An aperture simply refers to an opening. It is how wide an opening is. In this case it is going to be lenses.
The first rule of apertures is that the wider the opening, the more light will be coming into the camera.
The second rule builds upon this idea in that the more light coming into the camera, the less time is required for a proper exposure.
The third rule has to do with something called depth of field and a depth of field is a wall of focus. That is probably the best way to think of it, it is a wall of focus which runs parallel to the camera sensor. So, whatever the camera is looking at, there is this parallel wall and this wall usually is going to exist around the place where you are focusing.
The interesting thing about aperture is the thickness of this wall depends on the size of the aperture. And the rule is this, the whiter the aperture, the thinner your depth of filed or the thinner this wall will focus.
Now, the smaller your aperture, the thicker the wall, one way we can measure this is to take a ruler, lay it flat on the table pointing at us and to take a picture of it and at a very wide aperture we will see that only a few of those stick marks are in focus and at a very, very small aperture, many of those stick marks are in focus.
So coming back to this third rule, the whiter the aperture the thinner your depth of field, the smaller your aperture the thicker your depth of field.
The fourth and final rule has to do with how apertures are measured. Apertures are measure in something called F-stops. Now, this term is actually a ratio of the focal length of the lens divided by the diameter of the opening of the shutter blades. Say for example you have a lens with a focal length of 20 millimeters. And the opening of the iris with the shutter blade stop down is two millimeters. Well, 20 millimeters divided by two millimeters is 10.0 or an aperture of 10.0.
Now, something that is really confusing when students are learning about apertures is that the numbers are counterintuitive. The smaller the number that you have, the larger the opening and the higher the F-stop number, the smaller the opening.
So, those are the four rules of apertures and we will talk a little bit more about those as we go on.