No digital camera can capture every tone available in a scene. In other words you can’t record what you see with your eyes. Some detail in dark areas is lost because not enough photons are captured by the sensor, and details in light areas vanish when pixels in a sensor are flooded with more light than they can handle.
The range from the darkest detailed picture elements to the brightest is called the dynamic range, and the goal of proper exposure is to make sure that the most important tones in an image fall into that range.
Getting a good exposure is relatively easy, but getting the best exposure can be tricky.
Your camera should be equipped with a built-in 'Meter'. The Metering System will automatically work out the correct exposure by taking an average of all the tones in the complete image. Knowing how your digital camera meters light is critical for achieving consistent and accurate exposures.
Matrix metering (evaluative/multi-zone metering): In many cameras this is the default metering mode. The camera collects information from multiple locations in the scene, which can range from a few to more than 1000 locations. A number of factors are taken into consideration, including the following: Autofocus point, distance to subject, areas in focus or out of focus, colors/hues of the scene, and backlighting. Multi-zone tends to bias its exposure towards the autofocus point being used (whilst taking into account other areas of the frame too), thus ensuring that the point of interest has been exposed for properly. A database of many thousands of exposures is pre-stored in the camera, and the processor can use a selective pattern to determine what is being photographed.
For example, if the matrix system determines that the upper half of your image is light and the bottom half is dark, it may decide that you’re shooting a landscape and will optimize exposure so there will be detail in the foreground, allowing the sky to be a little too light, if necessary.
Center-weighted average metering: In this mode, the camera puts the most emphasis on the information in the center of the scene, but takes into account the illumination in the rest of the frame.
This mode is a good choice if you know your main subject will be in the middle of the frame, but the other areas of the picture have some importance too.
Spot metering: With spot metering the camera will only measure a very small area of the scene. This will typically be the very center of the scene, but some cameras allow the user to select a different off-center spot (usually the autofocus point).
This setting is a good choice if you are at a concert, where you want to expose for the spot-lit performer and don’t want the inky dark surroundings to affect the settings.
Partial metering: This mode meters a larger area than spot metering, and is generally used when very bright or very dark areas on the edges of the frame would otherwise influence the metering unduly.
One of the most common applications of partial metering is a portrait of someone who is backlit. Metering off of their face can help avoid making the subject look like an underexposed silhouette against the bright background.
Digital cameras have an adjustment system called EV (exposure value) compensation. With these modes, you can specify a little more or a little less exposure than the ideal exposure that your camera’s light-measuring system determines. This allows for manual corrections if you observe a metering mode to be consistently under or overexposing.
It’s very easy to use: simply press the EV button on your camera and use a command dial or left/right cursor key to add more exposure (+EV) or reduce exposure (-EV) using 1/2 or 1/3-value
Increments (depending on how you set it in your camera preferences). Most digital cameras let you fine tune exposure about +/- 2EV (some have 3 or even 5 levels).
+0.5 EV would give half a stop more light to the metered exposure, +1.0 EV would add a full stop more, and so forth. After taking a picture, look at the image you’ve just taken and determine if you need to add or subtract EV.
How does exposure compensation work? Let’s say you set the camera to aperture priority. You have chosen your desired aperture, but find that your image is constantly dark. By adding +1 or +1/2 stops via the exposure compensation on your camera, you will lighten the image overall. If we use this in aperture priority, the camera will use a slower shutter speed to compensate, or a lower ISO (if you have auto ISO enabled).
While in shutter priority mode, the camera will open the aperture (or change the ISO if Auto ISO is enabled), allowing more light to reach the sensor. The reverse is true if you are finding your image is too light, dialing in a negative exposure compensation number will darken your image.
Exposure compensation is simply an instruction to tell the camera to behave as though the amount of light received was either more or less than the measured amount.
In other words, dialing in +1 or –1 (or any other value) simply sais “OK camera, now that you’ve calculated the correct exposure, I want you to take that exposure, and under/overexpose it by this amount” (+1 or –1 or whatever you dial in). It’s that simple!
One thing you need to know is that your camera will ALWAYS expose for 18% grey (neutral grey). If the camera is aimed directly at any object lighter or darker than middle grey, the camera's light meter will incorrectly calculate under or over-exposure.
Example: Let's imagine two cats. A black one and a white one. The black cat is sitting on a pile of coal and the white cat is sitting on snow. You point your camera at the black cat on the pile of coal and take a picture. Then you point your camera at the white cat sitting on the snow and snap away.
What do these look like? Well, unless you have compensated the exposure they will both look pretty much the same. The black cat and coal will look grey, and the white cat and snow will also look grey.
Why? Because light meters integrate the light that they see to produce an exposure centered on neutral gray. Everything that is darker than grey (in our case, black) will be overexposed to look grey, and everything that is brighter (white cat) will be underexposed, again, to appear grey. Because that’s what the camera thinks is correct.
But, of course we're more clever than our cameras. We understand this, and we know how to use our camera's exposure compensation (EV/EC) control. We know that we need to decrease our exposure a stop or two so that the cat and coal look truly black, and increase our exposure a stop or two so that the white cat and snow indeed look white.
Other exposure adjustments
Scene modes: Scene modes are the modes offered on many dSLRs that are similar to the scene options found in other digital cameras. They set up your camera to use specific exposure options (long exposures for Night Scene or Night Portrait mode), set shutter speed/aperture preferences for action in Sports mode, as well as adjust parameters like color richness and sharpness.
Exposure Lock: The exposure settings are fixed when you press a special exposure lock button or simply press the shutter release down halfway. Such a lock gives you the freedom to set exposure and then reframe the photo any way you like without worrying that the settings will change.
Bracketing: Bracketing is the general technique of taking several shots of the same subject using different camera settings. It means you have the ability to be able to take three or more shots of the same scene each with differing exposure, white balance or flash values. By taking multiple exposures of the same this, you are also able to make some creative and useful HDR Images. Do a Google search for HDR imaging and check out some of the results.
Metering modes are the options for collecting exposure information and calculating an exposure. Shooting modes tell your camera how to apply that information.
Auto: Full auto mode handles all the settings for you, with no options for adjustment. This is the mode you use when you hand your camera to a passerby and ask to have your picture taken.
Programmed (P): Your dSLR uses its built-in smarts to analyze your scene and come up with a recommended exposure. However, you can override the camera’s selections with EV settings, equivalent exposure shifts, or other tweaks.
Shutter priority (Tv): In this mode the exposure system keeps the shutter speed you’ve chosen and varies only the aperture to achieve the correct exposure. This is good for sports photography or when you have a fast moving subject. If the light conditions change enough that your selected shutter speed won’t produce a correct exposure with the available aperture, you’re alerted with a LO or HI indicator in the viewfinder. That means your camera has opened the aperture as wide as possible, but the light levels are still too low to produce a correctly exposed image with the current shutter speed, or it closed the aperture down to it’s minimum size (large number) but the image is still too bright.
Aperture priority (Av): This mode lets you choose the aperture (f-stop), letting the camera select the shutter speed needed for correct exposure. If you notice that the shutter speed is too slow or high, you can adjust the f-stop you’ve dialed in to one that’s larger or smaller.
This mode is useful when you want to control the depth of field. Also this is the mode that I use 90% of the time. You just dial in a large aperture (small number) for a shallow depth of field or a small aperture (large number) for a wide depth of field, and snap away while your camera automatically adjusts the shutter speed for a correct exposure.
Manual exposure (M): Use manual exposure when you want to set both the shutter speed and aperture yourself. This gives you complete control over the exposure of your photo. You can still use your dSLR’s metering system to determine the “correct” exposure and then set it yourself, or set some other exposure entirely for a creative reasons/special effects. This mode is the “hardest” to master, since nothing is controlled by the camera.
Trying out alternate modes is not as scary as you might think. You must take this step if you want to grow as a photographer. With digital cameras you can shoot as much as you like, it costs you nothing; if you don’t like the picture, you just delete it. So go out and experiment.
After all, experience is the best teacher!