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If you've watched Foundations of Photography: Exposure, then you know that in photographic terms, exposure is a measure of the brightness of light, and you know that on your camera, you control how much light is captured by altering shutter speed, aperture, and ISO. Through careful choice of how you alter these parameters, you can not only control the overall brightness in a scene, but how deep the focus is, whether motion is frozen, or blurry, and more. Exposure compensation is a control that lets you adjust exposure to alter overall brightness without control of any specific parameter.
So, for example, you can use your camera's exposure compensation control to specify a one-stop brightening of a scene. Your camera will automatically adjust shutter speed, and aperture, and possibly ISO to get that one stop of additional brightness, but you won't know which parameter it's changing, or how much, to get this alteration. Now, why would you be willing to give up control over those specific parameters? Because a lot of times you don't have any particular depth of field or motion stopping goals in mind; you simply want good overall brightness. Also, a one- or two-stop difference usually is not going to make a huge difference in motion stopping, or depth of field, but it might make a huge difference in brightness.
So exposure compensation gives you a really quick way to brighten or darken an image, without having to think about specific parameters. It gives you a quick fix for backlight situations, shooting at dusk, restoring tone to dark objects, and many, many other situations, which are all explained in Foundations of Photography: Exposure. Exposure compensation is very easy to adjust on the Mark III, but before you can adjust exposure compensation, you have to have the camera metered. So I'm going to half-press the shutter button, and it meters in at 1/100 of a second at f/4.
This is my exposure compensation meter right here. To dial in exposure compensation, I simply turn the rear wheel. I am going to meter again, and as I turn to the right, you can see the meter going up; that's one stop of positive exposure compensation, that's two, that's three, and note that as I'm turning it, my shutter speed is changing. That's how the camera has decided to get its exposure compensation. In other words, I'm saying now I want the image to be one stop brighter, and so it's gone from 1/100 of a second to 1/50 of a second. That's a doubling; that's one stop.
It chose shutter speed in this case, because it can't actually alter the aperture, because this lens has a maximum aperture of f/4, so the aperture cannot go any wider. In other situations, it might choose to perform an adjustment to aperture, or it might choose to do both. Now, I'm currently set on ISO 100, which means that the camera is not allowed to change ISO. That's why it's making its adjustments to shutter speed and/or aperture. Watch what happens if I change ISO to Auto.
So I'm metered at 1/125, at f/4, at ISO 125. So the metering, right away, is different. It's bumped up the ISO a little bit, so that it could get the shutter speed a little bit faster, and the reason it's decided to go for a slightly faster shutter speed is I'm going to have a better chance of getting a sharp image while shooting handheld. Now, if I adjust my ISO, look; it's not touching shutter speed at all this time. It's altering the ISO; it's cranking it up. It's cranking it all the way up to a 1000 at three stops over.
And it's interesting, because if you go look at other -- at older Canon cameras, they won't do this; the ones they have auto ISO. They won't go quite as far with the ISO. Canon knows that on the Mark III they can really crank the ISO up a good amount before they start seeing noise. So nowadays, a lot of times it's really going to be doing ISO changes if you're in auto ISO, rather than fiddling with your shutter speed, or aperture. That will guarantee that they're not messing up your depth of field or motion stopping that you might have dialed in.
Now, of course, I can also go to the other direction, and lower my exposure compensation. This is one stop under now, and if you notice here, it's not making changes to ISO, because ISO can't go any lower, and it's not going to -- it's not altering my shutter speed. This time it's chosen to go with aperture, and now it's doing both. Here you can see it altering both shutter speed and aperture to get a metering that matches what I've dialed in. If I'm back to here, of course, I'm back to what the camera thinks is correct. So again, this is a way of getting over or under exposure, without worrying about how I'm getting there; the camera is making those decisions.
Now, there might be times where I know I want a particular aperture, but I still want to use exposure compensation to get the adjustment I want. I can do that in aperture priority mode. Similarly, I can lock down my shutter speed in shutter priority mode. We'll be looking at those situations when we discuss priority modes. In the meantime, I think you'll find that exposure compensation is an everyday feature; it's something you are going to use a lot. It's a great way to get your image brighter or darker, especially if you don't have particular depth of field or motion stopping ideas in mind.
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