Making it quite clear what your subject is in a photograph - depth of field
Some people describe it as making your subject POP. It's when you want the background to be blurred, basically.
The apple was shot F4.5, focal length 95mm
Actor Henry Luxemberg F5.6, focal length 200mm
Essentially. The larger the mm (the focal length) number and the smaller the F stop number, the more the area in front and behind the subject will be blurred.
This shot of a wedding is that rule in reverse. Large F stop number, small focal length number.
F11 (larger F stop number), focal length 13mm. Almost everything is in focus in this photograph. Easy.
