Re: Draft: Matching lens focal length and precise focusing

[T3D Dr. George A. Themelis <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

>From: T3D John Ohrt  <johrt@xxxxxxx>
>Please comment!!!!! You won't hurt my feelings!

OK, you asked!

>If you have questions, please join the tech-3d group and pose them
>there.  "What does this mean?" is a very common question there.  The
>members range from novice to practising optical professionals and access
>to the list is free.

Perhaps this commentary could be omitted, especially the part about who
are the members of tech-3d and how expensive it is to join. I think our
leader (John B) asked that this post is kept short and focused.

>The specifications for a 3d system
>
>A system specification describes the end to end (or net) tolerance.  For
>example, if your system comprises a camera and a viewing aid,  the
>specification for the maximum difference of magnification percieved by
>the viewer includes the effects of both the camera and viewing aid.  Do
>note that "cheating" is allowed.  In this example, an enlarger can be
>used during the processing phase to correct for known camera errors in
>some situations, such as holmes cards.

Not clear for me.

>The specs quoted are largely from MIL-HDBK 141,  Optical System Design.
>While most of us joke about MIL SPECS, this particular handbook was
>written by civil optical systems engineers and scientists to support
>military training.  Many optics specialists consider this the best
>single optical design handbook written.  It is well cross-referenced.

Too many words here... Is this commentary necessary?

>When possible, some specs were cross checked against other sources.  The
>agreements were either matched or were close.

Excuse me?  What does "the agreements were matched" mean???

>These specifications are for operational systems that may have to be
>operated for long periods of time.  If a system doesn't meet these
>specifications, some people may have not perceive the 3d effects or may
>tire very quickly.  There are many 3d system types and it is rare that
>one will not excell at some application.  So if you are on a budget, do
>mention that when asking questions.  The photo-3d group can guide you to
>approaches that work, and hopefully one that fits your budget.

I am sorry but I have a hard time following these writings... What was
the question again?  Why don't you clearly state what the objective is?
I still don't understand what you are trying to explain. It has not
been defined!  Anybody with a short attention span (John B's definition
of the average photo-3d reader), has pressed the <Delete> key at this
point.

>Unequal Magnification:   0.5% max,  some people can tolerate 2%
>
>Pupillary adjustment:      50 to 76 mm
>
>Rotation or skew:          minimize  (tiring)

Are you going to define this last term?

>Illumination:              10% max    (avoid rapid changes in
>illumination level)

What "illumination" are we talking about?

>Vertical imbalance:        0.5 prism diopter
>
>Horizontal imbalance:      0.25 prism diopter  ( 0.33 tolerable at low
>magnifications)

At this point, I am not sure what are these quoted specs.  Note
that I have not followed earlier discussion on the subject...


>Lens Matching
>
>As you may have noticed, the system tolerance for magnification is
>0.5%.  Depending on your system, you have two different sets of matched
>lens may be involved and the tolerances of both sets combined must be
>less than 0.5%.

What is the definition of the _combined_ tolerance?

>Note that the difference in magnification is the issue.  That is, you
>don't have to measure the magnification for each lens in the set.  All
>you need to know is the variance, lens to lens.
>
>One basis for doing this is to use each lens to observe the same target
     ^^^^^^^^^^^^^^^^^^^^
basis for doing what?

>for the same location.  The target is then measured at the focal plane.
>The percent difference in length measured at the focal plane is the same
>as the percent difference in magnification.  Exactly how you make the
>measurement depends on your lens/camera and other factors.  That is the
>subject of another technical note.  By the way, if you compare the
>measured length to the target dimension and accurately measure the
>distance to target, you can calculate the precise focal length.

Too many words, not clear definitions...

>Precision Focusing
>
>There are many applications that do not require precision focusing, but
>there are some that do, and others that could greatly benefit from the
>technique.  The preceeding test is one that benefits from precision
>focusing.

Which preceeding test?

>Some cameras are highly calibrated and others, such as large format, can
>be adapted to provide precision focusing.
>
>Cameras that use rolls of film are a different matter.  Sure the
>distance to subject marks are in the ballpark, but rarely calibrated for
>one simple reason.  No matter if you look through a viewfinder, a range
>finder, an SLR or whatever, you really don't know where the film is.
>
>
>____________________________     <- film pressure plate
>|| ---------------------- ||     <- film
>||  ||                ||  ||
>||  ||                ||  ||
>||  ||                ||  ||
>
>                       ^   ^
>                       |   |
>                       |   -------- Pressure plate rail (1 of 2)
>                       ------------ Film rail (1 of 2)
>
>As shown, the film is restrained by the film pressure plate and the film
>rails.  The pressure plate is spring loaded and presses firmly against
>the pressure plate rails.  The resulting gap between the pressure plate
>and the film rails is usually wide enough to handle the thickest and
>least flexible films, but narrow enough to handle the thinnest and most
>flexible films.  As you might expect, the thinnest films can be some of
>the finest grain films.  Fine grain film is desirable for 3d work as the
>viewing of 3d is more sensitive to grain than the more common 2d
>viewing.  On top of this, is that the ideal image plane lies within the
>film, but where varies from film to film.
>
>Is it expensive to determine a precise focus.  Not really, but the "how
>to" is the subject of another article.  Some methods are inexpensive.
>
>>John Ohrt,  Regina, SK, Canada

Is that it?  At this point I have to recheck the title of this posting...
Are these two subjects (matching of FL and precice focusing) part of the
same problem or two unrelated problems?

Frankly, I did not understand much from this first reading... Do I need
to take a paper and pencil and try and read this a couple of times to
figure out what the issues are and what are the solutions?  I am left
with one basic question: "what does that mean"... should I ask tech-3d?

Am I too negative?  Is it just me???  Somebody, please, say something!!!

-- George Themelis, not in a particularly good mood tonight...


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