P3D Re: Rever DOF scale: stereo fantasy

[Paul Talbot <ptww@xxxxxxxxx>]

Michael Kersenbrock and I wrote:
> 
> > > >
> > > > pt: Be very wary of those DOF scales on the Revere!
> > >
> > > mk: And the Realist Cover?  Not much different than the Revere!
> > pt:
> >  I beg to differ.  The distance between 3 feet and 4 feet is huge
> > when you are talking about focus.  On the DOF scale on the Realist,
> mk: 
> Revere (as quoted):  3.5 ft to INF
> My Realist (cover):  4.0 ft to INF
> 
> If one wants infinity AND something at 3.49 feet, then
> yes, there's a difference, but if one steps back
> just a bit I find it hard to see those two "specs"
> as being wildly different.

You have to look at it from the other direction to appreciate
the difference.  Assume your closest object is 3.5 feet and
you can't change that.  By framing the subject or other
composition technique, you are able to limit the far point.
Where should you set the far point?  Per the Revere DOF scale
it can be at infinity.  Per the Realist DOF scale it can be
no further than about 13 to 14 feet.  That's a big difference.

For comparison, using f/16 and setting the near point at 3, 3.5,
and 4 feet gives approximate maximum far points of:
      3:    8 feet
      3.5:  13-14 feet (approx)
      4:    a little over 25 feet

> If one has that FULL
> range in a set of slides, mounting them
> properly for projection might be tricky I suspect.

Agree.  That is why I repeated Piper's advice of limiting the depth
range to that which can be kept within the f/8 marks.

> So in a practical sense, they are identical even if one
> is fibbing more than the other (or not) whereas one
> wouldn't be USING the full indicated range anyway.

Disagree.  I carried out the f/16 example just because it
was started by someone else.  The Revere and Realist DOF
scales are very different for all f/stops.  If you are
keeping your depth within the f/8 marks as recommended,
the same issue of poor focus (and too much depth) with
the Revere will be present.

> Tiny
> distance numbers on mechanical knobs, IMO, are at best
> "rough guidelines", not something exact.

Accepted.

> The design,
> fabrication, and calibration (over time) tolerances
> seem unlikely to be tremendously precise.

The numbers on both the wheel and the camera body are
engraved and/or painted.  It is hard to imagine how they
might migrate over time.

> Anyway, it usually isn't a good idea to push the edges
> of "specs" unless one "gotta".  :-)

True, but with the Revere you can be past the edge and not
even realize it, if you don't consider the inaccuracy of the
built-in DOF scale.

This is *not* Revere bashing.  I have one and I like it.  But
the DOF scale is plainly unreliable.  When I asked on P3D some
time back what other Revere owners do about this, someone
suggested using the DOF scale that is two stops wider than the
actual shooting aperture.  I'm still looking for a better fix,
because with that approach I can never be sure how close to
the edge I really am.

Paul Talbot


------------------------------