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Characteristics of photographic lenses.. Lens speed

Lens speed
We must differentiate between lens speed and shutter speed. The speed of the lens is the relative aperture of the lens, and the fast lens can take an acceptable picture in dim light because it has a large aperture, and the slow lens needs more light. As for the speed of the moving veil, it is the period of time during which the shutter remains open to expose the film to light.
Lens speed is determined by two things:
1. The size of the lens aperture, which determines the amount of light that is allowed to enter through the lens at a specific shutter speed.
2- The focal length, which determines the area of ​​the image over which the light is transmitted.

To specify the speed of a lens, it is sufficient to refer to two things: its focal length and its aperture diameter, and this is what the number represents. f - number ( f ) on photographic lenses, which is the number, which represents the ratio of the focal length to the actual aperture diameter of the lens, i.e. the focal length of the lens divided by its diameter.
For example, a lens whose focal length is 8 inches and its aperture diameter is 1 inch, its speed is: 1/8 = 8/ F. A lens whose focal length is 4 inches, and whose aperture is half an inch in diameter, has a speed equal to: 4/1/2 --- 1/4 x 2 = 8 -- ( 8 f).
It is important to note here. We have two lenses, one with an opening of one inch and the other with an opening of half an inch. Despite that, they have the same speed, and therefore they require one exposure time.

To understand this fact, we must remember what we explained about the focal length before. A 4-inch lens has a focal length equal to half of an 8-inch lens. Thus, it projects an image whose dimensions are half the dimensions of the image projected by the lens, 8 inches.
So, to diffuse the same amount of light onto this smaller image, a 4-inch lens would need an aperture equal to half of an 8-inch lens.

From the equation: focal length / lens aperture = f-number
We conclude: the larger the lens aperture, the smaller the f-number.

The numbers denoting the number (f), i.e. (f-number), are placed on the bracelet that controls the diaphragm, in such a way that each number of them requires a shutter speed equal to twice the smaller number that follows it.

On a typical lens, for example, we find the numbers as follows:

f22, f16, f11, f8, 5,6, f4, f2, 8, f2

In which f2 is twice as fast as 2,8, f 5,6, f is twice as fast as f8, and so on to the end of the series, each number twice as fast as the next.
If the required exposure for a particular scene is 1/60 of a second at an aperture of f4, the same exposure can be obtained with 1/30 of a second at an aperture of 5.6f, or by 1/15 of a second at an aperture of f8.
Figure (2) shows this constant relationship between the shutter speed and the aperture value.