02 EV Compensation
Even though your DSLR appears to have a highly sophisticated electronic brain for calculating exposures, it has a crucial weakness compared to the brain in your own head. It’s incapable of understanding what an object is and, therefore, whether it’s inherently dark or light.
That means that if you want snow to come out looking white and black cats to look black, you have to override your camera’s automatic exposure.
Snow scenes, for example, require extra exposure, or positive EV (Exposure Value) Compensation. This may seem paradoxical; after all, if snow is so bright, shouldn’t you be reducing the exposure, not increasing it?
It’s important to remember that the camera’s automatic exposure system has already reduced the exposure. And anyway, that’s not the point. Snow is supposed to look white and not the even, mid-grey that exposure systems will try to produce.
What the camera has done is to compensate for something it shouldn’t have. You have to undo its mistake by increasing the exposure again.
The same thing happens in reverse with dark or black subjects. The camera measures lower than usual light levels, so it increases the exposure to compensate. But it shouldn’t have done – what you’re photographing is supposed to be dark!
But exactly how much EV Compensation should you apply? Here are some suggested starting points, although each subject will be different so be prepared to make slight adjustments as required.
For snow scenes, try +1EV. This should make the snow a cleaner white, without too much risk of over-exposure and burn-out. Black subjects such as steam trains might need about -1EV compensation, but because they’re rarely going to be shot against a black background, -0.7EV might give you better results.
Applying EV compensation
To apply EV Compensation, press the EV Compensation button. On most cameras this can be found on the top or the back of the body. While the button is pressed, turn the control dial to add or subtract from the camera’s exposure value.
By default, cameras apply these corrections in steps of 0.3EV (one third of a stop), though you can usually change this to 0.5EV increments.
The camera displays an exposure bar on the LCD and/or in the viewfinder, with the measured exposure in the centre and index markers either side corresponding to different levels of EV Compensation.
Some models with Live View can also display a real-time histogram, which helps you get the correct exposure. Once you’ve set an EV Compensation value, it remains in force indefinitely, so don’t forget to set it back to zero when you’ve finished.
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