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Noell,

> My question would be what is the 130 mm which I see as the maximum
> distance where I get a sharp image looking through the objective lens?
> Can that reliably be treated as the focal length of that lens?

Only approximately. The better way, I suppose is to focus the Sun image
on a sheet of paper, and look at the distance (from the lens
to the paper) when the spot on the paper
is smallest and sharpest.

For such small focus length it should work well.

Alex.

  If so,
> things fall into place.
>
> Surplus Shed lists lens by type, diameter, and focal length. No diopters.
> The ID of the lens holder is 18.2 mm. They have a Plano Concave of 12.5 mm
> & -43 and other PCV’s and DCV’s  that range from 15 to 17 mm and -33
> to -39 f. No exact 18.2 & -43 but close enough.
> Regards, Noell
>
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