The aperture is the opening in the lens diaphragm, which controls the amount of light that passes through the lens and on the sensor. Funny fact: it's actually based on the human eye. The iris to be more specific. You noticed for example how the iris gets tiny when there is a bright light? Take a flash light and get it close to your eyes in a mirror. See how your iris gets smaller. That's because the eye knows there's too much light, so it reduces the amount of it. In this way, you can still see around, rather than everything being white. In a darker room, the iris opens and gets bigger to allow more light to come in, so we can see the couch. 

The aperture openings are, as you might have noticed, represented by some numbers: 1, 1.4, 2, 2.8, 4, 5.6........32

We will try now to understand what exactly those numbers are trying to tell us.

The largest aperture is 1. largest because it is the max opening. It means the diaphragm is open at its widest. Until now, it's easy. After 1, it gets confusing for a lot of people. But no worries. It's not that complicated, trust me.

Ok..now that we have the largest opening which is 1, the next step is called a Full Stop, which will basically reduce the amount of light in half. Unfortunately for us, half of the largest aperture 1, is not 1*2. To understand this, you must know that cutting the amount of light in half, means reducing the diaphragm diameter with a size which will let in only half of the light in. To spare you the mathematics, each reduction has a number attached to it, so that everyone can easily determine how much light they need and want. Those numbers are not random and even if they have a lot of math behind them, there is a simple way to follow them.

1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32. 

This row of numbers is determined using a simple formula: sqrt(2) * x

If we have 1, the next would be: sqrt(2)*1 = 1.4

the next one  sqrt(2)*1.4 = 2 (rounded up from 1.96) 

the next one sqrt(2)*2= 4

the next one sqrt(2)*4=5.6....I can go on like this until the last one, but I am pretty much sure that you already got it.

There it is..simple. each of these numbers represent a full stop (F-stop). so If you have an aperture of 2.8 and I tell you to reduce it with 2 full stops, you know that this means: exactly, you are right. 5.6. Fours stops would be 11.

Told you it's not that hard.

Why you should understand the aperture

The aperture is important because it helps you determine the exposure. It's part of the triangle: Aperture, Shutter Speed, ISO. For example if you have a shutter speed of 1/200 and f/2.8 and ISO 200 and you realize that the amount of light is too much and you want to reduce it with a full stop, you know that you can use f4 and the exposure is adjusted. Once you start using your camera often, you'll get used to the amount of light you might need in certain situations and rather than go through all apertures to see which exposure works for you, you can just set the closest to your needs from the beginning.

Don't get confused

It might get confusing because the smallest the number, the larger the aperture is. But think of it like this: if you have an apple and you divide it by 1, you'll still have the entire apple. If you divide it by 2, you only get half of it. It gets smaller. It's the same with the Aperture. You just take the opening and divide it until it get really tiny. The bigger the number you divide it with, the smaller it gets.

riginal post here