# Focal length of binoculars



## jamesino (Apr 27, 2008)

A sample binocular measurement would consists of: magnification x objective diameter, eg. 3 x 60.

Suppose my binocular is 3 x 60 (3x magnification), what would be the equivalent focal length I need from a telephoto lens to achieve the same degree of magnification?


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## prodigy2k7 (Apr 27, 2008)

http://answers.yahoo.com/question/index?qid=20080219210949AA8YMpm
Yahoo Answers, someone talks about it.


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## jamesino (Apr 27, 2008)

The page you referred to talks about the zoom factors for zoom lenses. However, on a pair of regular binoculars, the focal length does not change and is analogous to a prime lens in photography. My question is, if a binocular has a 3x magnification, what is the mm equivalent for it on a lens?


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## prodigy2k7 (Apr 27, 2008)

He said "There is not a lens-to-distance equation"


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## prodigy2k7 (Apr 27, 2008)

http://photography-on-the.net/forum/archive/index.php/t-130859.html

2nd reply says

"12x zoom can mean any old thing - all it says is that the longest focal length is 12x what the shortest one is. A large "x" zoom range is easier to do with the short focal lengths of the digital P&S cameras. Getting a good large zoom range with a lens for a DSLR is much harder. Most 12x zooms for DSLR aren't worth the price - unless you absolutely can't possibly change lenses, you're better off getting several lenses to cover the same range. The theoretical limit for Canon, considering shortest to longest lenses, and without teleconverters, is something like 150x (8 mm fisheye to 1200 mm tele)."


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## jamesino (Apr 28, 2008)

So essentially there's no way for me to tell the focal length of my binoculars?


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## JimmyO (Apr 28, 2008)

Istead of trying  to find its  "focal length", try finding the angle of view.


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## Helen B (Apr 28, 2008)

This is a lot easier to draw out than to explain in words.

Binoculars and telescopes are not designed to form an image like a camera lens does - they are like two lenses together. Each of the two lenses has a focal length, but the combination does not have a focal length.

The first 'lens', the objective, forms an image and then the second lens, the eyepiece, allows you to view that image. If the focal length of the objective is, say, 150 mm then an image of a distant object will be formed 150 mm behind the lens (in simple terms). Now if the eyepiece has a focal length of 15 mm, and the eyepiece lens is 15 mm from the image formed by the objective, the eyepiece will form an image of the first image 'at infinity' - and that is what you see. The image you are looking at is a virtual image far away in front (at infinity). Therefore the system as a whole has no focal length, even though the two main components do. The magnification is 150/15, the ratio of the two focal lengths, ie 10x.  

If you remove the eyepiece, you have a long-focus lens.

If you take a picture of the image formed by the eyepiece, you can magnify the camera lens' focal length by the binocular magnification. This is what digibinning is (the binocular variant of digiscoping).

The 3x or whatever you see in a specification of a point-and-shoot is not the same thing - it is merely the zoom ratio of the lens, ie the longest focal length divided by the shortest.

Best,
Helen


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