عدسة آلة التصوير .. كتاب آلة التصوير

Chapter two

Camera lens

The lens is the main component of the camera, and it is the first factor in determining the value of the camera. The lens may consist of a single piece of glass, as is the case in box cameras, or it may consist of more than one piece, some concave and the other convex (Fig.

Each lens has a focal point where the rays passing through it converge. The focus is defined as the point located on the axis of the lens in which the rays parallel to the axis gather (Fig. 34) after they enter from the convex lens. This point is known as the original focus. The rays parallel to the axis are those that are emitted or reflected by a body located in infinity.

The focal length of a convex lens is the distance between the original focus and the center of the lens (Fig. 35), and the focal length of a simple convex lens depends on the refractive index of light and the radius of curvature on both sides of the lens.

The ability of lenses to collect light rays varies according to their focal length. Short focal length lenses are more able to collect rays at a point close to them than lenses with a long focal length (Fig. 36).

The function of the photographic lens is to create an image of the body, and this image is inverted. The area of ​​the image depends on the length of the object, the focal length of the lens, and the distance of the object from the lens. Likewise, the distance of the object from the lens, the area of ​​the image becomes smaller, so if the object is located in an infinity, an image is formed for it at a distance exactly equal to the focal length of the lens, and the image is reduced and inverted, and the closer the object is From the lens, the area occupied by its image is greater than the negative, so that if the object reaches a distance from the lens equal to twice its focal length, a real, inverted image is formed for it on the other side of the lens, at a distance equal to twice its focal length as well, and the dimensions of the image at that time are exactly equal to the dimensions of the body. (i.e. the length and width are equal in both cases).

Focal number. No. f:

The diaphragm aperture, as previously mentioned, is associated with numbers called (focal numbers) and is usually symbolized by the letter f, which indicates the ratio of the focal length of the lens and the diameter of the diaphragm aperture (Fig. 37).

That is, the focal number = the focal length of the lens over the diameter of the diaphragm aperture

In colloquial language, this relationship is expressed as “the power of the lens.” For example, it is said: (a lens whose power is 18). They mean by that the ratio between the diameter of the aperture of the diaphragm, and the reader may ask why the link between the focal length of the lens and the diameter of the aperture of the diaphragm and make it a basis for estimating the focal number .

In response to that, we say that the reason for the connection between them is that they are the two factors that determine the intensity of illumination of the image formed by the lens, as the focal length of the lens affects the intensity of the illumination of the image, and the diameter of the aperture of the diaphragm also affects it. The shorter the focal length, the greater the intensity of the image luminance, and the greater the diameter of the diaphragm aperture, the intensity of its luminance also increases, i.e. the lens’s ability to form a more luminous image increases, and vice versa.

From the foregoing, we see that the first factor that determines the intensity of illumination of the image is the focal number, and on this depends the determination of the length or lack of time that should be taken to expose the photosensitive film. This period of time is what is known as the “exposure period.” If the image is highly luminous, the exposure time is reduced, and vice versa. If the image’s luminous intensity decreases, the exposure time must be increased.

As mentioned, it is sometimes called (focal number).f. No Another common name is (the power of the lens), and it is an inaccurate designation, as it means another meaning, which we will explain later (1). It is sometimes called scientifically more accurate names such as speed or relative aperture, as this number is nothing more than the ratio between two values, the focal length over the diameter of the actual diaphragm aperture, as it is sometimes called stops.

As the diameter of the diaphragm aperture changes in one lens while the focal length remains constant, so the focal number must always change according to the increase or decrease in the diameter of the diaphragm aperture.

By looking at the ratio indicating the focal number, which is the diameter of the diaphragm aperture, we find that the numerator, which is the “focal length”, is always constant, and that the denominator, which is “the diameter of the diaphragm aperture”, is always variable. The larger the diameter of the diaphragm aperture (or in other words, the greater the ability of the lens to receive a larger amount of light), the previous ratio decreases, so the numerical value of the focal number decreases, and therefore if the focal number is equal to 4 f, this number indicates a lens that is more capable of Creating images that are brighter than the lens with focal number 48, and in the following table there is an explanation for this:

We see from this table that the diameter of the diaphragm aperture is inversely proportional to the numerical value of the focal number No. f, the smaller focal number indicates that the lens is able to receive a larger amount of rays (Fig. 6, page 27), and thus is able to form a more luminous image.