In a camera, there is an exposure meter which can be found on the top lcd or at the viewfinder. This meter indicates the exposure value (EV) of the images. Generally, (+) shows a brighter image and (-) shows a darker image. The number on the meter represents the number-stop increment or decrement. For most cameras, the compensation level ranges from -2.0 EV to +2.0 EV (-2 stops to +2 stops). Some cameras do have wider compensation, e.g. from -5.0 EV to +5.0 EV. So, the EV increases by 1/3 stop, or 1/2 stop. According the picture below, the increment goes like this;
-2.0, -1.7, -1.3, -1.0, -0.7, -0.3, 0.0, +0.3, +0.7, +1.0, +1.3, +1.7, +2.0
Therefore, any adjustment you make to the ISO, shutter speed and aperture will affect the EV. Let’s say the current setting is 1/125 sec at f4.0 gives 0.0 EV. Changing the shutter speed to 1/60 sec (aperture remains constant) means light is reduced by 1 stop thus giving +1.0 EV (image gets brighter).
Why exposure compensation?
Exposure compensation is necessary because 0.0 EV does not mean the image is correctly exposed. So, this is not the value that you should always aim.
Light meters inside the cameras are reflected light meters, and are calibrated to calculate correct exposures for subjects with 18% light reflectance (middle gray). So, subjects that are black or white or some other colour that deviates greatly from 18% reflectance can prevent the camera from calculating the correct exposure.
For an example, if the frame is filled with a dark subject (low reflectance), the camera think the scene is too dark and to achieve the same reflectance as middle gray, the camera will expose the sensor for longer than it should. Therefore, at 0.0 EV, the result is an overexposed image. To fix this, you must reduce the exposure level by applying minus compensation. On the other hand, if the frame is filled with a white subject (high reflectance), the camera think the scene is too bright and to achieve the same reflectance as middle gray, the camera will expose the sensor for shorter than it should. Therefore, at 0.0 EV, the result is an underexposed image. To fix this, you must increase the exposure level by applying plus compensation. So, use plus compensation when the scene has lots of bright colours and minus compensation when the scene has lots of dark colours. In other words, “minus to darken, plus to lighten“.
Well, exposure compensation is applied to make sure you get the correct exposure. And there are various ways to ensure you get the kind of exposure that you want. So, lets know them.
1. Know the lighting condition
If you know the lighting condition (light intensity), you can apply compensation accordingly. This is particularly evident when shooting against bright background, frequently in sunset or backlight situation (the lighting source is in the frame).
A backlit scene is difficult to approach because the brightness difference in the scene exceeds the digital camera’s exposure range, which is generally around 5.0 EV. If no compensation is done, the subjects will appear too dark. Therefore, applying plus compensation will properly expose the subject. Well, if you wish the subject to appear as silhouette, then no compensation is required.
So, like I previously mentioned, use plus compensation when the scene is bright and use minus compensation when the scene is dark. Anyway, through experience, you will know when you meet certain conditions, you know how much compensation you need to apply.
2. Use Auto Exposure Bracketing (AEB)
Sometimes, you are in a situation when you are absolutely not sure that minus or plus compensation is the correct adjustment. Or in a quickly shifting scenes, the time is not enough to manually adjust exposure compensation.
So, instead of manually adjusting exposure compensation before each shot, AEB is a nice feature to use. It enables the camera to take 3 consecutive images, each with different exposure. So, what do you need to do is set up the amount of exposure compensation prior to shooting, press the shutter button 3 times and let camera do the rest. Tadaa!
For and example, if you set the AEB to 1.0 EV, then the first shot is standard exposure, the second is compensated by -1.0 EV, and the third shot is compensated by +1.0 EV. Then, you can pick your favourite exposure later. That’s easy and you save time!
In addition, to make AEB even easier to use, set the camera’s drive mode to continuous shooting. So, instead of pressing the shutter button 3 times, you only need to press it once for all 3 shots to be taken.
3. Try compensating at 0.7 EV increments
One of the digital photography’s benefit is that you can shoot as many pictures you want at different exposure levels. Then, you can delete the ones that are bad. In terms of how widely you should spread out the exposures, 3 shots at 0.3 EV increment may not show much different, whereas 3 shots at 1.0 EV increment might be too different.
So, start practicing exposure compensation with +/- 0.7 increment. There is a high chance that you will get the correct exposure.
4. Check histogram
Viewing the image on the LCD screen can sometimes be unreliable especially outdoors in bright light when the light reflecting from the LCD can prove deceptive. You might think the image is okay, but when you get back home and recheck the image, it is underexposed. Damn! So, using the histogram is a far more reliable method to assess your images.
Histogram is a visual representation of an image’s tonal range. The horizontal axis indicates the picture extent from pure black (far left) to pure white (far right). The vertical axis shows how many pixels have that particular value. So, from histogram, you may determine whether the image is made up of predominantly light, dark or mid-tones.
When assessing a histogram, it is important to consider the brightness of the subject itself. So, you need to know what kind of histogram that you expect. Since it is hard to tell how a good histogram looks like, it is best to avoid histogram that is too skewed to the left (underexposed) or too skewed to the right (overexposed).
In conclusion, it is worth to memorize this rule “minus to darken, plus to lighten”. Since this is digital photography, you can shoot a lot of images with different exposures so you can get familiar with exposure compensation. And of course, do enjoy photography!