In the field of photogrammetry, the optical system, usually represented by the glass
lens, is used for metric purposes. Therefore, the aberration characteristics of such
a lens, inducing deviations from projective imaging, has to be well known. However,
the most important property of the metric lens is the stability of its glass and
mechanical elements, ensuring long-term reliability of the measured parameters.
In case of a focus-variable lens, the repeatability of the lens setup is important
as well. Lenses with a fixed focal length are usually considered as “fixed” though,
in fact, most of them contain one or more movable glass elements, providing the
focusing function. In cases where the lens is not equipped with fixing screws, the
repeatability of the calibration parameters should be known. This paper derives
simple mathematical formulas that can be used for measuring the repeatability of
the focus-variable lenses, and gives a demonstrative example of such measuring.
The given procedure has the advantage that only demanded parameters are estimated,
hence, no unwanted correlations with the additional parameters exist. The
test arrangement enables us to measure each demanded magnification of the optical
system, which is important in close-range photogrammetry.
