- f/1.0 (exotic, expensive)
- f/1.8 (very fast, but affordable)
- f/2.8 (fast for zoom lenses)
The best way I have heard this explained is as follows: The smaller the number, (f/1.0 for example) the smaller the number of things in focus. The higher the number, the higher the number of things in focus. (f/11.0 for example)
The reason it is confusing is because you forget it's a fraction. But that's why there is always "f/" in front of the numbers. Those numbers, f/1.0, f/1.8, f/2.8, those are denominators. And as we learned in elementary school, the BIGGER the denominator, the SMALLER the number's real value... (1/4 is smaller than 1/2, etc... So f/4 is smaller than f/2...)
(WARNING: The next paragraph is the really complicated part; skip it if your brain hurts already!)
Okay if you want to know EVERYTHING, the F in "f/1.8" stands for focal length. On this lens for example my focal length is 50mm. That was about standard for 35mm film, standard meaning it's about the same magnification as our human eyes. So if I set that f/1.8 lens to f/2, for example, 50mm/2 equals 25mm. This means 25mm is the diameter of that apeture opening. If we changed the apeture all the way to f/22, 50mm/22 equals about 2.2mm. That's one tiny opening! (See the picture below.)
Advantages to a "FAST" apeture:
1.) The bigger the opening in the lens, the more light comes in, giving you a brighter exposure. This either allows you to shoot in dimmer light, or shoot in normal light with a really fast shutter speed.
2.) The bigger the opening of the apeture, the more stuff will blur in the background and foreground. A blurry background is one of the key parts of almost every professional portrait. It's called "background separation."