Aperture - this is one of the main parameters that you need to pay attention to when choosing a lens. The aperture ratio of the optical device indicates the level of attenuation of the light stream.
In other words, this parameter shows how much of the luminous flux the lens system of a given lens can let through. The light flux passing through an optical device is partially scattered and reflected from the lenses, partially absorbed by the materials from which the lenses are made (optical plastic, glass). As a result, it can significantly weaken due to the indicated physical characteristics.
However, this is not the only parameter that determines the aperture ratio, which you need to pay attention to when choosing a lens. The aperture ratio of an optical device depends on the maximum open aperture. The stronger the aperture is open, the more light enters the lens. And so the lens, whose aperture ratio is f / 1.8 or 1: 1.8, is considered to be faster than the f / 2.8 or 1: 2.8 lens. Often, in order to simplify, the aperture ratio of an optical device is determined by the level of the maximum open aperture, which allows you to set this lens. However, the luminosity is an internal characteristic of the construct of the optical system, and the value of the relative aperture of the diaphragm performs only part of the functions associated with determining the luminosity of the device. However, from a practical point of view, it is easier to compare lenses by such a parameter as the maximum possible value of the open aperture. So these two concepts can be considered identical.
The aperture ratio of the optical system is always indicated in the technical specifications of the lens description, and they are also marked on the body of the device near the front lens. For example, the aperture of the Canon 24-70 f / 2.8 lens. This means that shooting with such a lens allows you to open the aperture to a maximum value of 2.8. This means that the device will not allow shooting with aperture of 2.0, 1.8, etc.
A lens whose aperture allows you to shoot at aperture level from f / 1.2 to f / 2.8 is considered aperture. Devices with a level from f / 3.5 to f / 6.3 do not belong to fast lenses, they are usually called “dark”, since they allow a small amount of light flux through. Some leaders in the production of optical systems (for example, Leica and Carl Zeiss), produce models of devices with a level of aperture level from f / 0.7 to f / 0.95. The most common fast lenses among photographers are devices with a value from f / 1.4 to f / 2.8.
Now consider what the aperture parameter affects. All the advantages of such lenses are related to the aperture value. This is because when shooting, the aperture affects the amount of light transmitted through the lens and the depth of field in the frame. Thus, the aperture ratio of the digital camera lens affects the ability to shoot in low light. The more the aperture is open, the smaller the depth of field of the frame will be, that is, objects that do not fall into the focus area will blur more.