23A
21
15
58
80A
Filters, Contrast and the Moon
Eric Douglass (modified from Selenology: Vol. 17; No. 1/2; p. 21-39)
Some time ago I performed a study designed to answer the question of which
filters provide the best contrast in lunar viewing. The study's design was
simple: take photographs of both a highland region and a maria using a wide
variety of filters, and then measure the contrast using a computer histogram.
The maria region chosen was about Hyginus Rille, while the highland region was
in the Cayley Formation. Filters tested were a red (23A), orange (21), yellow
(15), green (58), and blue (80A). Final photographs are shown below (note: all
are heavily contrast enhanced, and so do not represent what one actually
'sees'). The results of the study showed that in the maria region, both the 23A
and 80A provided the best contrast, while in the highland region, there were no
significant differences.
It is of note that I also took colored pictures of a cratered region and showed
it to non-astronomers. When asked which had the highest level of contrast,
greater than 90% chose the red colored photograph. This shows that there is a
subjective experience of greater contrast when using a red filter.
In conclusion, this study showed measurable differences in contrast when using
filters on a maria region of the moon--differences which are likely appreciable
in actual viewing situations. Those with the highest level of contrast were the
red and blue filters. However, on the highland regions, there were no
significant differences in contrast. Further, there is a psychological overlay
by which the vast majority of people feel that they see greater contrast with
red filters.
Hyginus Rille Region
Cayley Formation Region
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Filters 23A |