"daysleeper" wrote in message news:93688168007137.NC-1.54.daysleeper@news...
Hi!
Is there a simple way to determine the focal lenght of the binocular
lense, without taking it apart?
I own 15 years old russian 20x60 Tento binoculars. Maybe some of you
know the FL of that one...
I guess it's around 400mm (F6.6), but I might be very wrong

Clear & steady skies!

Hi
There is a way based on the change eye piece location with focus.
First focus on some item at infinity. A star or the Moon works
well. Measure the height of the eyepiece from the body.
Next focus on something as close as you can. Measure the change
in the eye piece position. Also measure the distance to the object
from the front of the binoculars. Use the following equation to determine
the focal length:

1/F = 1/A + 1/F+d

Where A is the distance to the close object and d is the delta
measurement of the eyepiece position. F is the focal length.
This equation reduces to:

F^2 +dF -dA = 0

Use the quadratic equation to solve.
Dwight