P3D Re: Rever DOF scale: stereo fantasy

[michaelk@xxxxxxxxxxxxxxxxxx (Michael Kersenbrock)]

> 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

My Realist says that f/16 with a near point of 4-feet,
the far point is at Infinity!  This particular (older)
Realist doesn't even have a DOF scale on the focus knob
(has a chart under the lens cover).

That was the crux of my original point (Realist maybe fibs
a little too (not your realist, my realist)).

> 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.

If you mean that one's expectations of DOF may be
exaggerated if one believes camera markings, then 
I agree and I think this applies to most if not all
cameras.  Not just the Revere (of which I have one
too).  Especially old ones not recently tested.

The Vivid has a cool little mechanical DOF "computer"
that's been called "generous" in it's DOF prediction.
But I love it anyway.  :-)


> > 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.

If it says "10 feet", then forever and ever, when
that point is used with the focus knob, the camera will
be focused at 10 feet (or whatever it started life with)?

I seem to recall people wanting to use ground-glass to
adjust their camera's focus.  There would be no point
in doing so if the adjustment screws/nuts in cameras
never need adjusting again once they leave the factory.

It's isn't the paint that migrates, it's the focus
mechanism the markings claim to represent that might get out
of adjustment (and make the markings fib).

So if it tells you that your DOF is "3.5 feet to 13" feet
at a particular setting, it REALLY may be 4.5 feet to 20 feet
or some such because your camera isn't at the setting you
think it is.  So the problem may not technically be that the
DOF was improperly calculated, but in a practical user sense,
the DOF isn't what is marked and something at 4 feet that
was thought to be in focus, isn't.  But if a couple stops
of margin are added in,  one is more likely to be covered.

> > 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.

I quite agree, but I think that it applies to Realists as well,
unless the particular camera has been recently tested and/or
calibrated.  At least based on my  Realists.  :-)

> 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.

I think that rule of thumb is probably a good one for all
50's stereo cameras (especially a FED too because its )*&)&_(^&
focus knob won't be where you left it :-).

I've about a dozen or so 50's stereo cameras and I wouldn't
trust the "edges" of the DOF indications on any of them unless
my experience with the particular one justified it.

But then also, there's a school of thought that one stops down
the camera "all the way" and does everything possible NOT to
ever change the adjustment (and a person I know who does
this takes magnificient stereo photos).  :-)


Mike K.

> 
> Paul Talbot
> 
> 


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