Photography basics – Aperture

The aperture controls how much light is let in by the lens.
 
Lenses have variable aperture, making it possible to let in a lot of light for low light conditions or let in only a fraction of the light for those sunny days. This is one of the ways to adjust the exposure of a photograph, or in other words to control the amount of light that falls on the sensor or film.
 

The aperture is set by controlling the diaphragm inside the lens. The diaphragm is a number of adjustable blades that form a hole. Since it is adjustable, the size of the hole, and thus how much light passes through it, can be adjusted.
 
The aperture is specified by the f-number. Confusingly, as the f-number gets smaller; the amount of light that passes through the lens gets larger.
 
On most lenses, the aperture can only be set to fixed points; a regular progression of f-numbers looks like the following:
 
f 22 – f 16 – f 11 – f 8 – f 5.6 – f 4 – f 2.8 – f 2 – f 1.4

 Here are some examples of different apertures from a Nikkor 28mm f2.8 Series E lens. These pictures show you what the film sees when the different apertures are set.

This looks like a weird set of numbers, but in fact there is some logic behind them.
 
As you go from left to right on this set, every next f-number doubles the amount of light entering the lens, also called one stop.
 
This is related to the meaning of the numbers. The f-number specifies the fraction of the diameter on the front element of the lens that is used to gather the light. The fraction is related to the focal length of the lens, thus a 50mm lens set to an aperture of f 2 will use 25 mm on the lens diameter to gather the light. Since the area of a circle is proportional to the square of the diameter, the ratio for a doubling of light is square root of 2 or ~ 1.4. That is the reason for the weird set of f-numbers.
 
A lens that gathers more light gives you more freedom for choosing your shutter speed and ISO setting.
 
Therefore lenses which have low f-numbers are preferable. Unfortunately, the lower the minimum f-number on the lens, the larger, heavier and more expensive it is. This is because it needs more glass to gather more light.
 
The aperture also affects depth of field, the range in your scene where the objects are in focus. The larger the aperture (low f-number) the narrower the depth of field, and the smaller the aperture the wider is the depth of field.
 
The aperture lets you control how much light to gather and also how much of your scene is in focus.