In order to have a proper exposure, how much light needs to hit the image sensor of your camera when you press the shutter release? This is the basic question of exposure, and two factors determine the answer: ƒ-stop and shutter speed. You can let the camera choose these for you or choose them yourself in one of your camera's manual modes. Either way, the amount of natural light at your location must first be measured—and measured accurately.
There’s a common misunderstanding that if you capture images in RAW format as opposed to TIFF or JPEG, you don’t have to worry about proper exposure because you can fix any problems later in editing. This certainly isn’t true. The image sensor of your camera is only capable of handling a certain range of exposure, from light to dark. Go past that in either direction, and the sensor simply isn’t going to capture those details. And no editing program can fix what was never there to begin with. The goal is to capture the best possible images initially, so you maximize your camera’s capabilities and have minimal problems when editing them in your computer. Here are some tips to help you do that.
Measuring Light
All digital SLRs and many advanced compact cameras
on the market today use sophisticated metering systems that take light readings from many different points in the frame, but those automatic interpretations can sometimes give you unsatisfactory results. The information gathered by the built-in meter is sent to a microchip where it’s analyzed and compared to a large database, so the camera can make a recommendation of what it "thinks" the optimum exposure should be. Much of the time, these built-in metering systems deliver excellent results, but they have limitations.
In complex lighting situations, the built-in metering system can lead to a blown exposure. And no amount of Photoshop manipulation is going to rescue the poor tonal definition or lack of detail in the shadows and highlights.
Let’s say you’re photographing forest landscapes in winter and a fresh blanket of snow has fallen. You point your camera at the scene to get an exposure reading, and one of two things may happen.
If you favor the trees, the metering system likely will interpret all the dark foliage to mean the ambient light is low and a longer-than-necessary exposure is needed. The resulting photograph will be washed out, and the snow and the trees will lack the right tonalities. Remember, the image sensor of your camera is only capable of handling a certain range of brightness. Go past that in either direction, and the sensor won’t capture enough image detail in the original exposure to make a good print.
If you frame for the snow, the meter will interpret this as excessive light. This happens because the light meter of your camera is reading reflected light. Even though the snow and the trees are illuminated with the same sunlight, their reflective qualities differ greatly. The metering system doesn’t "know" this, so if you go with an ƒ-stop and shutter speed that the camera tells you is a correct exposure, your image will be very underexposed.
Exposure Lock
One way to deal with a complex lighting situation is to use exposure lock. If the scene you’re trying to photograph has a wide range of light and dark objects, for example, metering something that’s midtone (or close to middle gray) often will be a much better interpretation of the natural light. Sometimes metering the sky is a good option, as long as you’re not facing the sun. It’s always wise to experiment. Try a number of different places to point your camera and lock the exposure before you reframe and take each shot.
Histograms
The single best tool you have at your disposal for determining the right combination of ƒ-stop and shutter speed is the histogram. Not all digital cameras have this feature, but if yours does, you definitely should use it because a histogram shows the exact range of tones captured by the image sensor. A photo’s exposure can look good on the LCD monitor, but upon later review on your computer, it actually can be quite poor. The goal always should be to get the best possible photo in the first place, and the histogram makes this easy.
Represented as a graph, a histogram looks like a mountain range. The left side depicts the darkest parts of the photo and the right side depicts the lightest. Anything beyond the left edge is pure black and anything beyond the right edge is pure white—both are outside the range of the image sensor.
Every histogram will be different and there’s no "correct" shape. Depending on which is more prevalent in your photograph, shadows or highlights, the histogram visually may favor one side or the other. Ideally, the slope of dark or light tones should end at or before the edge of the graph. What you have to watch for is the slope of the graph being abruptly cut off at either edge. When this happens, this means details aren’t being captured by the sensor.
To make a quick adjustment if you see a problem, you can use the ± exposure compensation control. This feature allows you to manually adjust the brightness of the auto-exposure value set by the camera before you take the next shot.
If you’re shooting in one of your camera's manual modes, work from the camera’s recommended settings for ƒ-stop and/or shutter speed, then vary them from there based on what the images look like on the histogram. Refine your settings and experiment. There’s certainly more than one way to get a proper exposure, with each having a different tonal range. The same scene could be captured with any number of ƒ-stop and shutter-speed combinations.
ƒ-Stops And Shutter Speeds
Each ƒ-stop number (ƒ/4, ƒ/5.6, ƒ/8, ƒ/11, etc.) of your camera represents a different diameter of aperture in the lens. The larger the ƒ-stop number, the smaller the opening in the lens, and vice versa. This may seem counterintuitive at first, but the reason for this is that the numbers are derived from a ratio between the focal length of a lens and the size of the aperture.
If you have a 50mm lens, for example, and the aperture is set at 25mm wide, then the ƒ-stop would be 50 divided by 25, or ƒ/2. The range of ƒ-stops will vary for different lenses, but the ƒ-numbers are all derived from this ratio.
Each ƒ-stop is used in concert with a shutter speed that will control how long light reaches the image sensor (1/25 sec., 1/60 sec., 1/250 sec., etc.) when you press the shutter release. The combination of these two settings—the size of the opening in the lens and how long the shutter stays open—is the exposure. Which shutter speed will best complement a particular ƒ-stop will depend on the lighting conditions and how you want to capture a scene.
If you want to freeze the motion of a bird in flight, a fast shutter speed is required and the ƒ-stop selected will have to complement this setting properly based on the available light. If you want to convey the movement of a bird with some slight blurring, a slower shutter speed is needed. With the right ƒ-stop and shutter speed, each image would have a proper exposure. It just depends what aesthetic qualities you want in your photograph, which includes depth of field.
Depth Of Field And ƒ-Stops
Remember, as the ƒ-stop number increases, the aperture gets smaller, giving your photograph increased depth of field. This is why landscape photographers frequently use ƒ/16 or ƒ/22; those settings will produce great depth of field and help emphasize distance. Everything in front of and beyond the focal point will be acceptably sharp, replicating how the human eye would view a scene.
Conversely, as the ƒ-stop number decreases, the aperture gets larger and the depth of field decreases. Everything in front of and beyond your focal point will be less and less in focus the larger the aperture. The amount also will depend on your distance from the focal point.
If you focus a wide-angle lens on something two feet away, an ƒ-stop of ƒ/2.8 will reduce the background to color and texture. Most of the detail beyond the focal point will be so out of focus that you won’t be able to tell what it is. Now, if you move farther away, say four feet, and focus on your subject again, an ƒ-stop of ƒ/2.8 simply will make the background less in focus than your subject. Whatever is behind your subject still will be visually discernable.
Depth of field also is affected by the focal length you select. Short focal lengths provide more depth of field than longer focal lengths. If you select an ƒ-stop of ƒ/11, for example, a focal length of 28mm is going to give you more depth than a focal length of 50mm.
Balancing Act
With every photograph you take, it’s always a balancing act of many different variables and compromises. You may need to experiment with different modes on your camera to find the right balance of ƒ-stop and shutter speed for the best possible exposure. And again, if your camera has the histogram feature, definitely use it. It doesn’t take a lot of time to study one, and it’s the best way to prevent poor exposures. You can make adjustments and refine your settings in the field so when you start editing images on your computer, you’ll have few problems, if any, to deal with.
Cardinal Sin, I would use a light meter!