غالق آلة التصوير ١-a .. كتاب آلة التصوير

Chapter Ten

Camera shutter

The diaphragm aperture controls - as we learned - the intensity of the light that is allowed to pass to the sensitive layer, while the shutter controls the period in which the light is allowed to pass to it, and from here it was necessary to choose a good shutter for cameras, especially for those equipped for accurate research or for recording fast movements .

The study of the camera shutter is of great importance, no less than the study of the lens or the aperture of the diaphragm, as it performs half the function in the exposure process. This does not concern us in this section to study the mechanical composition of the shutter in an extensive study, and therefore we will present this topic in an urgent glance, given that we give the first consideration to studying the effect of the type of shutter and its mechanical composition on the exposure process.

We have already mentioned in the first chapter that the shutter in modern photocopiers falls into any of the following two groups according to its place in the photocopier:

(a) Creator of the Diaphragm Shutter:

It may be in front of or behind the lens in inexpensive cameras, and it may be located between the pieces of glass that make up the lens in more expensive cameras. Therefore it is sometimes called as . Between lens shutter.

(b) Focal Plane Shutter: It is always located in front of the focal plane of the photographic lens.

The two previous types differ in the way they expose themselves to light. We will now talk about each of them in detail.

First: Diaphragm shutter

This shutter consists of several metal foils or from Fiivre, called blades (Fig. 152, p. 270). The shutter opens and closes by a spring that performs a precise mechanical operation that differs from one type to another.

Before opening the shutter, charging (or Setting as this process is sometimes called) must be done. The preparation or reconstruction process is carried out by means of an arm called the Lever. Here, we divide the genus (Ghalaq Al-Dia Farajm) into two parts as well, according to the manner in which the reconstruction process takes place (1).

(a) Self Setting Shutter:

It is sometimes called the automatic aging shutter. This type is distinguished by the fact that the shutter trigger or shutter lever performs the reconstruction process itself, which is automatically pressed. The shutter opens directly and then closes, and exposure takes place during the opening and closing phases.

(b) A shutter that requires pre-setting: this type has two levers or arms, the first performs the setting process, while the second, which is the trigger, performs the opening and closing of the shutter, i.e. it performs the exposure process.

The first type, which is self-repairing, is distinguished by its ease of use, as the photographer only needs to press one arm in order for the exposure process to take place, in addition to its cheapness due to its simplicity. On the other hand, the second type is distinguished from the first by its accuracy and can be set to determine any exposure time estimated by the photographer (from one second to 1/500 of a second usually). The two speeds denoted by the letters T and B (2) are also permitted. As for the first type, it does not allow limited speeds, which may be only two speeds, B, 1/25 of a second, as is the case with the rotary shutter (Fig. 150).
It may sometimes allow more than two speeds, as is the case in the type known as the Multi Speed ​​Shutter, through which it is possible to obtain speeds T, B, 1/25, 1/50, 1/200 m of a second, and speeds ranging from B, are rarely allowed. 1/25 of a second, which are the speeds needed for a slow exposure, and it is also impossible to estimate by it a high speed such as 1/500 of a second that is needed to photograph fast motion.

Disadvantages of the self-repairing shutter:

The following is wrong with this shutter:

(a) Weakness of the spring: This means that it is impossible to use it for fast exposure (such as 1/500 of a second), and its weakness increases with the longer the period of use, so its speed decreases. If it were assumed that the shutter had been prepared for a speed of 1/25 of a second when the camera was manufactured, then its speed will decrease with the passage of time and frequent use, and the exposure time will become 1/10 of a second, for example. Very much of the first kind.

(b) It is free of brakes: In order to control the shutter speed, a way must be found to stop the spring for a period of time after the shutter is opened, and this period is the same as the exposure time that you specify. For example, if we estimated the duration of exposure to light in half a second, then we must find a way to stop the movement of the spring for half a second after opening it, or reduce the speed of its movement, so that it takes half a second for the light to pass through the lens to the sensitive layer. Accordingly, it is imperative to find a brake for the spring, which is in fact the means that controls the estimation of the exposure time.

Since the self-repairing shutter is free of this braking, the manufacturers circumvent the way to control the speed by changing the degree of spring tension, but it is an unpleasant method, as it is damaged over time, so the actual exposure speed does not become equal to the speed written on the machine, in addition to this. The inability of this method to fully control any speed we desire.

The brake industry went through several stages, which we describe as follows:

(a) The brake pads were initially made of leather or rubber, and frequent use is sufficient to break them, wear them out, or weaken their resistance, which results in damage to the shutter or its failure to select correct speeds.

(b) Then the previous brake was replaced by another “air brake”, which is a piston that moves inside a sealed cylinder. Although the discovery of this type is. . It has made remarkable progress in adjusting the exposure speed, but it is also criticized for the extreme care it requires, as it is damaged by any slight trace of dust or oil. However, it may be easier for the other to obtain different shutter speeds, so if the piston moves slowly and the distance is short, the shutter speed decreases, and conversely, its speed increases if the piston moves in the cylinder quickly and for a long distance.

(c) Modern cameras got rid of the previous types of brakes and replaced them with brakes that operate in a mechanical way similar to those that control the movement of the parts of a wristwatch. As low as a second, 1/2 second, or 1/5 second, for example.

Shutter opening and closing motions:

Both movements of opening the shutter to begin exposing the film, and then closing it after it has finished, take a period of time. The time taken for the opening movement is almost equal to that taken for the locking movement.

And if we imagine the mechanical movement by which the modern shutter opens, we would find that the light penetrates first, as soon as the shutter begins to open, from a hole in the middle of it completely (Fig. 152). The opening was gradually narrowed during the movement of the lock, and finally the passage of light ceased completely (Fig. 153).

If we imagine those successive developments that take only a fraction of a second, we can divide them as follows:

(a) As soon as the shutter is opened, light enters through a small hole in the middle. Due to the narrowness of this aperture, an image is formed in which there is a very large depth of field, but it is an image with very little illumination due to the lack of the amount of light that is sufficient to form an acceptable image.

(b) The aperture through which the light penetrates widens gradually until it reaches its maximum width, so an image with less depth in the field is formed than the first (due to the widening of the aperture), but it is an image with acceptable illumination as it has been exposed. This is the picture that counts and is the one we finally see.