أساس حساب سرعة الغالق المناسبة لسرعة حركة الجسم .. كتاب آلة التصوير

The basis for calculating the appropriate shutter speed for the speed of movement of the body:

The movement of the body in nature is related to the movement of its image on the film, and the following factors control the extent of the movement of the image on the film:

(a) The speed of movement of the body, as the speed of movement of the image increases as the speed of movement of the body increases.

(b) The focal length of the lens: the speed of its movement increases with the longer the focal length.

(c) Duration of exposure: The distance that the image moves increases as the duration of the exposure increases.

(d) Distance of the object from the lens: The speed of movement of the image decreases as the distance of the object from the lens increases.

Based on the previous factors, the range of movement of the image (1) is estimated as follows:

Image movement = object speed x focal length x exposure time over the distance between the object and the lens

And since the extent of the movement of the image on the film is what determines whether we will get a sharp or shaky image in the end, it is obvious that the sharpness of the final image will increase very much whenever the range of movement of the image of the body on the film decreases.

We have previously learned, in the course of studying the diameter of the mixing circle (pages 181, 182), that the eye cannot differentiate between a single point and between two points that are only a millimeter apart if we look at them at a distance equal to the normal vision distance of the human eye (which is 25 cm). ) And this is what finally called for making the permissible mixing circle diameter = the average focal length over 1000

Taking these previous principles, it was also agreed that the permissible amount of movement of the image on the film is the same = the average focal length over 1000, so that the image can be considered sharp and unshakable, as the eye will not differentiate at that time. (fixed) and between the light point whose image has moved by that small percentage
(which is the focal length averaged over 1000)

Based on the foregoing, the equation that we mention later was developed to estimate the appropriate shutter speed for motion recording, based on the following factors:

(a) The distance between the moving object and the lens.

(b) The focal length of the lens.

(c) the speed of movement of the body.

(d) The permissible amount of movement of the image on the film (which is equal to the diameter of the permissible mixing circle).

Shutter speed = the distance between the body and the lens (in metres) x the diameter of the mixing circle (in centimetres) on
Movement speed (meters per second) X focal length of the lens (in centimeters)

This equation applies if the moving body is in a horizontal direction with respect to the machine. If its direction is inclined (45 degrees), then the shutter speed is multiplied by (2/3).

If its direction is perpendicular to the machine, then we multiply the shutter speed by (1/3).

Example :

A train is running at a speed of 108 kph and is 40 meters away from the lens. Assuming that the focal length of the lens is 10 cm, and that it is of medium focal length, what is the appropriate shutter speed to record this movement if the object runs once in a horizontal direction, and the other in a horizontal direction? slanted, and a third in a vertical direction?

the solution :

The speed of movement of the train in meters per second = 108,000 divided by 60 x 60 = 30 meters per second.

The diameter of the mixing circle = 10/1000 = 1/100 of a centimeter.

By applying the following equation, you can get the appropriate shutter speed for photographing the train if it was running in a horizontal direction in relation to the position of the machine:

Shutter speed = the distance between the body and the lens (in meters) x the diameter of the mixing circle (in centimeters) on
Movement speed (meters per second) X lens focal length (cm)

= 40 meters x 1/100 of a centimeter by 30 meters x 10 centimeters
= 40 x 1/100 x 1/30 x 1/10 = 40/000, 30

If the train is running in an oblique direction (45), then the shutter speed will decrease to 1/750 x 3/2 = 1/500 of a second.

And if the train is running in a perpendicular direction to the machine, then the shutter speed drops to 1/750 x 3/1 = 1/250 of a second.

Moment of rest:

If we look carefully at the movement of some bodies (for example, a child swinging), we will find that there are moments when the body stops moving, so the swing actually moves from one side to the other, but there are two moments in which its movement speed decreases, or even almost stops, and that is when the swing reaches its maximum Two heights and before heading towards the descent. The moment when the swing direction is changed to the other direction is the moment of rest.

Moments of motion stillness often occur in bodies that move in a periodic motion, that is, in which the body moves in a certain direction and then stops to return to the other direction (and this happens to all of us while walking if we notice the movement of our hands).

There are also other moving bodies that do not move in a periodic motion (i.e. they do not return to the other direction), but rather their movement decreases at certain moments, such as the cases of jumping over barriers and dams. The body moves in a certain direction and at a certain speed, even if it rises to the top of the dam or barrier, the speed of its movement slows down. Then it goes downhill at a speed almost equal to its speed before jumping or more. And if the photographer was able to record the image during the moment of stillness of movement or the moment of change of direction (and this does not require anything other than simple exercise), then he would not need a very high shutter speed (Fig. 169).