Re: [ATM] Eyepiece

["vladimir" <svla@isp.com>]

Saqib Sajjad wrote: 

> i saw couple of microscope eyepieces available in
> market.my question is can an eyepiece of a microscope
> be used for telescope's eyepiece ? and the
> eyepieces(3) had written 20x,40x and one had 80x
> written on it. there was nothing else written on it.
> so how do i know its focal length?

The least object distance at which an average eye can properly focus
is about 10" (250mm). An eyepiece does
the trick of reducing this distance to its focal length. Thus,
eyepiece magnification is given by m=250/f, with "f" being
the ep f.l in mm. A 20X ep has 12.5mm f.l., 40X 6.25mm, and so on. 

Vlad

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