A few months ago, if you had asked me “what is the point of exposure compensation?”, I would probably have replied “if you’re shooting in RAW, nothing”. And I would have been very wrong. I know I’ve been talking about the Fuji X-T1 a lot recently, but it did change the way I work in many ways. And one of them taught me to make sense of that fairly obscure thing that is exposure compensation. I mean, many cameras have a dedicated dial for it, must be important, right?
There’s one thing to remember about digital cameras: they try to help you by telling you what a correct exposure is. The camera’s light metre looks at the scene (or part thereof), and tries to figure out which exposure settings are “correct” for the scene. And by correct, the camera means that the area of the scene it measures should have an average level of luminance of 18%, a.k.a neutral grey.
Even when you shoot in manual, the camera still tells you what it thinks the correct exposure is, and tells you how far from it you are with the settings you have chosen, and encourages you to line up that mark on the 0 of the light metre. There are potentially two problems with this.
For starters, what the camera thinks is correct exposure isn’t necessarily what is appropriate for the shot, especially when you consider that the camera can make mistakes. For instance, if you have a lot of bright white-coloured objects in the scene (clouds, snow, etc.), the camera will try to expose that to neutral grey. But then it’s not white any more, it’s grey. That’s why one of the most common pieces of advice when shooting a subject in a snowy environment is to slightly “overexpose” the shot, which actually is really just countering the camera’s overzealous quest to neutral grey.
This is basically the whole purpose of exposure compensation. It’s a “let’s agree to disagree, but I’ve got the final say” dial. That said, when shooting in full manual mode, it’s usually quicker to change the aperture or shutter speed than try to set the exposure compensation. At the end of the day, setting the exposure compensation to +1 stop and zeroing out the light meter is going to have the same effect as not changing it and lining up the meter with the +1 mark.
The only way to know whether you should disagree with your camera’s assessment is by looking at the scene. If you’re looking through an optical viewfinder, you won’t really know until you take a test shot. Though the more you use your camera, the more you’ll be able to predict if the camera is over or underexposing and correct accordingly before taking the shot. Cameras with an EVF are a bit simpler as what you see is (pretty much) what you get.
The second issue is actually the reason why your camera disagrees with you: When assessing the brightness of the scene, you might not be considering the same area of the scene as your camera. This is where the different metering types come in. Some systems will look at the whole scene and figure out the average brightness, some will only look at the centre of the frame and discard the rest. In order to reduce the need for exposure compensation you can change the metering system to let the camera know which area of the frame you care about and/or use features such as exposure lock to get the camera to meter exactly where you want and then frame your shot.
The other way to deal with this is to actually embrace exposure compensation.
When shooting with the X-T1, I’m usually in A-mode, and then I use the exposure to make adjustments to the shutter speed. Those adjustments aren’t motivated by the desire to achieve a specific shutter speed, but by by what I see in the viewfinder, supported by the histogram, as it’s available in the viewfinder as well. If I think the image should be brighter, I might do a +1EV, and vice versa. One of the reasons to use exposure compensation is that the camera as no idea how well you can recover shadows, or whether you’re going to do it at all.
Keep in mind however, that exposure compensation is relative to what the camera is metering. For the same frame, you might need +1 EV on spot metering or -3 EV on centre-weighted average metering to get the same result. Also this obviously mostly applies to available light photography. Unless you’re using TTL flashes, the camera cannot metre light that isn’t there (yet).
Whether you’re doing it “manually” or using the dedicated dial, exposure compensation is a good way to teach you to assess the frame before taking the shots and it will get you to work with your camera rather than fight against it.