Rich, The first number is the magnification of the bino. In a 15X70 the magnification is 15. In the 12X30, the magnification is 12. The second number is the size of the objective lens in millimeters. The 15X70s have 70 mm objectives. The 12X30s have 30 mm objectives. If you divide the objective diameter by the magnification you get the exit pupil. Exit pupil is the size of the pencil of light coming out the eyepiece end. In the case if the 15X70s the exit pupil is 4.66 millimeters. A completely dark adapted eye is as large as 8 millimeters in diameter, although some of us old guys can only manage 4 or 5 millimeters. You should make sure the exit pupil is not larger than the eye opening IF you are interested in light gain. If you are using them in the daylight, the exit pupil is somewhat irrelevant. The larger the diameter of the objective, the more resolution (more detail) in the image. The 70 mm binos will have 2.33 times more detail than the 30 mm binos. Hope this helps a bit. Brent --- Rich Allen <rico@ricosweb.com> wrote:
I've been following these threads about astronomy suitable binoculars with interest. I'm curious to learn about how the power rating translate. What's the difference between a 12x30 vs. 15x70, for example? Are there any good web sites that explain this? My initial Google searches came up with lots of irrelevant hits. Thanks!
Rich Allen
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