All modern consumer-level cameras come equipped with a light meter. And a good thing too, because without a meter photography would be at best, a game of educated guesses, and at worse, a festival of complete and utter frustration. But if I had to guess, I’d say that this most-important piece of photography equipment is probably the most taken-for-granted of anything that comes equipped on a camera. You change your shutter speed, aperture and ISO pretty regularly. You probably also change your white balance setting and your focusing mode. But you may not pay a whole lot of attention to your meter.
How meters work
To fully understand why this matters, you first need to understand how your metering system works.
All internal camera meters measure reflected light, which means that they’re not actually reading the light that falls on a scene, but the light that reflects off of that scene. That means that the best they can really do is guess, because not everything in the world is exactly as reflective as everything else in the world. Snow, for example, is highly reflective—that’s why it’s so painful to try to look at it on a sunny day without a really good pair of sunglasses. A field of flowers, on the other hand, is not very reflective at all. So what’s a meter to do when it really isn’t able to tell the difference between a scene that has a lot of reflected light and a scene that doesn’t have very much reflected light at all? That’s where 18 percent gray comes in.
All internal meters are designed to assume that everything in any given scene averages out to roughly middle gray, otherwise known as “18 percent.” Don’t let that number fool you—I know it seems like middle gray ought to be 50 percent, but that 18 percent is actually referring to the amount of light that reflects off of that particular tone, rather than where it falls between black and white. And middle gray doesn’t have to be technically gray, either, the term can also refer to a color that falls somewhere in the middle between highlight and shadow. But the main takeaway from this is that your meter isn’t smart enough to know the difference between black and white, because it works in averages. So in its mind, everything in a scene should average out to middle gray, whether it’s that brand new blanket of snow or an unbroken stretch of new tarmac.
Matrix (evaluative) metering
Most cameras have two or three metering modes to choose from, with the default setting generally some version of a matrix or evaluative system. This is the sort of catch-all mode—it works really well in most situations.
Matrix (evaluative) metering works by averaging out all the reflected light in any given scene. It’s designed to assume that everything in any given scene is going to average out to roughly an 18 percent gray. With matrix metering, each scene gets divided up into zones, and the meter finds the highlights and shadows in each zone. It then averages all that information together and makes and educated guess about what settings you’ll need to use to get a good exposure of that scene. The behind-the-scenes stuff is actually pretty interesting, though not required learning (don’t worry, there will not be a quiz). Depending on your camera there may be just a few zones or a few thousand of them. And other factors come into play, too, such as focus point, the distance between you and your subject and, for some cameras, a database full of information about the best exposure settings for thousands of other images.
OK so why do you need to know all that? The short answer is that you don’t. It should, however, give you some confidence in your matrix metering system, but don’t let that lull you into a false sense of security. Despite all those bells and whistles, there are certain situations where your metering system is going to get it dead wrong. There are many more situations where it’s going to get it a little bit wrong. You want to be able to identify those situations so that you can choose another metering mode if you know that matrix metering probably isn’t going to give you great results.
dog zone by Flickr user hezur
The second and equally important metering mode that most modern cameras have is spot metering. This is really the opposite of matrix metering, because instead of taking an average of all the reflected light in a scene, this system will take a reading off of a single point: the one right in the middle of the frame. Now, spot metering works in middle gray, too, so for best results you need to point it at something that falls in that 18 percent gray range. That can be tricky to do for the uninitiated. It’s hard enough picking out an 18 percent gray from, say, a pile of gray rocks—but how do you find it in a world of colors? The answer is that you don’t, at least not at first. Instead, buy a photographer’s gray card—they’re inexpensive and you can clip them to your camera bag or slide them into a side pocket. Whenever you need spot metering, place the gray card in the scene and take a spot reading from it. Lock in your settings, then remove the card and take the shot.
Now, there will come a time when you don’t have your card with you or you really just can’t be bothered to get it out, or maybe you just don’t have time. When that happens, you will need to have an idea of what you might be able to meter off in your environment. So to help you out in that situation, here’s a short list of things you might find in your environment that will be more or less an 18 percent gray.
Brown Bag (with staple) by Flickr user Jeffrey Beall
- Healthy, medium-colored grass
- A brown paper bag
- A clear blue sky (with your camera turned away from the sun)
- Your gray camera bag (some are deliberately made 18 percent gray for just that reason)
But here’s something you can do to guarantee you’ll have an object to meter off of in every scene—use your hand. The average Caucasian hand is one stop brighter than 18 percent gray. To find out where your hand lies on the scale, compare it to an 18 percent gray card. First take a reading off the card, then (while in the same lighting conditions), take a reading off of your hand. Figure out how many stops above or below 18 percent gray it is, and then you’ll have a gray card with you wherever you go. Hint: use the palm of your hand instead of the back of your hand, in case your color changes in the summer because of time spent in the sun.
Drawing by Flickr user JelleS
Center-weighted metering is probably the most under-utilized of the three metering modes most cameras have. It’s kind of like a cross between spot metering and matrix metering in that it also uses the center of the frame, and it also works in averages. But instead of considering everything in the scene the way matrix metering does, it assigns an importance of somewhere between 60 and 80 percent to the light that is concentrated in the center of the frame. This area is larger than the spot that spot metering uses, and the system does also consider the corners, though they are given less importance. Some cameras will let you control the size of the circular area used for center weighted metering, which can be handy in certain situations.
Center-weighted metering is good for portraits, because when you’re capturing a head shot you want the camera to give the most importance to your subject’s face. For the most part you’ll be completely ignoring what is in the background, which means that you may get underexposed or over exposed backgrounds, but in portraits that’s far less important than getting a well-exposed face. Center-weighted metering is also particularly useful for high-contrast situations such as full sun. And it works well for backlit subjects, too.
It’s harder to get used to these concepts than it is to get used to fundamentals like aperture and shutter speed, so if you want to take your time and stick with matrix metering for a while, I really don’t blame you. But there is going to come a time when you’re no longer going to be happy with those less-than-perfect results, and when that happens you’ll want to refer back to this lesson and start thinking about the ways that your meter has been hindering you—and the ways that it can be used to help you. It’s always a good idea to try to get that perfect exposure in-camera, and choosing the right metering mode for the right situation is one very good way to make sure you can achieve that. Yes, it requires some practice and learning how to develop an eye for the light, but that’s something you need if photography is a pursuit you’re passionate about.
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