Measure the outside lens-to-lens distance of the binoculars. We need to find the length of the light path, Fo + Fe. So, we know two formulas with the same terms and we know magnification. Also, Fo + Fe is the total length of the light path. In binoculars, the same as in telescopes, magnification is focal length of objective (Fo) divided by focal length of eyepiece (Fe), or mag = Fo / Fe. That would be relevant to the discussion if we were performing this calculation because we were concerned about chromatic aberration. Some binoculars are triplet apochromats, the Oberwerk BT100 for example. The short f-numbers of these systems certainly help explain why chromatic aberration can be present. Most common porro prism binoculars are fast Achromats. I wrote this a year or two ago, embedded in an article published in the CN Reviews (4 Way Binocular Comparison, published Jan 2003), but I've corrected it here to account for refractive index and I've cleaned it up a bit and now repost it here.Īlthough this is not a particularly useful measurement in a fixed system, knowing how to calculate the focal length of binoculars may help you to understand a little more about how fast the optics are operating. I once owned a pair of 15x70s that had been converted to 25x70s by doing the exact same thing. This question was asked in another forum: "How can I determine the focal length of my binoculars"? This individual wanted to change the magnification of his binocular by replacing the eyepieces, but didn't know what focal length eyepieces to use as replacements.
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