Reducing the overall brightness of the photograph being captured is what exposure compensation is all about. Apart from this, the exposure compensation can also come in handy for a lot of other interesting techniques as well.
(A dry river-bed. For scenes such as these, built in light-meter of the camera can’t be blindly trusted)
Before moving on to the exposure compensation, it is important to understand the term Exposure Value or simply EV. Exposure value is a number that represents all the available combinations of a camera’s shutter speed and f-top (aperture), in such a manner so that all available combinations yield the same exposure, if the light conditions remain the same.
If an exposure of 1/500 sec at f/11 aperture gives the correct brightness of the image, then letting the shutter to open for even shorter duration will have to be compensated by opening the aperture to let in more light. So a faster shutter speed of 1/1000 at a larger aperture opening of f/8 will give the same brightness of the image. These both the combinations of shutter speed and aperture fall under the exposure value of 16. (For understanding shutter speed and aperture, please see this article first – Basics of Exposure)
Keeping the aperture same as f/11 but increasing the shutter speed in the above example to 1/1000 will reduce the amount of light entering the camera, thereby producing a darker image.
(Table to give an idea about various shutter-speed and aperture combinations available for an Exposure Value.)
This is the method where Exposure Value to be captured by the camera is increased or decreased. All modern day cameras have built in light meters (Metering Modes), that tell the camera about the correct exposure required to capture the scene in front. The calculation done by these meters are not always correct. With the facility of preview screens and histograms (Understanding Histograms), I highly recommend checking the exposure to confirm if everything is fine. Exposure compensation can help when the camera calculations are not correct and the histogram depicts wrong exposure. Sometimes even when these exposure calculations by the camera are correct, the photographer might be interested in brightening or darkening the overall image. In such scenarios, the camera needs to be told to change the exposure. This is done by setting the exposure compensation. Minus settings, reduce the brightness of the image and plus settings increase the brightness of the image.
The exposure compensation is given as a plus or minus number which represents the increase or decrease in the exposure value. A ‘-1’ exposure compensation is effectively a reduction of one exposure value. Many cameras directly mention this as -1 EV. Easier to understand! Cameras either have a button for this with a logo showing a ‘+/-‘ symbol or sometimes it can be found in the menu.
The good news is that this feature (exposure compensation) is present in even the simplest of compact cameras too. A range starting from -2 EV and going upto +2 EV is common in compact cameras. ‘0’ stands for no exposure compensation.
(Trishul and Nanda Devi Peaks – An exposure compensation of -2 EV was required to prevent the snow covered peaks from getting over-exposed)
Camera Modes and Exposure Compensation
The way exposure compensation changes the overall exposure value differs from camera mode to camera mode.
In Aperture Priority, the user sets the aperture and camera calculates the shutterspeed. When exposure compensation is dialed in, the shutter speed is calculated by considering the exposure compensation too. The aperture remains what the user sets but the shutterspeed changes depending on the other settings including exposure compensation. The photograph above was captured in aperture priority. The initial calculated shutter speed for an aperture setting of f/8 was 1/500 sec. Dialing in -2EV kept the aperture same but pushed the shutter speed to 1/2000.
In Shutter Priority, the shutter speed is set by the user. The camera calculates the aperture. When exposure compensation is dialed in, the aperture calculation is done accordingly. The change in exposure is brought about by changing the aperture in this case.
Now comes the interesting scenario. In Manual Mode, both the shutter speed and aperture is set manually. When exposure compensation is dialed in, it does not automatically change any of these. Instead it gives a meter reading according to the exposure compensation. It is therefore upto the photographer to use that reading or not. I do not recommend using exposure compensation setting while in manual mode. It is easier to simply change the aperture or shutterspeed while keeping an eye on the amount of over or under-exposure shown in the meter.
In Auto or Program mode, the pair of shutter speed and aperture are determined by the camera. On dialing in the exposure compensation, the camera shifts the pair accordingly so that the pair falls in the suitable exposure value as per the compensation.
Flash and Exposure Compensation
Flash is also a part of the camera calculations when the exposure compensation is used. The dialled in exposure compensation also affects the flash output.
There is however a dedicated compensation for flash which is aptly called ‘flash compensation’. This also moves in the same manner as exposure compensation but affects only the flash output. The logo for this resembles the exposure compensation but with a small ‘flash’ icon too along with the ‘+/-‘ logo.
When can this be used? As I always say, the limits are there in our imagination. One simple experiment to demonstrate this effect can be a photograph of a subject located close-by, maybe a flower?
Capturing everything the way camera calculates will give a good photograph of the flower. Dialing in the exposure compensation can reduce or increase the overall brightness of the image depending on the setting. Now let us set the exposure compensation to -1 EV and flash compensation to +1 EV. The exposure compensation will darken the overall scene by 1 f-stop or exposure value. The flash compensation on the other hand will brighten the flash output to give a 1 stop overexposure. The flash power diminishes quickly as the distance from the flash increases. The flash compensation will nullify the effect of exposure compensation on the flower. So, the flower now stands out in front of the darkened background.
Flash compensation is usually used to balance the fill-light or when a balance between the ambient light and the flash is required. For most other scenarios, exposure compensation should be used.
(This is a basic article. Flash compensation is actually extremely useful in studio setups and while balancing multiple flash units. I will write about this in future.)