I just recently learned that 30x Optical Zoom on a Camera and 30x Magnification on Binoculars are different things.

Suppose if I have got 30x50 Binoculars, then what would be its equivalent number of an Optical Zoom that I can expect?

  • \$\begingroup\$ Only the "30" in the name of a 30X50 binocular has anything to do with magnification. The "50" indicates the objective lenses are 50mm in diameter. The diameter of the objectives affect how bright the view through the binoculars is. \$\endgroup\$
    – Michael C
    Commented May 4, 2017 at 3:28

1 Answer 1


Astronomers typically cite 50mm as magnification 1. Given this criterion, 10X in binoculars corresponds to a 500mm telephoto lens on a camera. Using this same logic, a 30X binocular or telescope corresponds to a 1,500mm telephoto lens.

The 50mm = magnification 1 has been cited many times and always there are doubting Thomases. I myself assumed the 50mm rule of thumb stemmed from 50mm being the “normal” focal length of the venerable 35mm film camera. So I did a little research. A 50mm camera lens, when used to image the moon (or sun), projects a circular image 0.5mm in diameter. This size image of the moon will be just barely discernable as a tiny circular point when viewed with the unaided eye when held at standard reading distance. Thus astronomers use the 50mm focal length to reference magnification 1.

  • \$\begingroup\$ That's fine and all, but I don't think that connects to binoculars, and furthermore, I really don't think it's a good idea to introduce the idea that this is a "conversion rule" people should use to convert binocular specifications to lens specifications. People are confused enough as it is. \$\endgroup\$
    – mattdm
    Commented May 4, 2017 at 7:46
  • \$\begingroup\$ 50 mm is about the focus length of the human eye (actually, rather 45 mm). As a result, looking through a camera with a 50 mm lens looks about the same as looking through no camera (try it with one eye on the camera finder and the other eye open). \$\endgroup\$
    – Aganju
    Commented May 4, 2017 at 10:50
  • \$\begingroup\$ You can use 50mm, and any other, lens both to magnify and to belittle the projection. 1:1 ratio is in between. \$\endgroup\$
    – Crowley
    Commented May 4, 2017 at 10:55

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