Draft: Matching lens focal length and precise focusing

[T3D John Ohrt <johrt@xxxxxxx>]

Please comment!!!!! You won't hurt my feelings!

Draft:  Matching lens focal length and precise focusing

Introduction

Originally a "matching lens" discussion expanded into a more global
inquiry to include "precise focusing"

This is the first Technical Note from the Tech-3d group on this subject.  
Amongst other data it lists 3d system level specifications to ensure that 
you view the following discussions in perspective.

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.


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.

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.

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

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 excel 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.

Unequal Magnification:   0.5% max,  some people can tolerate 2%

Pupillary adjustment:      50 to 76 mm

Rotation or skew:          minimize  (tiring)

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

Vertical imbalance:        0.5 prism diopter

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


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%.

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 obsurve the same target
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.


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.

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


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End of TECH-3D Digest 132
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