I have a kit lens for my D80 which is Nikon 18-135mm telephoto.
zooms like 18-135mm is 135/18 = 7.5x zoom.
that means you can zoom 7.5x nearer to your subject.
so if your subject is like 1000m away from your camera,
if you zoom 2x from 18mm wide, you use 18x2 = 36mm
that means (assumption) you are like halfway nearer to the subject (from your point of view using 18mm) and that is equal to 500 m foot zoom (zoom using your foot by walking nearer to the subject)
doubling the zoom further or using 72mm or 4x zoom,
another 250 m nearer to the subject.
at the longer end; 135mm or 7.5x zoom,
you have traveled a distance of (1000/x)*(x-1) where x is 7.5
equal to 867 m nearer to your subject.
Foot zoom = (s/x)*(x-1) ; where s is the distance of the subject from your FOV (field of view).
so if your subject is only 10 m away from you and you zoom to 135mm
its equal to foot zoom of 8.67 m (when you are using 18mm lens).
Magnification
With 35 mm film cameras, the 35 mm refers to the film size, not to be confused with the lens size. A 50 mm lens is usually considered the normal or neutral size, with a magnification factor of one.
A 50 mm lens has no magnification. So, as the size (millimeters) of the lens become larger, the magnification of the image becomes greater. A 100 mm lens gives 2 times magnification, and makes the subject appear 2 times closer and gives less viewing area; a 200 mm lens gives 4 times magnification, and so on.
A 25 mm lens would make the subject look farther away by a factor of 2 and give a wider viewing area. Fisheye lenses, usually about 8 to 15 mm, give a very wide view and negative magnification. The increased area appears as a distorted "fisheye" view.
Foot Zoom
If the subject is 100 meters away.
Then using 100mm telephoto means, you can have the same field of view with 50mm lens if you walk a distance calculated by our equation below:
Foot zoom = (s/x)*(x-1) ; where s is the distance of the subject from your FOV (field of view) and x is the magnification (with reference to 50mm as neutral or no zero magnification)
=(100/2)*(2-1) = 50 meters equivalent foot zoom.
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