[MF3D.FORUM:1144] Re: Sputnik focus fix

[Paul Talbot <ptww@xxxxxxxxx>]

Paul Bigelow wrote:

> Has anyone set the focus at, say, 2 meters
> (measured exactly) with the aperature wide
> open and then checked for focus at infinity
> and see if the distance scale and camera focus
> tracked accurately?

Yes, I did that a few days ago.  I was going to
follow up my advice to Harold Baize once I
recovered from being up to 4 and 5 am working
(mostly) on Sputniks!

My finding was that although the viewer lens and
the two taking lenses track each other reasonably
well, the distance scale on the Sputnik is out
of synch with the lenses.  With the lenses set to
focus at 5 meters, with the distance scale in synch,
it is not possible to focus the lenses to infinity.
The little stop pin on the distance dial prevents
the lenses from turning far enough to reach infinity.

> Every camera (with focus) I can remember has
> infinity at the end of travel.  In that case, the
> manufacturers must have set set focus at
> infinity.

On the Sputnik and probably many other cameras, the
"end of travel" is determined first by a stop pin or
similar arrangement.  The end of travel as determined
by the threading of the lenses allows them focus "beyond
infinity."

> The most critical focus would be with
> the aperatures completely open.

Yes, absolutely.

Richard Rylander wrote:
> 
> 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...

A close by image is much easier to check for focus, as the image
on the ground glass is brighter and clearer and not obscured by
atmospheric haze.  A good flat-surfaced, high-contrast, well-lit
close image is easily set up, and a lot easier to see on the
ground glass.

>  ####
> 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.

Fortunately on the Sputniks I checked the focus tracking among
the three lenses seemed fairly consistent.  That makes sense to
me, as about all that should be required is thread spacing that
is consistent among the three, and roughly matching FL.  But
as noted above, there is a problem with the distance scale band.

> 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.

Certainly true on the Sputnik; if you agree the distance marks to
the measured near distance, the stop will prevent you from setting
the focus to 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.

That pretty well sums up what I was going to write as the way
I'd settled on to do the focus adjustment.  Took me a long time
to get there, though!  :-(

I picked up a 30x focusing loupe to work on the Sputniks.  It's
amazing how much more accurately you can check the focus with the
30x loupe compared to the basic 8x loupe.  I actually use the 8x
loupe to get the rough focus first, and then nail it down using
the 30x loupe.

Paul Talbot