خصائص عدسات التصوير .. البعد البؤري ( Focal Length ) .. كتاب فن التصوير الفوتوغرافي

Characteristics of photographic lenses

Focal Length

One of the basic characteristics of a lens is its focal length. The focal length of a lens can be defined as follows:
“It is the distance from the center of the lens of the focused (clear) image of a subject that is very far away or at infinity.”

The focal length of the lens is measured indirectly, because it is difficult to determine the center of the lens. This center is usually close to its center of mass in relatively simple lenses, and in some lenses with a large angle, it is in the back of the lens. As for telephoto lenses, it is in the front part of the lens or in front of it.
The focal length of a lens determines the size of the image projected by that lens. A lens with a focal length of 6 inches gives an image twice as large as a lens with a focal length of 3 inches. (Fig. 8). This is one of the most important features of lenses, and you should always remember them

The focal length of the lens is indicated in inches, or millimeters (an inch is approximately 2.5 mm). Pictures 9 to 12 represent four shots taken for the same subject and in the same situation, using four lenses with different focal lengths.
Photo 9 was taken with a 50mm focal length lens, Photo 10 with a 90mm lens, Photo 11 with a 135mm lens, and Photo 12 with a 200mm lens.
Note that the dimensions of the woman in picture 12 are four times the dimensions of her in picture 9.