Exposure Compensation – Technical Fundamentals Course

In this post I want to talk about what happens when your exposure meter gets things wrong, which sadly is quite a lot of the time! To get around this you’ll need to use exposure compensation, or, failing that, exposure lock.

The exposure meter in your camera has been calibrated to what’s called “18% grey”.  You’ll be glad to hear this isn’t one of the fifty shades, but a mathematical average of exposures arrived at after decades of study and thousands upon thousands of images in that same enormous room in Tokyo full of white coated lab-technicians to find a mean average exposure tone.  This is what 18% grey looks like – here is a Kodak grey card – perfectly calibrated to 18% grey so you know where we’re starting from.

A white balance/grey card
A “grey card” perfectly calibrated to 18% grey

This is all very clever, and in practice it means your meter usually does a pretty good job of getting the exposure right.  Of course, when the scene you’re shooting is way above or beyond average, it tends to run into trouble. It gets stuck in a slightly counter intuitive way – if the scene is very dark, your camera will often give you an exposure that’s too light, and vice versa.  The reason for this is that the meter takes in all the tonal information in a scene, and tries to average everything out to that 18% grey. If the scene is overwhelmingly darker than 18%, it of course tries to make it lighter, and does the opposite for a very bright scene.

The way to get round this is firstly with foreknowledge of how your camera behaves, which I’ve just given you, and secondly to use either manual mode, or exposure compensation if you’re in an automatic mode.

Exposure Compensation Button
The exposure compensation button

Most cameras will have an exposure compensation button somewhere on them – look for something marked with a + and – sign.  They work very simply, just frame the shot up in an automatic mode, then (this will vary slightly from camera to camera) hold the compensation button whilst dialling it up or down, to force the exposure higher or lower.  If the scene in front of you is very dark, you’ll want to darken the result the meter is producing, so slide it towards the – direction, and the opposite way if you’re standing in the middle of a field of snow on a bright sunny day.  You’ve just outsmarted all those lab technicians in Tokyo. Well done you! The exact amount you’ll want to do this will vary from shot to shot, and there’s no way I can sit here now and tell you exactly what compensation will work for each shot – you’ll need to experiment I’m afraid!

There’s actually one more method for compensating for what you suspect will be an inaccurate exposure.  Your camera may well have an “exposure lock”, which is a very simple tool that allows you to read exposure from one part of the scene, or a slightly different scene, lock that exposure in the camera and then recompose your original shot and shoot.

AE-L Exposure lock button
Exposure Lock button (AE-L) on a Nikon DSLR

One of the most common ways to use this is if you’re shooting a landscape shot.  If you point your camera out across a beautiful scene and shoot, you may often find that the image comes back quite dark.  The reason for this is that the sky is (usually…) much brighter than the ground, and in an attempt to bring the exposure back to that perfect 18% grey, the camera has decided to lower the exposure quite a lot.  That’s fine for the sky, but you now can’t see any detail in the ground, or possibly in the people who were standing there so proudly with the mountainous vista they’ve just climbed stretching away behind them. They won’t be happy when the find you’ve underexposed them! 

Exposure lock lets you quickly solve this.  First, frame your shot up, and get an exposure.  Now, reframe the shot until the ground, or the people, or whatever you want to be correctly exposed is filling the frame.  Hold down the exposure lock (you may just need to tap it – varies from camera to camera) and now reframe back to your original image and shoot.  The exposure lock will have exposed your ground/people correctly, and whilst your sky may now be a bit bright, at least you’ll be able to see their proud faces properly.

Of course, this only applies to an automatic exposure mode of some sort.  If you’re shooting in manual, then you choose what exposure values to use, and deal with the consequences!

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