Aperture is one of the component of exposure triangle. It is a lens, not a camera parameter, and is one of the factors for why we have such a difference in prices. The smaller number on Aperture, the more expensive lens. Also the lenses are identified by the smallest f-number available and their focal length.
Along with shutter speed and ISO and depending on what your final result want to be, brighter or darker images, or just a shallow depth of field with great bokeh, aperture is key of understanding how your camera works.
The f number: It is (f), the focal length, divided by diameter of the aperture of the lens. In picture below you can observe a f11 aperture for my Mamiya film camera. This means aperture is 11 times smaller than the focal length, in my case a 135mm lens.
The following usual values found written on the lenses are referred to as F-STOPS:
1/2.8 1/4 1/5.6 1/8 1/11 1/16
In the image above, there is a 50mm fixed lens with aperture ranging from f1.7 to f16.
Each f-stop represent a change in light intensity of a factor of two.
An aperture of f2.8 has an area double than area of f4, this means double the photons entering the camera sensor and double the image brightness.
f4 has an area double than f5.6, so double the brightness.
To better understand this values, you have to calculate the radius (r) of an aperture versus the radius (R) of double the aperture.
So double the area and you get a radius of R = r×√2. This is also the reason behind this strange numbers.
Take for example f2.8 and multiply by 1.42, which is the approximate of square root of 2, and you will get f4:
1/2.8 = 1/4×√2;
2.8×√2 = 4;
The purpose of this article is not to enter into physical details of lens optics, but just understand enough what is the idea behind Aperture parameter. In the end, when you are out there shooting, what it matters is to control the result of your desired photo.
I use Aperture to control two conditions:
Each F-stop can increase or decrease by a factor of two the light entering the camera sensor or film.
2. Depth of field
For this section I will have to make another article.