[camera] the concept of aperture coefficient and the role of aperture in camera

Time:2021-3-30

The so-called “aperture grating” of the lens is needed to control the light input of the lens.

Aperture apertures are located in the interior of the lens, usually composed of a plurality of movable metal blades (called aperture blades), which can make the (approximate) circular hole formed in the middle larger or smaller, so as to achieve the purpose of controlling the amount of light passing through.

We use “aperture” to describe the light transmission ability of the lens, and the aperture is controlled by the aperture.

For different lenses, the position of the diaphragm is different, the focal length is different, and the diameter of the incident pupil is also different.

In order to calculate the exposure and use a unified standard to measure the aperture of different lenses, the concept of “relative aperture” is adopted.

Relative aperture = [lens focal length] / [pupil diameter] = f / D

For example, if the focal length of a lens is 50 mm and the diameter of the incident pupil is 25 mm, the relative aperture of the lens is 50 / 25 = 2.

Generally, the relative aperture can be expressed by adding [f /] in front of the relative aperture, such as F / 1.4, f / 2, f / 2.8, etc., and 1:2 can also be used to express f / 2. Usually, the lens mark is more like 1:2.

In practical use, the term “relative aperture” is rarely used, and it is usually called “f-stops”, or “f-stop” for short.

On the marking of the lens, the maximum aperture coefficient of the lens is usually marked, as shown in the figure

Now the relative aperture of the marking lens is a series of standardized values:

f/1 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32 f/45 f/64

It can be seen that each value has a relationship with the adjacent value, indicating that the luminous flux of the latter value is half that of the former one, and the luminous flux of the former value is twice that of the latter one. Because according to the formula of circle area, the amount of light passing through the lens is inversely proportional to the square of F coefficient.

For example, the luminous flux of F / 5.6 is half that of F / 4 and twice that of F / 8.

For a lens with a maximum aperture of F / 2:


;

In the above table, changing from the previous number to the following number is called “f-coefficient change one”. Changing from the previous number to the following number (that is, increasing f-coefficient) is called “reducing / shrinking aperture”; conversely, it is called “increasing / widening aperture”.

On the lens, there is an aperture adjustment ring to control the actual aperture.

Due to the popularity of AF SLR, some brands of lenses have cancelled the aperture adjustment ring, and the aperture adjustment is controlled by the fuselage, such as Canon EF series, Minolta AF series, Nikon G series, Olympus Zuiko AF / digital series and Pentax J series.

The function of aperture

  • Control the amount of light input: because the aperture controls the amount of light input of the lens, shooting in dim light requires the use of a large aperture lens to get more light; while in bright occasions, the use of a small aperture does not lead to overexposure. In short, we can adjust the aperture to achieve the purpose of accurate exposure.
  • Control of depth of field: the function of aperture is to control the depth of field of the shot image besides the amount of light input. For depth of field and related calculation, see [concept and calculation of depth of field]. An example of depth of field is as follows
  • Control of image quality: due to the limitation of optical principle and manufacturing cost, the image quality of the camera lens is not the best when the aperture is fully open. Generally, after the aperture is reduced, the image quality is obviously improved. For example, the following figure is the MTF curve representing the image quality of the lens, where the black line corresponds to the maximum aperture and the blue line corresponds to f / 8. The higher the curve position, the straighter the image quality is.