Lens Increase

Since the lineal increase is directly proportional with the focal, if we take the normal length of each format as reference, we can immediately deduct the increase factor regarding sight by dividing the lens focal length by the normal for the format used. For example, a 200 mm provides an increase of four times (4 x) in universal format because this is the relation it as with normal focal length (50 mm). A 250 mm telephoto provides an increase of approximately 3 x for 6 x 6 format, because the normal of that format is 80 mm. The image of an object which with a 50 mm is registered in the image with a height of 2 mm, for example, photographed from the same place with a 500 mm (10 x) it will appear with a height of 20 mm.

Characteristics

Images made with a telephoto are characterized by a narrow angle of vision and a reduced depth of field , which will be lower the greater the focal. The way the three dimensions of space are registered (perspective) is characterized by the relative long distance that separates us from what is being photographed. It's evident that a medium or long telephoto is of great use when trying to photograph things that are far away and of impossible access, hard or dangerous: astronomy, far away landscapes, wild animals, etc.; since we can't get close and we want the image of the object to fill the picture, we use a telephoto. Short telephotos are very advisable when we don't wish to be very close to the scene because our presence might interrupt the situation: snapshots, children, informal portraits, domestic animals, etcetera