Macrostereo Formula

[P3D Paul S. Boyer <boyer@xxxxxxxxxxxxx>]

Proposed Handy Table for Stereo Close-ups

After some years of playing around with various formulas, I
think that I have reached the point where I have a simple
solution to the problem of calculating lens-base (or
interlens separation) for close-ups.

Most of the modern macro lenses for SLRs are marked
in magnification (or its reciprocal) rather than focal distance.
I have also found by experience that it is much easier to set the
desired magnification and then move in and out to focus
using a focusing rail, than to try focusing conventionally with
macro and near-macro subjects.  What I have needed is a formula
for interlens separation which uses M=magnification
instead of focal distance.

Heres the formula:

I = 106.68*w / F*(M+1)

where I is interlens separation, w = width of the
diapositive [36 for full-frame, 31 for European, and 23 for
Realist], F = focal length of the lens, and M = 
magnification.  All measurements are in mm.

I set up a simple Excel worksheet to make tables of values of
I for M.   For a given favorite macro lens and a favorite
format, F and w will be fixed, and M will be the only
independent variable.  I put values of M (3, 2, 1, 0) on
the vertical, and tenths of M on the horizontal axes of
my table, and let Excel fill in the values according to
changeable entries for F and w, which are entered in
adjacent cells.  The values are very realistic:
they assume that the target is right at the window.  If
one wishes to drop the target back from the window,
one shoul reduce I slightly.  Obtain I by moving the camera
laterally on a calibrated focusing rail between exposures.

Here is a simplified, abbreviated form of a chart for determining
interlens separation in mm from values of M by 2/10ths of M.

F = 100 mm
w = 23 mm

        0.0      0.2      0.4      0.6      0.8
3      6.1      5.8      5.6      5.3      5.1
2      8.2      7.7      7.2      6.8      6.5
1    12.3    11.2    10.2      9.4      8.8
0    24.5    20.4    17.5    15.3    13.6

For example, if the magnification is 2.4, the interlens separation,
with a 100-mm lens and for Realist format, would be 7.2 mm.

On my Minolta 100-mm macro lens the calibrations are according
to the reciprocal of M, so I have constructed the chart below.  I
have been more realistic in making this table by recognizing that
I probably cannot set interlens separation closer than 1 mm!

F = 100 mm
w = 23 mm
Values of I/mm for values of 1/M:
       0.0     0.2     0.4     0.6     0.8
1      12      13      14      15      16
2      16      17      17      18      18
3      18      19      19      19      19
4      20      20      20      20      20
5      20      21      21      21      21
6      21      21      21      21      21
7      22      22      22      22      22
8      22      22      22      22      22
9      22      22      22      22      22
10    22      22      22      22      22

I would welcome reports from users of this technique.
--Paul S. Boyer   <boyer@xxxxxxxxxxxxx>


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