عدسة آلة التصوير١_a .. كتاب أسس التصوير الضوئي

Camera lens

It is the lens in the basic piece of the camera and has the greatest impact on the value of the camera. The lens may be made up of a single piece of glass, as is the case in box cameras, or it may be made up of more than one piece, some concave and the other convex, and assembled in a special system so that they form a group

:: These pieces together «collection lens». Each lens has a focus in which the rays passing through it gather, and the focus is defined as the point located on the axis of the lens in which the rays parallel to the axis gather after they have escaped from the convex lens. This point is known as the "original focus". The focal length of a convex lens is the distance between the original focus and the center of the lens. The focal length of a simple convex lens depends on the refractive index of light and the radius of curvature of the two sides of the lens. The ability of the lenses to collect light rays varies according to their focal length.

A lens with a short focal length is more able to collect rays at a point close to it than a lens with a long focal length. The function of the photographic lens is to form an image of the body, and this image is inverted. The dimensions of the image differ according to the distance of the object from the lens, so the farther the object is from the lens, the smaller the area of ​​the image.
Opinion: The more space it occupies in the negative image (even 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 the focal length, and the dimensions of the image then are exactly equal to the dimensions of the body, the focal number) Focal number f No

f, that is, the focal number =: the diameter of the diaphragm aperture, and photographers express this relationship as the power of the lens) so the focal length is said to be its lens power. By this they mean that the ratio between the diameter of the aperture of the diameter of the diaphragm

As we mentioned, the diaphragm aperture is provided with numbers, and it is called “focal numbers” and is usually symbolized by the letter f. This focal number symbolizes the ratio between the focal length of the lens: diameter. diaphragm slot

The focal length of the lens

The reader may ask why the connection between the focal length of the lens and the diameter of the diaphragm aperture? And how important is that relationship?

In fact, the reason for the connection between them is that they determine the intensity of illumination of the image created by the lens, as the focal length of the lens affects the intensity of the illumination of the image, just as the aperture of the diaphragm affects it as well. The smaller the focal length, the greater the luminous intensity of the image, and the greater the diameter of the diaphragm aperture, the greater the ability of the lens to form an image.

more luminous, and vice versa: and since the widening or narrowing of the diaphragm aperture is actually the factor that controls the actual aperture of the lens, we made the diameter of the diaphragm aperture the second factor that determines the ability of the lens to form a highly luminous image and to show the importance of this relationship in Photography suffices to say that no
The previous two factors determine the brightness of the image, so based on knowing the ratio between them, you can determine the appropriate time to expose the photosensitive film.

We conclude from the foregoing that the term “focal number (f No.) determines the intensity of luminance of the image produced by the lens. It is called, in excess of the ordinary language of photographers, "the power of the lens."

The focal length of the lens is the power of the lens = the diameter of the actual diaphragm aperture

And if we look at another fact, which is that the diameter of the aperture of the diaphragm in one lens always changes, while the focal length of the lens remains constant, we find that the number. The focal length always changes for a single lens depending on the increase or decrease in the diameter of the diaphragm.

Looking at the previous law, we find that the numerator (which is the focal length) is always constant, and that the denominator (which is the diameter of the diameter of the diaphragm aperture) is the variable. The greater 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 lower the previous ratio, so the focal number decreases, and accordingly, if the focal number is equal to 40: this number indicates a lens that is more capable of forming a more luminous image. of a lens with a focal length of 8 f

The table shown after explains this