Copyright Michael Karbo, Denmark, Europe.
Chapter 19. Exposure compensations
If you are not satisfied with your automatic light metering, then you can make a compensation. There are usually two kinds of compensation in most cameras: Exposure compensation and flash compensation.
Both methods of compensation work quite simply. The function is activated either with a button or in the menu system. You select the number of diaphragm steps that the exposure is to be underexposed of overexposed with. Compensation can be made in steps of one-third diaphragm – typically on a scale from plus/minus 1/3 to 2 whole diaphragm steps.
The reason for compensation is that you are able to compensate for automatic light metering. If you think that the exposures are generally a little too dark then you can, for example, compensate with a +2/3 diaphragm step. The exposure will then be overexposed with what corresponds to 2/3 diaphragm.
Exposure compensation is often used to take a lot of shots of the same subject. The lighting can be altered a little every time an exposure is taken and finally the best version can be selected. This way of working is called bracketing, and it is built in as an automatic function in most cameras.
It can be difficult to take a good image of snow. The automatic light meter will almost always make the exposures too dark so that the snow looks grey, which doesn’t look good at all.
So it is a good idea to overexpose the image with about 1 to 1½ diaphragm. This is easily done with the function exposure compensation.
Figur 75. The snow landscape is overexposed in relation to the automatic light metering. The three small figures at the top display the LCD screen’s information about three snow images, where the two last are overexposed with the use of compensation.
As we have seen, diaphragm steps enable you to halve or double the amount of light for every step. We can, for example, choose to compensate for the camera’s light metering with +2/3 diaphragm steps. We want to overexpose the image a little bit compared with the camera’s calculations. It is, however, not necessary to change the diaphragm itself.
It is just as often the shutter speed, which is increased or decreased for the required compensation. If we want to compensation with a whole diaphragm’s overexposure, then we can do it by halving the shutter speed. It can also be done by doubling the ISO values (sensitivity).
With exposure compensation and the function bracketing (described later in the book) the exposure deviations are calculated in values, which correspond to diaphragm steps. They are called Exposure Values. A whole EV corresponds to either a halving or a doubling of the luminosity – i.e. the same value as a whole diaphragm step. EV values are generally used to describe exposure alterations.
An alteration +1 EV means that the amount of light is doubled, corresponding to the diaphragm being increased by one step. In practice, however, the diaphragm, the shutter and the ISO values can all be altered to compensate the exposure:
Figur 76. Examples of exposures, where adjusting the diaphragm and the light sensitivity has changed EV values.
Figur 77. This exposure is really meant to have a shutter speed of 1/80 second. So to compensate the exposure, the shutter speed has been altered.