[MF3D.FORUM:1143] Re: Sputnik focus fix

[Richard Rylander <rlrylander@xxxxxxx>]

Paul Talbot wrote:

>
> Most sources talk about setting focus based on infinity, but
> I'm not sure I understand why, except that it seems to be far
> less demanding.  Setting focus based on, say, 15 feet, requires
> far more precision.  It seems to me that two lenses that appear
> to be in synchronized focus at infinity could easily be off by
> several feet when focus is changed to a closer distance.
>
> So my tentative conclusion is synchronize focus at infinity
> for a "quick and dirty" solution, but synchronize focus at
> a much closer distance for pinpoint accuracy.  Am I going
> wrong somewhere?
>

If the goal is to get both taking lenses and the viewing lens to agree,
it shouldn't matter whether you use infinity or a nearby target   ####
AS LONG AS THE FOCUS TRACKS PROPERLY FOR ALL LENSES ####  -- This is NOT
a given with the Sputnik.  The viewing lens is a different [simpler]
construction than the taking lenses (it may also have a slightly
different focal length), and front cell focusing is used rather than
moving all lens elements en masse.

One important reason to adjust the focus using a target at infinity is
to guarantee that you CAN focus at infinity.  After fiddling with the
lenses at a near distance, it wouldn't surprising to find that you
cannot then focus at infinity.  Personally, I first adjust the viewing
lens for infinity focus at its far limit (and make sure the distance
scale agrees), then choose a subject at about 15 feet (as you suggest)
before adjusting the taking lenses to agree with the viewing lens.
Since most of my stereo photos are of nearby subjects (you don't get any
stereo effect at infinity), I want to ensure the best focus match here.
The two taking lenses of the particular Sputnik I have are fairly well
matched, but there is some minor focus discrepancy with the viewing lens
for infinity subjects - I rely on depth of field for those cases.

I hope this is helpful.

Richard Rylander