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Transactions of the Symposium on Photo Interpretation

It seems a risky task to judge the results of the measurements of each property
and to classify them as “Good”, “Tolerable” or “Poor”. In one way or
another it has to be done. We, therefore, attempted it, and proposed realistic
specifications. Misinterpretation of the resulting “Performance-sheets” of
stereoscopes cannot be avoided completely; we only hope that it will occur
less frequently than persistent misbelief in qualities of various stereoscopes
without knowing any facts except weight and price.
Specifications for mirror stereoscopes (pp. 48 and 49)
Each property is presented, together with its qualification. The sheets are
a guide for the testing agency, and may also be of value to the designer for
future development.
Performance of stereoscope
Each stereoscope possesses a set of sheets indicating how well each pro
perty conforms to the proposed specification. Results for new stereoscopes
will be published only after authorization by the manufacturer. No single
figure for the overall quality of the stereoscope can be given.
Use of specifications
The specifications should be judged according to the task for which the
stereoscope has to be used. This has consequences, not so much for the way
in which the results are specified (“Good” is good and “Poor” is poor), but
for the relative importance of the properties. Examples of when the qualifica
tion “Poor” is irrelevant are:
4. Observed area, when used for point transfer.
11. Eye-base (59-69 mm), when one has 65 mm eye base.
33. Illuminance at retina, when used for a short time only.
45. Useful focussing range, when one has normal eyes.
51. The Position of the eyepiece, when used for a short time only.
62. Protection against rough handling, when used by scientists.
On the other hand, a certain use can change the weight of some properties
(ie.g. in field work 62-66 are important) or even change some specifications
{.e.g. for point transfer, the useful magnification - in general 4 X - might be
6 X or more).
Conclusions as to the qualifications of a particular property
It is for the user to compare different stereoscopes, because only he knows
the relative importance of the properties listed.
We will now consider more closely some particular properties. Out of each
group one interesting property has been chosen.
2. The position of the Exit Pupil (the bright disk which can be seen when