العدسات الإضافية .. كتاب آلة التصوير

additional lenses

They are lenses that are used for different purposes and change the original focal length of the original lens, and are placed in front of the original lens either by turning it by means of a screw or by pressing it. These additional lenses may be positive or negative.

Positive additional lenses or (collective)

There are different names for these lenses, so they are sometimes called the Portrait lens, as well as the Close up lens, and whether we take this name or that, the reality is that it is a Positive Supplementary lens that shortens the focal length of the original lens, which gives the photographer the opportunity to photograph moving objects. At a close distance from the lens, and the image of the body is relatively larger than it would be if the photography had taken place without an additional lens (Fig. 98). However, it is criticized for the distortion it causes in the original proportions of the bodies that we photograph.

There are two ways to adjust the focusing distance in the case of using additional positive lenses, and we explain them as follows:

First: If the distance adjustment in the machine is done by means of a graduated division on which the distances are indicated, the Focusing Scale: (as in machines in which the image is not seen on frosted glass):

I put a converging lens, then from if I set the original lens at a distance of infinity, then . additionally, then the object at a distance equal to the focal length of the additional lens becomes perfectly detailed (In Focus). For example, if any original lens is set to infinity and an additional lens with a focal length of thirty centimeters is placed in front of it, the object becomes completely sharp in detail if it is located 30 cm away from the additional lens. If the photographer wants to make the distance between the body and the additional lens less than its focal length in order to obtain an image of the body from the above, he may follow one of two methods:

(a) Refer to the tables published by the manufacturers of cameras to find out the distance at which the distance is sharp whenever the distance of the object from the lens changes and when an additional lens is placed in front of the original lens.

(b) It is also possible for the photographer to estimate the distance that must be available between the body and the lens (after placing the additional lens) in order for the image to be completely sharp, by applying the following equation (1).

Where m = the distance at which the original lens was set.

p = focal length of the additional lens.

Q = the distance that must be available between the body and the additional lens in order for the image to be sharp.

Second: If the distance adjustment takes place while viewing the image:

Here, the process of adjusting the distance is much easier and more accurate if the camera is of one of the types in which the image is seen directly with the eye prior to shooting, whether that is by seeing through the camera lens itself, as in Single Reflex Cameras (Fig. 28, page 45). Or was the vision of the image through a lens other than the imaging lens, such as reflective machines. Twin Reflex Camera (fig. 26 p. 43). In this photograph, reference is made to an equation or to tables in order to set the original lens at a certain distance. Rather, it is easy to set the distance with the utmost precision by simply looking at the image through the frosted glass, for example.

The focal length of the additional lens is the maximum that cannot be exceeded by the distance of the object from the lens. If the focal length of the additional lens is 50 cm, for example, then this distance is considered the maximum distance that must be available between the body and the lens in the case of close-up photography.

In this regard, we point out that it is necessary to note the difference in parallax that occurs (Fig. 99, 100) between both imaging and vision lenses, especially in the case of using reflex cameras with twin lenses if the imaging is close-up.