LENS BRIGHTNESS
Between the objective lenses, or outside, aperture is placed. It is an arrangement of sickle-shaped lamellae, allowing the opening to be enlarged or reduced, through which light enters the camera a.
Depending on the amount of light, that passes through the lens at full aperture, the brightness of the lens is determined. This amount depends on the relative aperture of the lens. It is a number obtained by dividing the diameter of the entrance pupil by the focal length of the lens. The relative aperture is expressed as the ratio of unity to number, obtained by dividing the focal length by the diameter of the entrance pupil of the lens.
The illumination of the image depends not only on the diameter of the lens, but also from its focal length, the distance of the image from the lens. Double this distance (focal length) spreads the same amount of light over four times the area. The brightness of the lens is therefore proportional to the square of the relative aperture.
Because the frequency of taking photos in dimly lit rooms and in motion has increased (when the exposure time should be very short) – requirements for the brightness of lenses have increased significantly. Until recently, a lens with relative aperture 1 : 3,5 (considered a medium brightness lens today) was considered to be of great clarity. Today a lens with relative aperture is considered very fast 1 : 2. The relative openings have already reached the value 1 : 1,5, and even 1 : 0,7.
If we compare the apertures of lenses with relative aperture 1:2 i 1:3,5, then the brightness ratio will be
(1 : 2) 2 : (1 : 3,5) 2 = approx 3,
i.e. a lens with a relative aperture 1 :2 Is 3 times brighter than an aperture lens 1 : 3,5.
A sequence of numerical values is assumed when creating relative openings with the scale, in which each successive relative aperture is obtained by multiplying the previous one by 1/√2 . In this way, with each subsequent relative aperture the brightness decreases twice, so the exposure time has to be doubled.
Currently, the most common is the sequence of relative holes created on the above principle.
It happens sometimes, that, as a result of calculating the optical system of the lens, the largest relative aperture (provided on the frame) is not a number in the given string. In this case, the largest relative aperture is taken as the first relative aperture (not present in the sequence), and then follow this sequence. If the second relative aperture differs from the largest by less than 10%, then this value is omitted from the scale as slightly different from the first.
The following lens sequence is generally accepted (aperture): 1:1,4; 1:2; 1:2,8; 1:4; 1:5,6; 1:8; 1:11; 1:16; 1:22; 1 : 32 itd. Apertures not included in this sequence, like for example. 1 : 3,5 the 1 :.4,5 joins those closest to him.
These data relate to the situation, where the image is in the focal plane, so the sharp focus is “infinitely”. If, on the other hand, the photographed object is close to the lens, we need to increase the distance between the lens and the film, which reduces the brightness of the lens, although the aperture does not change.