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Message: <blockquote data-quote="Binastro" data-source="post: 3360129" data-attributes="member: 111403">Hi Red Shoulder.The field lens is the element furthest from the eye.What is the clear aperture of that?Measure the focal length of the objective and divide by 7. That will give you the fl of the eyepiece approx. It is likely the magnification will change slightly at the edges, but we can forget that.39 to 45mm is not the eyepiece fl. The Nikon Action VII has aspheric eyepieces and the magnification varies slightly across the field.The aspherics maybe a plastic on glass sandwich or just glass?Zeiss had a mechanical aspheriser in the 1930s, but not their invention.Here charity shops are full of faulty binoculars for a fiver. You could find box loads.It is lucky when you find a good one, and occasionally gems for very little money.In Sweden they have those Visby aspherical magnifiers made about the year 1,000.P.S.Red Shoulder.Did you take the eyelens out and are you saying that is 39 to 45mm focal  length?If so, I apologise.I thought you were quoting the whole eyepiece focal length.P.P.S.I think what you are saying is that the element nearest the eye is 18.9mm diameter and is an achromatic doublet of 42mm +/- 3mm focal length.If this is correct.Then one still has to find a doublet of the correct thickness. If lucky it might work.And that the eyepiece is 4 elements.I don't know when the Nikon Actions started using aspheric eyepieces.</blockquote>