Aperture and f-numbers
Inside most photographic lenses, you will see an approximately circular hole or "opening" located midway between the front and back elements.
A series of overlapping black metal blades, known as iris diaphragms, usually shrink the size of the aperture, continuously from full-lens diameter to just the center part of the lens.
It is adjusted with an adjustable ring or lever on the outside of the lens barrel.
With single-lens SLR cameras, you won't be able to see the aperture change, when you turn the ring unless you remove the lens from the device.
On these cameras, the aperture generally remains wide open until the real-time exposure, when it closes at the defined value.
There may be a preview button on the camera or lens that lets you close the opening ahead of time to verify the effect.
A series of relative aperture parameters can be sensed by a click and are displayed on a scale of numbers known as f-numbers.
Note that the smaller the relative opening, the greater the f value.
They typically run: f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22
The f-numbers follow what is an internationally accepted standard sequence concerning the luminosity of the image.
In the above sequence, f/1.4 is the widest aperture, allowing the greatest amount of light across for a brilliant image.
f/22 is the smallest, leaving only a fraction of the light through, allowing a greater depth of field and most helpful on very bright days.
Note that the opening scale extends beyond the above example with openings wider than f/1.4 and smaller than f/22 being often possible for many purposes.
Each change to the next highest figure halves the amount of light that passes through the objective.
And because the aperture is placed in the center of the lens, it attenuates or illuminates the entire image uniformly.
The f-number system means that any lens set to the same number provides standard image brightness, regardless of the camera's focal distance or size.