Bryan Peterson has written a book titled Understanding Exposure which is worth a read if you’re wanting to venture out of the Auto mode on your digital camera and experiment with its manual settings.
In it Bryan illustrates the three main elements that need to be considered when playing around with exposure by calling them ‘the exposure triangle’.
Each of the three aspects of the triangle relate to light and how it enters and interacts with the camera.
The three elements are:
ISO - the measure of a digital camera sensor’s sensitivity to light
Aperture - the size of the opening in the lens when a picture is taken
Shutter Speed - the amount of time that the shutter is open
It is at the intersection of these three elements that an image’s exposure is worked out.
Most importantly - a change in one of the elements will impact the others. This means that you can never really isolate just one of the elements alone but always need to have the others in the back of your mind.
Imagine your camera is like a window with shutters that open and close.
Aperture is the size of the window. If it’s bigger more light gets through and the room is brighter.
Shutter Speed is the amount of time that the shutters of the window are open. The longer you leave them open the more that comes in.
Now imagine that you’re inside the room and are wearing sunglasses (hopefully this isn’t too much of a stretch). Your eyes become desensitized to the light that comes in (it’s like a low ISO).
There are a number of ways of increasing the amount of light in the room (or at least how much it seems that there is. You could increase the time that the shutters are open (decrease shutter speed), you could increase the size of the window (increase aperture) or you could take off your sunglasses (make the ISO larger).