WHAT IS… Exposure Compensation?
Most digital cameras provide exposure compensation. What is exposure compensation — and how do we use it to obtain better exposed pictures? This OnePager™ tutorial makes it all clear.
WHAT – The Short Description
Exposure Compensation is a feature of a camera that allows you to adjust the exposure measured by its light meter. Usually, the range of adjustment goes from +2 to -2 EV in 1/3 steps.
WHAT – The Long Description
If you look at the QuickFact Sheet for a digital camera on this site, you’ll notice this entry:
|Exposure Compensation||+/- 2 EV in 1/3 EV steps|
This means that you can adjust the exposure measured by the light meter by telling the camera to allow more light in (positive exposure compensation) or to allow less light in (negative exposure compensation).
Technically, you could take note of the exposure measured by the light meter, and then switch to Manual mode and adjust it manually yourself.
Depending on how your digital camera deals with exposure compensation (and the shooting mode used), it may adjust the aperture while maintaining the shutter speed constant; it may adjust the shutter speed while maintaining the aperture constant; or, it may adjust both the aperture and shutter speed. On the digital camera I am using for this tutorial, the camera first adjusts the aperture while maintaining the shutter speed constant; when it can’t adjust the aperture anymore, it then adjusts the shutter speed.
|Camera Metered Exposure|
So, let’s say that the light meter measures a shutter speed of 1/125 sec. and an aperture of F4.0 for a correct exposure.
|+1EV Exposure Compensation|
When I dial in a +1EV exposure compensation, the meter opens up the aperture by 1 f-stop to F2.8, while still maintaining a shutter speed of 1/125 sec. In effect I have purposefully dialed in an overexposure.
|-1EV Exposure Compensation|
If I dial in a -1EV exposure compensation, the meter closes down the aperture by 1 f-stop to F5.6, while maintaining the same 1/125 sec. shutter speed. In effect I have purposefully dialed in an underexposure.
Why would anyone want to dial in an over or under exposure on purpose?
Because there are certain situations where the light meter of your digital camera can be “fooled.”
As an example, say you are shooting a scene where there is an abundance of light around your main subject (for example, at the beach on a sunny day, or surrounded by snow). In this case, using Weighted-Average metering or Multi-Pattern metering, your camera might be ‘deceived’ by the abundance of light and expose for it by closing down the aperture and/or using a faster shutter speed (assuming ISO is constant), with the result that the main subject is under-exposed. By dialing in a positive exposure compensation, you are making sure that your main subject is correctly exposed — though the surroundings would be overexposed.
Another example would be going the other extreme where the surrounding is too dark, and the camera exposes for the lack of light by either opening up the aperture and/or using a slower shutter speed (assuming ISO is constant), then the main subject is over-exposed. By dialing in a negative exposure compensation, you would in fact be under-exposing the surroundings, but properly exposing the main subject.
Positive Exposure Compensation
Dial in a positive exposure compensation when your main subject is surrounded by a very bright background. In the photo below, the camera will be “fooled” by the bright sky and therefore underexpose the tree. To compensate, I dialed in a +1.3EV exposure compensation (i.e. I overexposed by one and one-third stops) to ensure that I retain some detail in the tree.
Negative Exposure Compensation
Dial in a negative exposure compensation when your main subject is too bright compared to the surrounding. In the photo below, the sun was shining right onto the flower pod and, since the pod was so small, the camera took the darker surrounding into consideration and exposed for the background. The main subject was slightly overexposed and lost some detail. To compensate, I dialed in a -0.7EV exposure compensation (i.e. I underexposed by two-thirds of a stop) to ensure that I retain some detail in the pod.
[Yes, I could have switched the metering from Pattern to Spot and probably achieve the same result, but that is for another tutorial.]
In the picture below, the camera exposed for the area under the porch, correctly exposing that area but completely overexposing the area to the left (the structure and sky) to the point of losing all detail.
I could have dialed in a negative exposure compensation to compensate for the overexposure but since I do not know how much negative exposure compensation to dial in, I decide not to guess but simply point the camera to the sky and half-press the shutter release button to lock in the correct exposure for the sky. (Note that the camera locks in both the exposure and focus. A better option is to lock in the exposure only using the AE-L button.) I am now able to get some detail from the structure and sky, and notice that the area under the porch is now darker.
I think the structure on the left still lacks detail, so I dial in a negative exposure compensation, in this case -1.7EV. The structure on the left and the sky gain more detail (though the roof is still blown out in overexposure) while the area under the porch gets darker still. (Though the difference does not show up too clearly in these small prints, it does in the original size pictures.)
Which one of the three pictures is the correct one?
It’s really up to you and what you were trying to achieve.
If your goal was to capture detail under the porch, the first picture would be the correct one. If it were to capture detail in the structure to the left, then the last picture would be the correct one.
High Dynamic Range (HDR) Photography
What if you wanted the best of both worlds: to capture detail in the structure on the left as well as under the porch?
Many digital cameras has a HDR function where the camera will take these three or more exposures for you, then merge them to produce a final image that incorporates the best of all the exposures.
You can also perform this “Photomerge” or “HDR” yourself (and achieve a better result) in an image editing software. I loaded the three shots into Photoshop Elements 12 and used the Photomerge Exposure option to obtain the result below:
When do you use HDR? When your scene includes the high dynamic differences as in the example above. If you expose for the sky (background), your main subject is underexposed; if you expose for the main subject, the sky (background) is overexposed. The best HDR result is achieved when you take a number of shots that expose for all parts of your scene. At a minimum, you take 3 shots, one exposed for the main subject, one exposed for the brightest area and a third exposed for the darkest area.
Bracketing The Exposure
Many photographers will bracket their exposure when taking an important shot by taking one shot using the camera’s measured exposure, a second one at positive exposure compensation, and a third one at negative exposure compensation. This is called "bracketing your exposure" or simply "bracketing" — and is a good technique to use whenever you need to nail a particularly difficult-to-expose scene.
Automatic Exposure Bracketing (AEB)
Instead of manually bracketing and taking the 3 shots as described above, most digital cameras provide Automatic Exposure Bracketing (AEB) where the camera will take the three shots with one press of the shutter release button: one at the camera measured exposure, a second at a negative exposure compensation and a third at a positive exposure compensation. (You can usually specify in your camera’s settings how much under and over exposure you want your shots to be taken at, e.g. by 1/3 EV, 2/3 EV, 1 EV or 2 EV.) You would then load those three shots into your image editing software and perform a HDR photomerge.
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