Re: [APML] Proper Use of Varimagni Finder

[Robert Reeves <reeves10@swbell.net>]

> I have found that the varimagni finder is pretty much useless for
> astrophoto focusing- not enough magnification. For focusing piggyback
> shots, I just set the lenses at infinity.

Low power magnifiers like the Varimagnifinder and the Nikon DW-2 or DW-4 are
best used for composing the image.  Too low a magnification for determining
focus.  But then, there are too many variables to insure that at wide
apertures what you see focused in the viewfinder is really focused on the
film.  The mirror or focusing screen can be off slightly and still work
great for terrestrial photography but not for wide aperture work on pinpoint
light sources.
>
> You can determine if the infinity mark is accurate by attaching tape with
> many small marks (ie a ruler scale) to the lens and making a series of
> test exposures with a stationary tripod, each time moving the focus
> slightly. Then just use the mark that gives the thinnest line on the test
> shot.
>
> Torsten

A Hartmann mask will nail the focus point even better.  A quicky description
of how to use one to test a lens' focus is on my web site at
http://www.robertreeves.com/class12.htm

Robert Reeves                      reeves10@swbell.net
520 Rittiman Rd.                   www.robertreeves.com
San Antonio, Texas 78209    210-828-9036
USA                                     29.484  98.440  200 meters


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