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What is the focal length?

The focal length (FR or ƒ) is the distance between the optical center of the lens and the camera sensor. The larger the focal length, the larger the image scale projected by the lens onto the sensor, the smaller the focal length, the smaller the image scale. We can say that a lens with a large focal length enlarges objects, as if bringing them closer to the photographer, and with a smaller focal length – it reduces, moves objects away.

The focal length determines the angle of the image of the lens (angular field). For a lens with a large focal length, the image angle is narrow – enlarging objects, a telephoto lens fills the entire frame with them. A lens with a small focal length, in contrast, has a wide image angle and is able to cover a large amount of space. For example, a lens with a focal length of 50 mm has an angular field of 47 °, and with a focal length of 200 mm, it will provide an overview of only 12 °.

Depending on the focal length and, accordingly, the image angle, three main groups of lenses are distinguished: normal (or standard), telephoto lenses (telephoto lenses), and telephoto lenses (wide angle ones).

Normal lenses i.e. those that give the image closest in perspective to what the human eye sees have a focal length approximately equal to the diagonal of the frame, or slightly longer. So, the frame of a standard 35 mm film has dimensions 36 x 24 mm, and therefore, its diagonal is approximately equal to 43.3 mm. Lenses with a focal length of about 40-60 mm are considered normal. In fact, 50 mm is most often used. Such a lens is also called “fifty dollars.” The angular field of a standard lens lies in the range of 40-60 °.

The telephoto lens has a focal length greater than the diagonal of the frame. Such lenses are used for shooting distant objects, as well as in cases where the background can distract attention from the main subject, and a small angle of the image of the telephoto lens is necessary to isolate the object as much as possible, eliminating all superfluous from the frame.

A wide-angle lens has a focal length less than the diagonal of the frame. Its large image angle is indispensable when the background is important for the picture and you want to cover more space, emphasizing the perspective and the relationship between the plans.

At the present time, zoom lenses — the so-called zoom lenses, zoom lenses, or zoom lenses — have gained widespread popularity. Their convenience and practicality are obvious – one zoom can replace a whole bag of lenses. Of the minuses – the complexity of the design and, as a consequence, the high cost, large size and weight, as well as lower image quality compared to lenses with a fixed focal length.

Prospect management
The focal length of the lens along with the position of the camera affects the composition and perspective of the picture.

Imagine that you are taking a portrait of a man against the background of any distant objects – let it be a mountain, a forest edge or any man-made structures. We’ll take a few shots using lenses with different focal lengths, but at the same time try to keep the size of the person unchanged relative to the frame size.

When shooting with a normal lens, you will get a frame that has the most natural perspective, with background objects that decrease in proportion to their distance from the person in the foreground.

Normal lens
Photo taken with a standard lens.
If you take a telephoto lens, you have to step back to compensate for its magnifying ability and keep the scale of the portrait still. At the same time, background objects will increase in scale and will approach you. Why? Yes, because moving away an extra ten meters from a person who was originally located five meters from you, you have tripled the distance between you, and the distance to the background, which could be measured in tens, if not hundreds of meters, has not changed. Therefore, they say that telephoto lenses compress plans, eliminating perspective distortions. In fact, the lens has nothing to do with it – it just enlarges the image, not understanding where the background is, but where is the front, but this allows you to shoot an object at a greater distance, reducing the difference between the distances from you to different plans of the scene.

Telephoto lens
A telephoto lens brings the foreground and background together.
Telephoto lenses are great for taking portraits, as well as for macro photography, because, firstly, they depict all parts of the object at approximately the same scale, and secondly, due to the small angle of view, extraneous background elements can be excluded from the frame. However, telephoto lenses do a great job with landscape photography too, when you want to bring objects that are really distant in reality, giving rise to a surrealistic perspective. As for photography, here the telephoto lens is almost indispensable.

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