The aperture or f-stop is the opening in the lens through which light falls onto the image sensor.
At interchangeable lenses of DSLR cameras a small metal pin allows to change the aperture.
The picture above, taken with an old manual Nikon lens shows the principle that is still valid today.
When looking through the viewfinder of a DSLR camera the aperture is wide open to get the viewfinder image as bright as possible. When pressing the shutter release button some tiny mechanics will close down the opening to a specific aperture before the image is taken.
The aperture is given in „f-numbers“ which are fractions of the focal length and the diameter of the opening. The opening is the denominator so
smaller numbers mean a bigger opening and brighter pictures
bigger numbers mean a smaller opening and darker pictures.
The usual f-numbers are:
|1 – 1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 – 22 – 32|
|bigger aperture ←||→ smaller aperture|
This series is built in steps of full exposure values – every change from one number to the next will leave double or half the amount of light through the lens and will have the same effect on the exposure as doubling or halving the exposure time.
The actually used f-numbers have a finer distinction with intermediate values.
You may find these f-numbers with a simple f in front, e.g. „f8“ or a naming like „f ⁄ 8“.
The letter f is the common symbol for focal length in physics and the slash indicating a fraction. So „f⁄8“ means an aperture with a diameter that is one eigth of the focal length.