OK, so in the first part of this series I discussed focal length. In this part I'm going to discuss aperture and what those numbers mean. I was originally going to discuss how aperture effects depth of field but I'm going to leave depth of field as the subject of it's own part.
Before I get into what the aperture numbers mean and what they represent I first want to go over a little preliminary information. When we focus on a subject, in my example below it's the lady bug, light from a single point on the subject goes through every part of the lens. Depending on the angle the light enters the lens and leaves the lens it gets bent a certain amount. When the lens is moved closer to the film/sensor or farther away it brings different parts of the scene into focus. As you can see from my example light reflected by the tip of the antenna (yellow lines) moves through the lens and converges back into focus on the film. Similarly light reflected by the tip of the leg (green lines) moves through every part of the lens and is brought into focus on the film.
The light reflected off the subject, moving through the lens, and then hitting the film can be generalized as in this picture.
Now, what if we put a piece of metal with a hole drilled in it behind the lens. The hole would be smaller than the lens. This would block the light that tried to pass through the outside portions of the lens but let the light that was moving through the middle part of the lens through. Well, that's what an aperture is. It's a hole that you can make larger or smaller (it consists of overlapping blades). When you make the hole smaller it lets less light pass through. If you half the light getting through the aperture you must double the time the film is exposed to the light by the shutter being open in oder to get the same exposure. How do you determine how to half the light? Well, you cut in half the area of the hole made by the aperture!
If you don't quite follow don't worry, I'll use some pictures to help explain. But first I want to give a definition of f stop. f stop is what photographers commonly call aperture - you know, f2.8, f4, f8, etc. f stop is nothing more than the focal length of the lens divided by the diameter of the aperture. The smaller the diameter the larger the f stop number becomes.
Here is an example of a 100mm lens with a 50mm aperture diameter. Remember, the area of a circle is pi times the square of the radius:
Notice what happens when we decrease the aperture diameter to 35.36mm - our f stop goes from 2 to 2.8 but the area of the hole made by the aperture goes from 1963 square millimeters to half of that, 982 square millimeters.
Every "stop" or f stop lets through exactly half the light as the previous f stop. The standard progression for f stops is 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32. Since you are halving the light as you "stop down" or change the f stop from say f4 to f5.6 you need to double the time the shutter is left open. That's why shutter speeds at large apertures (or low f stop numbers) enable you to use fast shutter speeds and small apertures (or high f stop numbers) force you to use longer shutter speeds. But, you may ask, where did this formula for f stop come from - you know, the focal length divided by the diameter of the aperture? Well, without going into the whys of it, the ratio of the focal length and the diameter of the aperture determines how much light leaves a lens regardless of the actual value of focal length or aperture. In other words a 100mm lens and a 50mm aperture diameter is f2 and it lets the exact same amount of light through as a 50mm lens with an aperture diameter of 25mm. If you use a light meter to set your exposure then the meter can tell use to use 1/60 of a second shutter speed and f4 regardless of what focal length lens you use because f4 on all lenses lets in the exact same amount of light!
In my next installment of this series I'll explain depth of field and how the f stop, focal length, and distance to the subject all effect it.
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