9. THE CALCULATION OF THE STEREO BASE 
Prerequisite:
Deviation = constant
b_{N}  b_{F}
= = constant
For simplifications, assume:
b_{F} = b_{O}
The triangle with the opposite angles
are similar, therefore:
_{} 








_{} 














































_{} 









































Since the value of a', the image distance
in the camera, can hardly be measured, it is replaced with help of a wellknown
optical formula.
The assumption: a_{N}/a
= 1 distorts the result only ever so slightly. With a large amount of depth
of field t_{s}, a can be significantly larger
when a_{N} is big.
The value a_{N}/f then is very large. With a small a_{N} the value of a, the object distance = focal range, is not much larger, since then the depth of field t_{s}, dependent on the aperture, is also not large.
The value of a_{N}/f is then small too. Subtracting 1 or 0.8, for example, makes no difference.
According to the above equation, the stereo base b_{O} is dependent upon the selected stereo window distance from which is derived, from the focal length f and the nearpoint distance a_{N}. and usually f are constants so that only the nearpoint distance remains. The farpoint distance is not of importance and must be considered in the choice of the aperture.
If one calculates the enlargement factor V from the known formulas for the focal length f, object and image distance a, a' and substitutes them into the equation for the calculation of the base, this equation simplifies to b_{s} = /V.
Since both the depth of field and the exposure
factor are dependent on the enlargement factor, all the relations can be
depicted on graph 4. This is of special interest to macro and microstereophotography,
where a lot of work involves the enlargement factor. Graph 5 shows the
base dependent on the enlargement factor with different focal lengths =
picture diagonals.
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