:: Exposure basics
|Camera:||Canon EOS-1Ds Mark III|
Want to totally confuse your camera? Try auto-exposing a scene like this. (Details below.)
Cameras aren’t so bright
Your camera is stupid. Sorry (so is mine). And while I’ll gladly cite many examples, right now it’s just important that you understand that your camera thinks the entire world is a middle tone. Regardless of what its meter sees, your camera will do everything in its power to make it a middle tone. Sunlit snow? Asphalt at midnight? It doesn’t matter—if you let your camera decide, it will turn out gray.
Modern technology from camera manufacturers offers faux-intelligence to help overcome this limitation. Their solution, called “matrix” or “evaluative” metering, compares a scene to a large but finite internal database of choices, returning a metering decision based on the closest match. This works pretty well in conventional, “tourist” light, but often struggles mightily in the warm or dramatic light artistic photographers prefer. If you want to capture more than documentary “I was here” pictures, you’re much better off taking full control of your camera’s metering and exposure. Fortunately, this isn’t nearly as difficult as most people fear.
Laying the foundation
We control the amount of light in our images with our shutter speed, f-stop, and ISO settings, adding or subtracting “stops” of light by increasing or decreasing the shutter speed, f-stop, or ISO. A stop is the measurement of light photographers use, much like a cup (of sugar or almonds) is a measurement of volume that cooks use.
The beauty of metering is that a stop of light is a stop of light, whether you control it with the size of the:
- Aperture: The opening light passes through when the shutter opens, measured in f-stops
- Shutter speed: The time the light is allowed to pass through the aperture—slower shutter speeds mean more light; faster shutter speeds mean less light
- ISO: The sensitivity of the sensor or film to light
For example, let’s say you’re photographing fall leaves in a light breeze. You got the exposure right, but decide you need to double your shutter speed to freeze the leaves’ motion. Doubling the shutter speed reduces the amount of light by one stop. To replace that lost light, you could increase your aperture (f-stop) or ISO by one stop (or a combination of f-stop and ISO adjustments totaling one stop). Problem solved.
Your camera’s metering mode determines the amount of the frame the meter “sees.” Since most images have a range of tones from dark shadows to bright highlights, the meter will take an average of the tones it finds in its metering zone. The larger the area your meter sees, the greater the potential for a wide range of tones. I prefer “spot” metering because it’s the most precise, covering the smallest area of the frame possible, an imaginary circle in the center three or so percent (depending on the camera) of what’s visible in the viewfinder. Spot metering isn’t available in all cameras—in some the most precise (smallest metering zone) metering mode available is “partial,” which covers a little more of the scene, somewhere around ten percent. Check your manual for the metering modes it offers.
Don’t confuse the metering mode with the exposure mode. While the metering mode determines what the meter sees, the exposure mode determines the way the camera handles that information. Most SLR (single lens reflex) cameras offer manual, aperture priority, shutter priority, and a variety of program or automatic exposure modes. Serious landscape photographers should forego the full automatic/program modes in favor of manual (my recommendation) or aperture/shutter priority modes that offer more control.
If you select aperture priority mode (I’m a landscape photographer, so I’m assuming people reading this are too), you specify the aperture (f-stop) and the camera dials in the shutter speed that delivers a middle tone based on what the meter sees. But you’re not done—unless you really do want the middle tone result the camera desires (possible but unlikely), you then need to adjust the exposure, using exposure compensation (usually a button with a +/- symbol), to adjust the exposure for the amount you want it to be above or below a middle tone.
I prefer manual mode because it gives me more control, and I think it’s easier. In manual mode, after setting my aperture (based on the depth of field I want), I point my meter (in my 1DS Mk III that’s the center 3% of my viewfinder) at the area of the scene I want to meter and dial in whatever shutter speed gives me the amount of light I want that subject (where my meter points) to have. That’s it. (In manual mode you can ignore the exposure compensation button.)
So let’s review. Start by selecting your metering mode (the way your meters”sees” the scene: spot, partial, matrix, and so on), then take your camera out of auto exposure mode and put it in manual (my recommendation) or aperture priority (if you prefer) mode. (Remember, I’m a landscape photographer so I never use shutter priority; if you’re shooting action, you may want to consider shutter priority if you don’t like manual exposure.)
Before metering, set your camera to whatever aperture you decide your composition calls for. Then meter, remembering that your camera isn’t telling you what the exposure should be, it’s telling you the exposure that will make what it sees a middle tone. Finally, correct the meter’s middle-tone suggestion by dialing in the shutter speed (in manual mode) or exposure compensation (in aperture priority) that gives the correct exposure.
What’s the correct exposure? That’s a creative decision that’s entirely up to you—feel free to play until you’re comfortable with your results. The more you do it, the easier it gets.
Below are some sample images and the thought process I followed to get the exposure.
Now get to work
Don’t try to apply all this for the first time when you really, really want the shot. Instead, find a time when the results don’t matter and play with your camera to find out how much control you have over exposure. In fact, you can do this right now in your backyard or even sitting right there in your recliner. Meter something nearby, set an exposure, and click. Look at the result, adjust the exposure, and click again. Watch your histogram, and watch how its shape shifts right as you increase the exposure, or left as you decrease it. Continue doing this until you’re confident in your ability to make a scene brighter or darker, and can consistently achieve the exposure you expect.