How to determine binoculars focal lenght?

On 27 Aug 2003 19:05:38 -0700, (edz) wrote:

"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've measured several and then used the information to develop this
rule. It applies to standard porro prism binoculars that have no
special internal amplifying or reducing lenses.

Measure the outside dimension from objective glass to outside eye
piece glass (not rim to rim, glass to glass). Of course you need to
start with rim to rim, but you then subtract for lens recess from
rim.

Subtract 5mm for half the thickness of the objective lens. You can
skip this and still be fairly accurate.

Add for prism light path. Small binoculars 7x35s 8x42s, 10x50s, all
had about the same, 90mm to 100mm thru the prisms. Large
binoculars, 16x70s, 20x80s had about the same, between 120mm and
130mm thru the prism light path.

Use a prism light path value based on the size of your binocular and
add it to the out-to-out. You will not be off by any significant
amount if you are a little off here.

Divide the resultant total light path by (magnification + 1). For
any standard binocular, this will give you the focal length of the
eyepiece.

Remember the focal length of the binoc plus the focal length of the
eyepiece is the length of the light path. Subtract f of eyepiece
from total light path = focal length binocular.

You can divide focal length by objective and get the f# of your
binocular.


Thank you, it's very helpfull.

So, for my 20x60, the objective FL would be around 330mm (F5.5), and the
eyepiece FL 16.5mm. Makes sense

Clear & steady Skies!

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"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