Cameras usually measure light to render the scene in a way that everything or almost everything will be lit and properly exposed. The way they measure light is different than it is done professionally with the use of hand held light meters (This will be discussed in another article).
They take their information from the light that passes through the lens, and based on that, they calculate the proper exposure settings in any mode except Manual; and that is because in Manual you control the exposure values, therefore the light metering is up to you.
However, sometimes you will point the camera at scenes that will contain elements that will throw off the camera and will expose incorrectly.
The camera's in built light meter measures the light in a scene based on the light reflected from the subjects in it.
However, there are colors that absorb light, that bounce it off or absorb a part and reflect another.
The camera's metering system is based on the notion that almost all colors reflect roughly the same amount of light and it calculates exposure based on that.
However, it may have happened to you that you pointed your camera at a person with white clothes with plenty of light around it and the camera underexposed the image OR you pointed the camera at something mostly black or made up of a dark color and it overexposed the image. Or viceversa.
In the first case, there isnt enough light in the shot and in the second theres too much light and the colors look washed out.
At this point you either go postal on the camera, switch to Manual or fix the photo in post processing.
All of these options are valid (except go postal on the camera, maybe) but sometimes there isnt enough time to switch to Manual and retake the shot or you dont like sitting in front of a computer for hours fixing a shot that should have come up properly the first time.
Don't panic though, there is a way to fix it right away.
That way is called Exposure Compensation.
By exposure its meant the amount of light that will reach the sensor when you press the shutter.
Compensation in this case means to increase or decrease a value or values in order to properly expose a picture.
The values that directly affect exposure are Shutter Speed, Aperture and ISO.
So, if you are in any mode (except Manual) and you point your camera at a subject with bright colors (such as white) or dark colors (like black), the camera will think the following:
Bright colors: There is too much light in the picture, so I must select a fast shutter speed, a small aperture or low ISO in order to avoid overexposing the shot.
Dark colors: There isnt enough light in the picture, so I must select a slow shutter speed, big aperture or high ISO in order to avoid underexposing the shot.
The problem is, under this logic, the camera will underexpose in bright colors or overexpose in dark colors most of the time. It may actually not do this or it may happen that under bright colors you actually need to overexpose a bit and in dark underexpose a bit.
It all depends on the subject, amount of light available and the exposure values.
Exposure Compensation is a function that allows you to shift the exposure values and tell the camera to over or underexpose as you consider necessary.
In all Alpha DSLRs (and even in the cameras made by other manufacturers) the Exposure Compensation is represented by a button with these symbols: +/-. Usually the button is split in two; black on the plus side and white on the minus side.
In the Alpha DSLRs, this button is found at the back of the camera near the viewfinder (A100/200/300/350) or near the shutter button (A700/900).
Exposure compensation scale is based on a scale of exposure values (EV), the plus side illuminates the image and the minus side darkens the image.
When you press this button, either you will get a exclusive screen for you to move the cursor up or down the scale or the scale will light up if you use the Quick Navi screen.
As with any scale, there is a middle ground, in this case is 0, which is always the exposure recommended by the camera.
In the LCD screen looks like this:
-3 I I -2 I I -1 I I 0 I I 1 I I 2 I I 3+
In the viewfinder it will look like this:
-2 o o -1 o o 0 o o 1 o o 2+
The A700/900 go to -3 or +3, the A100/200/300/350 go from -2 to +2.
If you own a A700/900 don't worry if the scale in the viewfinder wont go to -3 or +3, if you compensate beyond -2 or +2, the camera will compensate to the value you select. The value will be displayed in the viewfinder as you select it, it will disappear once you press the shutter button, but the camera will compensate to the value you select.
Now, you may wonder why the LCD and viewfinder have I or o's between the numbers. Those are the EV steps.
"And what are those?" you may ask.
As I said before, the scale used to compensate its called EV scale, and the units to measure light in that scale are called steps.
As you compensate in either side of the scale, the shutter speed, aperture or ISO values change, it may be just one, two or the three of them.
The point is, whenever one of these values changes, the exposure changes.
The Alpha DSLRs have two ways of measuring in the EV scale: in 0.3 or 0.5 steps.
The A100/200/300/350 have their scales in 0.3 EV steps and the A700/900 can deal with 0.3 or 0.5 EV steps.
The I's in the LCD between the numbers or the o's in the viewfinder between numbers are the markers when you measure in 0.3 EV steps, but when you measure in 0.5 EV steps, the marker is placed between two I's or two o's.
0.3, 0.5, 0.7
So what's the difference between 0.3 and 0.5 EV steps?
The basic difference is the amount of steps you can compensate.
In 0.3 steps, the scale goes like this:
-3, -2.7, -2.3, -2.0, -1.7, -1.3, -1. -0.7, -0.3, 0, +0.3, +0.7, +1.0, +1.3, +1.7, +2.0. +2.3, +2.7, +3
In 0.5 EV steps, the scale goes like this:
-3, -2.5, -2.0, -1.5, -1.0, -0.5, 0, +0.5, +1.0, +1.5, +2.0, +2.5, +3.0
When you use 0.3 EV steps, the scale is larger because you can access 0.3 and 0.7 steps of compensation between numbers.
When you use 0.5 EV steps, you can only access halves between each number. The scale is smaller.
Differences between 0.3 and 0.5 EV steps
Now, if you use a A100/200/300/350, you dont have to worry about 0.5 EV steps because you can't access them, but if you want to know the difference between 0.3 and 0.5 scales, keep reading.
For those of you using a A700/900, there are differences when you use 0.3 or 0.5 EV step scales.
The use of both scales presents tradeoffs. Using a 0.3 EV step scale allows you to use a bigger scale whereas 0.5 is more limited.
The difference is that if you need precise increments, 0.5 EV works a lot better than 0.3 EV, because 0.3 EV presents you subtle changes that wont affect too much the exposure in case you screw up, but it will require you to use more steps to make a visible compensation. And thats another tradeoff, if you want subtle changes without introducing a harsh change in exposure, 0.5 EV wont work for you since that's what it will do, the changes will be more visible, so if you want just a slight touch of more light into the picture, 0.5 EV won't do the job since it will introduce more light in each step than 0.3 EV.
In 0.3 EV, the lack of a middle point between steps is compensated by the fact that you can use 0.7 EV as well, and that step may introduce enough light to make a difference or a little too much. It all depends on the scene.
In 0.5 EV the changes are more predictable, so if you require a visible change and you know how much, 0.5 EV will work better than 0.3 EV since it won't require you to decide whether to use a 0.3 or 0.7 step compensation, it will be just a half and you can learn what to expect. However, if you rather make exposures where the light is increased or decreased in a more gradual manner, chose 0.3 EV.
There is one more thing to keep in mind: the scale you chose for Exposure Compensation also affects the shutter speed and aperture range. If you chose 0.3 EV, the shutter speed and aperture will change in values of 1/3rd whereas if you chose 0.5 EV, the values will change in halves.
This is done in order to match the changes introduced in 0.3 or 0.5 EV steps.
And again, there is a tradeoff, having the speed or aperture shown in 1/3rd gives you a broader range of speed and aperture (limited by your camera's top speed or lens' aperture range) but it will take longer for you to get to the setting you want, which wont happen if you use 0.5 EV.
It's up to you which scale you use; 0.3 or 0.5 EV.
To change between those two in the A700: Menu-> Recording Menu (camera icon) Page 1-> Exposure step-> 0.3 or 0.5 EV
In the A900: Menu-> Recording Menu (camera icon) Page 2-> Exposure Step-> 0.3 or 0.5 EV
Notes of use and Closure
- The primary function of Exposure compensation is to illuminate or darken an image when the camera doesn't expose correctly because its pointed at a subject or scene mostly made of white or black.
- Exposure Compensation is useful, especially since Auto Mode tends to underexpose sometimes.
- It must be used with caution since it can ruin a photo if not used properly. Use only as much as you need to get the shot you want, if you over do it in any part, you may do a worse mess than the camera's.
- For pictures that require a drastic and precise compensation, use 0.5 EV, for pictures that require subtle and gradual changes use 0.3 EV.
- It doesn't work in Manual Mode, but will work in the rest of the modes, even Scene Modes.
- In Manual Mode, the Exposure Compensation scale changes to M.M.; which means Metered Manually, the camera still offers its opinion about the proper exposure to chose but the final choice its up to you and you can ignore the camera's recommendation.
- There is more range in compensation in 0.3 EV than in 0.5 EV, but in 0.5 EV the changes are more visible in each step.
- For some weird reason, in the A700 happened that when using Exposure Compensation; the exposure values DIDN'T change BUT the illumination in the picture DID change a bit.
Exposure Compensation is a useful tool when you're shooting in P,A,S or any Scene Mode (except in the A900, it doesn't have Scene Modes) and the shots aren't coming properly exposed. It allows you to do a quick tuning and let the camera do the rest, you just tell it how bright or dark you want the picture to be regardless of what it thinks.
This function will save you time in front of your computer and will give you more time to take more pictures.
One final advice: I strongly recommend you conduct experiments of your own with this function so you learn the changes it introduces in a picture and how does the camera behave when you use it, this way you will learn to know what to expect from this function and will give you a solid idea of how much to use and when to use it.