You already know that you need light to take a picture and that too little or too much light will spoil that picture, so how do you adjust the amount of light that your camera takes in, in order to capture your chosen moment?
The first thing to look at is shutter speed. This is the amount of time that a light blocking cover is moved out of the way to allow light to the sensor. Taking a hobby level digital single lens reflex camera as an example the shutter speed is adjustable from 1/4000th of a second up to 30 seconds. Obviously, the longer the shutter is open, the more light will be let in. If the camera is hand held and the sensor is open for 1/30th second or longer there is a real risk of not being able to hold the camera steady enough and a blurred image will be the result. This is known as camera shake. If you need longer time exposures, a tripod or other stabilising device will be required.
The second factor in the light stakes is aperture. The aperture is a mechanical device that forces the image to be seen through a larger or smaller hole depending on how it is set. Aperture is measured in ‘f’ stops and confusingly the smaller the f stop number, the larger the hole and consequently, the more light that is let through. Another function of aperture is ‘depth of field’. In other words how much of an area in front of and behind the focus point, is actually in focus.
If you are taking landscape pictures then your focus point is (for all intents and purposes) infinity and you want everything between you and the far horizon to be in focus, so you would want to use a smaller aperture, that is – a higher f stop. If however you want to photograph a bird sitting on a fence post, then the background scenery would just be a distraction, so selecting a wider aperture, a lower f stop, would give you what you wanted.
Widening the aperture means that you are letting more light in, so you can also select a faster shutter speed to get the ideal exposure, but what if you have a situation where you need a larger depth of field and a fast shutter speed – say at a car race where there is some distance between the first and last vehicles, but you want to picture them all together? You will need a fast shutter speed and wider aperture, but you still need to let in enough light. It is then we come to the third factor of the light triangle – film sensitivity or ISO.
In this digital age it seems strange to talk about film sensitivity, but it is the easiest way to explain the concept. In film days, the emulsion coating the film could be made more or less light sensitive and each level of sensitivity was given an ISO number. Low numbers were less sensitive to light but had a much finer grain size giving higher quality pictures. Higher numbered film was more useful for fast action photography but usually gave a more grainy result. The same is true of modern digital cameras but instead of having an ISO range of 25 to 1600 as in the days of film, the ISO range of digital cameras can be from 100 to 32000 making night time shots easily possible.