Understanding exposure

Do you want to get the best out of your dSLR? Then you have to understand the basic concept exposure. It is really not that hard. There are only three variables and they are easily explained.

Imagine you have to fill up a glass of water from a water tap. If you open the tap to its fullest you’ll have the glass filled in no time. However, if you only open the tap a little, so the water is only dripping out, it will take quite some time to fill up the glass. But either way in the end the glass will be full, right? So, the two factors in play here are the time the tap is open and how wide open the tap is. That’s the same with a camera!

The water tap represents the camera shutter and the lens aperture. The shutter speed is the amount of time the shutter is open that is, the amount of time light is collected onto the digital sensor (or film); longer shutter speed equals more light collected. The aperture is the size of the opening of the lens; the larger the aperture the more light is collected.

Lots of water coming from the water tap. This represents a large aperture.

Less water coming from the water tap. This represents a smaller aperture.

Shutter Speed

Shutter speed is expressed in fractions of a second, for example 1/4 of a second or 1/125 of a second. Of course, more light is collected a 1/4 s compared to 1/125 s. A doubling of the shutter speed will double the amount of light collected. This is very intuitive for most people.

Aperture

Aperture is expressed somewhat differently. It is expressed as fractions of the focal length of the lens, for example f/4 or f/32 where f is the focal length of the lens. Fortunately, you do not need to know the focal length, as the aperture is a relative number; both a 24mm lens and a 200mm lens have f/4 and f/32 apertures.

Just like shutter speed, f/4 collects more light than f/32. However, unlike shutter speed a doubling of the aperture will not result in a doubling of the light collected. One only needs to multiply by a factor of 1.4 (actually the square root of 2) in order to get a doubling of the light collected. This is because the aperture is the diameter of a circle and by multiplying the diameter by 1.4 we get twice the area.

The concept of aperture is harder for most people to grasp. But fear not, it is really not hard at all. I’ll show you some more practical examples along the way.

ISO

The third and last variable to exposure is called ISO. In the days of film this was called film speed and expressed in ASA. ISO is expressed as a number, for example 100 or 400. The ISO is a measure of the sensitivity of the digital sensor. In essence it is an amplifier so that the higher the ISO the brighter the image will be. However, this is as digital effect! The ISO setting does not change the amount of the light that is collected on the sensor. The ISO setting will amplify the amount of light. A doubling of the ISO results in a doubling of the brightness of the image.

In our analogy with the water tap, think of ISO as the number of people filling up glasses of water. If two people were filling up glasses of the water with the same settings (time and opening), you would get twice the amount of water. If four people were filling up glasses of water you would get four times the amount of water.

One glass of water. Representing ISO 100.

Twice the number of glasses. Representing ISO 200.

Four times the number of glasses. Representing ISO 400.

Whats next?
Now I think you need a little exercise.