Why is exposure so important?

It is simply because a wrong exposure will leave you a picture which is to light or too dark. That is probably not what you want – in most cases. There are however situations where the photographer wants to achieve special effects and moods. High key photos are one of these exceptions. In high key the light parts of the photo is overexposed giving an effect by purpose where the photo look overexposed. This is often used in portraits.

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Automatic or manual control
Modern digital cameras have automatic control which measures the light and ensure optimal exposed sensor. Actually you don’t need to know anything about it. OK, that is only partly true. As long the light conditions are good and the motive is straight forward the cameras automatic control manages this very well.

If your camera has possibilities of manual settings of aperture and shutter speed, you can influence on how the photo should be exposed – even in difficult light conditions. In difficult lighting conditions, the cameras auto settings might not be working well.

EV - what is it?
The combination of aperture, shutter and the sensors sensitivity (ISO) is what we call exposure value (EV). Theoretically the EV is defined as 0 (zero) at an aperture of f1, a shutter speed of 1s and ISO 100.

If you double the amount of light on the sensor by reducing the aperture f-value one step down (bigger lens opening) or reducing the shutter one step down (longer time), the EV will increase with 1.

Let us say your cameras metering for a certain photo is set to a shutter of 1/60 and aperture f5.6 at ISO 100. If you double the shutter speed (from 1/60 to 1/125) and keep the aperture and ISO the same, the sensor will be exposed for half the amount of light. The picture will be “under exposed” – too dark. To get a correct exposed picture in this example, the aperture can be lowered from f5.6 to f4 or the ISO increased to 200.

ISO only in bad light conditions
While the aperture and shutter decides how much light is let into the sensor, the ISO adjust the sensors sensitivity to the light. Increasing the sensitivity of the sensor is made electronically by amplifying the signal. This process might introduce digital noise.

Different aperture and shutter settings can be used for creative effects. Increasing the ISO is just like a last solution if the light conditions are very dark. Remember higher ISO introduces more noise to the photo, so try keeping it as low as possible


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