Today’s digital camera’s are wonders of modern technology, encompassing miniature optical, mechanical and electrical systems, and even a CPU. All of these marvels can be had for as little as a hundred dollars or you can pay upwards of several thousands. Today I want to focus on the metering system in these cameras – not so much on what they do, but on what you can do when you don’t like what they do.
The metering system in your camera assesses the scene you are shooting and does it’s best to determine what settings will provide you with the best exposure. Depending on the mode (P, S, A, M) you’ve selected, the camera may select the aperture, the shutter speed or the ISO, all of the above or none of the above, leaving it to you. Most times, it does a pretty good job, and typically the more expensive cameras will do a better job than the really inexpensive ones. But the metering system’s settings might not be to your liking and this is where exposure compensation comes into play.
Metering systems are designed to lighten dark scenes and darken light scenes, and capture everything at a standard level of brightness, known as 18% gray. This works fine in most cases, however, not all. Sometimes, the metering system can’t handle complex lighting situations where there are both light and dark areas in the frame. Depending on the balance of light and dark areas in the scene, the meter might end up over-lightening (overexposing) or over-darkening (underexposing) the scene. The metering system can also struggle when faced with a scene that is predominantly either bright or dark. For example, this image was shot in aperture priority mode, at f/2.8, 1/50 second, with available light and no exposure compensation. The image of the cup is not as white as in reality, because the meter tried to render it as 18% gray. More on this in a bit…
If your camera has exposure compensation adjustments (most DSLR’s and some point and shoots do), then you can change the exposure the meter arrives at by forcing the camera to take in more or less light, resulting in a correct exposure. Exposure compensation is typically measured in Exposure Values (or EV’s), which represent increases or decreases of light in stops:
- One positive EV is equivalent to one positive exposure stop, or a doubling of the amount of light that enters the camera.
- One negative EV is equivalent to one negative exposure stop, or reducing by half the amount of light coming in.
Cameras typically allow selecting EV’s in 1/2 or 1/3 of a stop increments, usually up to plus 3 or minus 3 total stops.
When you decide to change the EV as you’re about to make an image, your camera will adjust the exposure settings depending on what mode you are in. If you are in Aperture Priority, it will adjust the shutter speed; if you are in Shutter Priority, it will adjust the aperture; if you are in Program mode, it could adjust either aperture or shutter speed or a combination of both. In Auto mode, exposure compensation will not work, and in Manual mode it will have no effect.
So an example of how you could use exposure compensation to correct the white cup image above would be to increase the exposure compensation by dialing in a positive EV. I chose an EV of +1:
Everything else remained the same. The resulting exposure was f/2.8 (no change), 1/25 second (one half the shutter speed of the original). You can see that the cup is whiter but so is the black background. I prefer the whiter cup but I also want a darker background, so the easy solution is to darken the background by increasing the shadows in Picasa, a basic photo-editor.
One of the most common examples of when you would use exposure compensation would be snow scenes. Because your camera is striving to capture everything as 18% gray, that snow that looks white to you will appear in your image as… you guessed it, gray, because your camera will underexpose the scene. For example, this was taken without exposure compensation at f/7.1, 1/200 second:
Better. I shot these in Program mode, so the camera opened the aperture to f/6.3 and slowed the shutter speed to 1/160 second. This second image is better, as it brightens the scene and still portrays the gray and overcast day that it was, but I could have gone to +1 EV or even +1 and 1/3 to brighten it further.
Another typical use of exposure compensation is when shooting a sunset. In that case, where you may be looking at a darker foreground and sky but with bright light from the setting sun, the meter will usually overexpose, trying to bring everything up to 18% gray.
For example, this sunset was taken without exposure compensation in Aperture Priority mode at at f/2.2, 1/400 second:
Not bad, but it could be better. So I dialed in an EV of –1 and the camera exposed the scene at f/2.2 (no change) and 1/1000 second. This faster shutter speed resulted in a darker sky and water, and really accentuated the colors of the sunset.
One question you should be asking about now is “So if I do need to use exposure compensation, how much EV should I dial in?” and the answer is simply, I don’t know. In fact, only you will know that answer and that will come by experimenting. Digital images are free to make, so make many. Look at your LCD screen and if the image you just made and it’s histogram are too dark or too gray, EV in a positive number. If it’s too light, EV in a negative number. Start with 2/3, look at the result and adjust from there. With more experience, you’ll begin to look at a scene and know immediately, this calls for exposure compensation of –2/3, for instance. And you’ll be right. But in the meantime, experiment.
Another tip is to use auto-exposure bracketing (AEB). This will take multiple images in succession, changing the exposure by set amounts that you choose. So for example, a bracketed set of images might be taken at –1, 0, and +1 EV, allowing you to choose the best one when you get home. AEB will be a future post but for now, check your camera manual to see if and how to do it.
Other reasons to exposure compensate are because the metering system in your camera is not working correctly (it could be off just a bit, resulting in consistently over- or under-exposed images), or you may simply have a different artistic interpretation of the scene than what your camera rendered. In either case, it’s exposure compensation to the rescue.
Generally, an exposure compensation feature is designated on your camera by the [+/-] symbol, highlighted in yellow in the picture at the top of this post. If you don’t see that, check your camera’s manual to see if it’s labeled differently and how to access it.
As you venture away from shooting in pure Auto mode, you’ll enjoy the added control you can exercise over your images with exposure control. And in time, it will become second nature.
Thanks for visiting 2 Guys Photo. Posted by Ed