ‘sees’ in a scene and assumes that when it all gets averaged out the result should be not too dark, not too light. Just a mid-tone. If it averages out lighter than a mid-tone, then the camera decides there must be too much light coming into the lens and so it reduces the exposure. And if things average out dark then it increases the exposure.
Because most scenes contain a mix of light and dark, this averaging-out method works pretty well and that’s why the exposure will be pretty much right, more often than not. But what if you’re photographing a scene that only has light-toned things in it? Like for example, a white rabbit on snow.
A bright sky caused this photo to be badly underexposed.
Much better. Exposure compensation (+ 1 stop) made everything lighter. But the colours look kind of washed out. Now, what could be causing that?
The camera will ‘see’ nothing but white and when it averages that
all out, it ends up with — you guessed it — white! It decides that just can’t be right
because it’s been programmed to think everything should average out mid-toned grey.
So it reduces the exposure. The result will be a photo that is badly underexposed, with a
grey rabbit against grey snow. Now the camera’s happy, but you’re not.
So, what can you do?
The good news is your digital SLR (and some of the smarter compacts) will most likely have a function called exposure compensation.
At this point you’ll need to look at your camera’s manual to find out which knobs and dials activate it. But once you know how to turn on that setting, here’s what you do.
First of all, you’ll see that exposure compensation is measured in stops. Stops is a word that refers to your aperture settings.
If your camera keeps making things too dark, you increase your exposure by setting your camera’s exposure compensation to a positive number of stops.
And if you want to decrease your exposure (make things darker) you set it to a negative value.
Usually, one stop will make a world of difference to your photo and you can test if you need more or less by taking a shot and glancing at your histogram.
As you take more and more photos, you’ll soon start recognising the kinds of situations where your camera is likely to get things wrong, when exposure compensation is needed. The examples below illustrate some of the things to look out for.
Extremely bright background
Here’s how the camera exposed the photo of this Little Pied Cormorant at its default (automatic) setting. It is clearly very underexposed.
… and here’s how the same scene photographed with +2 stops of exposure compensation. While I was adjusting the camera the bird was also kind enough to turn its head slightly, which improved the shot a little more.
When I saw the Little Pied Cormorant shown at right it was an auto-exposure nightmare. Not only was the bird back-lit, meaning that the parts of the bird visible to me were in shadow, but sunlight was reflecting right off that water around it, throwing up a huge amount of glare. That bright glare was going to play havoc with the camera’s automatic exposure settings. Leaving the camera with its default settings would have given me the underexposed photo shown at right.
Now, I could perhaps have used a flash to light up the front of the bird, but the bird was a long way away and my flash was an even longer way away (at home). I knew the little pop-up flash on the camera wouldn’t be enough.
I knew the camera would make things underexposed, resulting in the bird being a silhouette, so I set exposure compensation to +2 stops. The result was the next version shown here, where the water is now pushed almost to white, and the bird is correctly exposed.
Bringing out the detail in a white texture
When an area of white texture is hit by bright sunlight, the camera will often ‘blow out’ the detail, which is to say it over-exposes the shot so badly that big chunks of the image go completely white, with no image detail captured at all. Once an area of white is overexposed that much then no amount of pulling levers in your image-editing software is going to get that detail back. So at times like that, it can be a good idea to deliberately underexpose your shot a little.
In this picture of an egret, I didn’t want to lose that white feather texture so I set my camera’s exposure compensation to minus one stop. Thanks to that setting, those feathers turned out fine.
How do you know if you should make things darker or lighter?
The easiest way is to take a test shot and see how it looks. Checking your histogram if often a good idea too.
However, a very general bunch of guidelines are:
- If the scene contains nothing but white objects on white backgrounds (white rabbit in the snow), make it lighter (to counteract the camera’s tendency to underexpose this kind of shot). That means a positive number of stops in exposure compensation.
- If the scene contains nothing but dark objects on a dark background (black bug on a black wall) then make it darker (to counteract the camera’s tendency to overexpose this kind of shot). That means a negative number of stops in exposure compensation.
- If your highlight alert is flashing (I describe highlight alerts towards the end of this article about histograms) then you need to reduce the exposure with a negative number of stops in exposure compensation.
- To reveal the detail in a dark texture make it lighter (positive number of stops in exposure compensation)
- To show up detail in a light texture make it darker (negative number)
Too confusing? Then just fire off that test shot and work your way from there. You’ll quickly get a feel for this kind of thing.
