Unidentified Binocular Eyepiece.

[Guest]

Hi Folks

Would there be an eyepiece guru who could advise on how to ascertain the focal length of a "bodged" binocular eyepiece.  I received some advice from Scarp on SGL regarding exit pupil most suited to nebulae viewing. His opinion was that a 40mn eyepiece would provide said exit pupil on my setup.  I have an eyepiece converted from retired binos which does give me a field of view larger than my Vixen NLV 25mm eyepiece.  As to actually quantifying the number I have no idea.  Any opinions ?  

[michael.h.f.wilkinson]

I extracted a couple of EPs from a defunct pair of 7x50 binoculars, and from the 200 mm focal length of the objectives (easily measured by projecting the moon on a piece of paper and measuring the distance between lens and paper), I was able to work out a focal length of around 28mm for the EPs (which seemed to fit the fact that they offered a bit less magnification than my 25mm Ortho).

[Guest]

Ah yes, I can see how that works, 200/28 gives approx x7.  Alas, the rest of the binos are long lost but I do recall they were an old set of 10x50's. 

Thanks. 

[Peter Drew]

Eyepieces from average 10x50 binoculars are usually in the 18mm-20mm focus range.     ?

[Guest]

Thanks folks. I've another set of de-collimated 10x50's that I'm going to strip down and use Michael's method. (They took a knock and went slightly out. Tried to readjust the prisms: you can guess the rest....) 

Cheers all.