In my last article I introduced the idea of exposure and some of the things every photographer should consider when trying to properly expose a photograph. In this article, I’m going to explain how to use the two technical tools that are built right into your digital camera for that express purpose.
It all starts with the light meter. . .
How your camera’s light meter works
With the exception of some specialized cameras, all contemporary digital and film cameras have a built-in light meter. These light meters (called TTL meters because they measure “Through The Lens”) work by measuring the light reflecting off the scene in front of the camera.
When you click the shutter, the camera’s onboard light meter first measures the reflected light coming through the lens, then computes how bright that light is, and finally adjusts the shutter speed, aperture and ISO. In a sense, the camera is able to “see” and respond to the scene by using the light meter.
Now, while the camera’s light meter is certainly useful, there IS a catch!
Your light meter’s baseline is 18% gray. . . that’s the average of what it expects to see when you point it at something. It doesn’t expect 100% reflected light (which would be pure white) or 0% reflected light (pure black). Instead, it expects a certain average—about 18%, according to laboratory tests run by scientists who study this sort of thing.
However there are plenty of real world situations which break all the rules and contradict this nice idea of the world being 18% gray. So what’s a photographer to do? How do you properly expose a scene that is either darker or lighter than the “average”?
Using your camera’s EV compensation scale
For all the situations that aren’t 18% gray, most cameras will let you override the calculated light meter exposure. This can be done by using EV compensation. The EV scale is quite handy and can be found in your camera’s top or back LCD display.
Most cameras will allow you to use EV compensation regardless of whether you’re shooting in auto, aperture- or shutter-priority, or manual.
The camera’s EV compensation scale generally reads between -2 and +2, with 0 being the “correct” exposure according to the light meter. If you adjust the calculated EV by +1, that’s overexposing by 1 f-stop. Likewise, if you adjust the calculated EV by -0.3, that’s underexposing by 1/3 of a stop.
As different cameras require pushing different buttons and turning different knobs, I’ll leave the exact method that you’ll need to employ on YOUR camera, to you.
The point is, though, that if you’re photographing a bright snowy landscape, you’ll need to tell the camera to “overexpose” the image. If you’re photographing a dark forest, you’ll want to tell the camera to “underexpose” the image. . . your camera will assume the world is 18% gray until you tell it that, actually, snow is very bright and shady forests are very dim. This will give your photograph the proper lightness or darkness that a viewer would expect from a field of snow, or a forest of gloomy trees.
When you’re in a normal situation of course, just taking photographs of people in even lighting, you’ll be fine with “0” EV.
Understanding your light meter settings
It’s important to know that the camera’s onboard light meters can measure incoming light from different parts of the image. Most cameras typically have 3 choices: spot metering, center-weighted metering and matrix/evaluative metering.
Spot metering measures just a small percentage of the scene from the very center of the image frame, while center-weighted metering measures from points throughout the frame, but weights the readings from the center of the image frame higher than those points around the periphery.
Matrix metering (also called evaluative metering) is the most sophisticated metering mode on SLRs and DSLRs, and uses several different variables and algorithms to determine the correct reading.
When to use spot metering
If you want be certain that a critical detail is properly read by the light meter, spot mode can be very useful to keep that detail in your photograph from being under- or overexposed.
Spot metering only takes a reading from the center of the image frame, right where the central AF point is located. This can give you a precise reading of a small part of the scene you’re photographing—just remember that the rest of the scene isn’t being calculated into the exposure.
A simple way to use spot metering mode on many cameras is to depress the shutter button halfway, which will force the camera to auto focus and calculate the exposure. Then, while still holding the button halfway down, you can recompose the image as you like before pressing the button all the way down to take the picture.
When to use center-weighted metering
If you’re working in relatively even light and photographing a scene where your subject is roughly in the middle of the frame, center-weighted metering is the appropriate choice. It can also be useful when you are photographing a scene without extreme contrast.
The logic behind center-weighted metering is that, generally, the critical information in a photo is largely in the center—in other words, the light meter will concentrate on measuring the light on the person in the middle of the frame, and use that information to expose the entire photo, even if it means the scenery on the outside edges will be over- or underexposed.
When to use matrix/evaluative metering
If you’re photographing a quick-moving subject, or when you’re trying to work quickly, matrix/evaluative metering is very helpful.
With matrix metering, the light meter communicates with the autofocus system to see which focus points are being used, how far various objects are from the camera and what the lighting is throughout the scene. Then, that information is cross-referenced with the camera’s database of similar scenarios and the exposure is calculated accordingly.
Like the first two metering modes explained above, this mode works much the same regardless of which brand of camera you’re using and despite the somewhat-secretive processes involved in calculating the exposure.
Ultimately, the TTL light meter and EV compensation scale are two basic tools built right into your camera—which means they won’t cost you a dime to use! They are relatively simple to master (although it will take some hands-on practice) and they’re also quite powerful.
Get comfortable switching between metering modes and dialing in EV compensation now. When you need them, you’ll be ready to get the shot, and your photos will be properly exposed.