People feel very different about using the F word. Some use it in almost every sentence, some cover up the ears of their kids when someone starts pronouncing the F word.
Aperture
In the field of photography, the F word is no curse. You can freely say F8 or F11 without offending anyone. It is simply a setting on your camera. The setting controls the aperture of the lens. Large aperture, a lot of light coming in, small aperture little light coming in.
Focus
Apart from the light that comes in it also controls the depth of field. The depth of field is the part of the image that will be in focus.
The easiest way of thinking about this is by picturing the number of F's in front of your lens. The F's represent the area of in focus elements.
For example:
With F2 you picture two F's in front of your lens. That is just a small area.
With F11 you picture eleven F's in front of your lens. That is a much larger area.
Live View
Most camera's are not able to show you the depth of field in live view. In that case it is best to take a test shot and adjust the F setting to your needs.
Changing F number in Practice
With this setup I took several images keeping everything the same except for the F number. See how the image changes when the F number gets larger.
First shot:
Aperture F8
Shutterspeed 1 sec
The fly with just its face and some hair on its head in focus.
Second shot:
Aperture F11
Shutterspeed 2 sec
More of the fly's back is in focus and also the background becomes more in focus.
Shutterspeed
While changing the F number from small to large, you see the shutterspeed increases as well. A large F number means less light coming in. For the camera to still make a good picture, the shutter needs to stay open longer. Therefor these pictures were taken with the camera on a tripod.
Third shot:
Aperture F14
Shutterspeed 5/2 sec
The front of the wings start to get in focus and so do the front paws
Forth shot:
Aperture F20
Shutterspeed 5 sec
The left wing is starting to show some more details
Fifth shot:
Aperture F20
Shutterspeed 6 sec
I didn't want to bother you with the mess on my desk so I put the yellow paper as background. The F number is the same but the shutterspeed changed. Eventhough being a bright color, yellow reflects less light than the earlier white background. This makes the shutterspeed increase. There is also more shadow.
Sixth shot:
Aperture F36
Shutterspeed 15 sec
This is the end of the F range my camera can handle. Largest depth of field for this setup.
Setup
Improvement
The area that is in focus spreads from 1/3 before the point you focused on til 2/3 behind that point. Taking pictures while changing the part you focus on can improve the image.
Click to see at full size