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Macrostereo Formula
 From: P3D Paul S. Boyer <boyer@xxxxxxxxxxxxx>
 Subject: Macrostereo Formula
 Date: Sun, 26 Oct 1997 20:44:59 0500 (EST)
Proposed Handy Table for Stereo Closeups
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 lensbase (or
interlens separation) for closeups.
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 nearmacro 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 fullframe, 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 100mm lens and for Realist format, would be 7.2 mm.
On my Minolta 100mm 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>

End of PHOTO3D Digest 2377
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