العوامل التي يجب وضعها محل الاعتبار عند تقدير سرعة الغالق ١-a .. كتاب آلة التصوير

Factors to consider when estimating shutter speed

If the object that will be photographed is stationary (Fig. 163), then there will be no difficulties in estimating the shutter speed, since its speed should be 1/2 of a second or a second or 1/1000 of it. But if the object that is being photographed is moving, the shutter must be fast in order to be able to freeze the movement.” If it does not reach the appropriate limit for the speed of the body’s movement, a shaky image of the moving body will be produced (Fig. 164).

Estimating the shutter speed in the case of photographing moving objects depends on the following factors:

The focal length of the lens:

In order to realize the effect of this factor, we return to what we have previously mentioned (on page 99) regarding the effect of the focal length of the lens in increasing or decreasing the relative area occupied by the image on the negative (or frosted glass). We know that this relative area increases with the increase in the focal length. If the lens has a long focal length or is a telephoto lens, the image of the body will occupy a relatively large area on the negative. As a consequence of increasing this relative area, the speed of the image carrier on frosted glass or sensitive film also increases (Fig. 165), while its movement speed decreases if this relative area decreases. As this relative area is affected by the focal length of the lens, so we finally find that it is an important factor in estimating the speed of movement of the image on the negative, which calls for increasing the exposure speed (i.e. shutter speed) if the lens has a long focal length or a telephoto one than if it is a long focal length or obtuse. The angle (assuming that the velocity of the body's movement remains constant and the rest of the other factors that we will mention below).

2- The distance of the moving object from the camera lens:

The relative area occupied by the image is also affected by another factor, which is the distance of the moving object from the lens of the camera, as this area increases whenever the body is close to the camera, which calls for an increase in the shutter speed when photographing a moving object close to the camera than if it was far from it (Fig. 167_a b) Assuming that all other factors remain constant. In order to bring this closer to mind, we imagine a fast sea train one meter away from us. Can we count the windows of a single carriage, for example, or read what is written on it? There is no doubt that it is difficult to achieve this desire, but it is easier if we stand ten meters away from the train moving at the same speed. If we applied the previous example of the eye seeing a moving object to the lens seeing it, we would find a similarity between the two cases, which calls for an increase in the shutter speed (i.e. the exposure speed) as the object approaches the lens.

3- The direction of body movement relative to the position of the camera:

Objects moving in a vertical direction on the camera do not require estimating a high shutter speed like those that run at the same speed and move in a horizontal direction. As for objects that move in an oblique direction, they mediate between the two previous cases.
(Fig. 167 b, c, d).

4- The speed of movement of the body itself:

We knew before that fast-moving objects require a fast shutter, unlike slow ones. It is not sufficient in the context of photography to know only the speed of a specific object in kilometers per hour, for example, but we must also know what is appropriate to the speed of movement of this body.

As the speed of movement of some parts of the body (such as the legs, hands, and head) differs from the speed of movement of the body itself (the torso) when photographing sports movements, for example, so it is wrong to estimate the shutter speed according to the speed of movement.