"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.

Example:
Oberwerk 15x70s

Glass out-to-out = 252mm
Prism path = 110mm
subtract 5mm for half objective thickness
Total light path approx 357mm

Mag is 15x, so divide by 16
357 / 16 = 22.3
The eyepiece has a focal length of 22.3mm

357 - 22.3 = 335mm
The binocular has a focal length of 335mm.

335mm / 70mm = 4.78
The binocular has an f# of 4.78


Works every time.

edz