Re: lens coloration

[P3D John Weiler <jweiler@xxxxxxx>]

Bob Howard writes:

>>Cool rendering of lenses is really often the true rendering the 
>>film gives if not filtered (sensitive to blue), but many lenses with 
>>a lot of elements act like light yellow filter due to tint of cement 
>>or intentional dying of cement (Leitz Absorban cement) to give UV or 
>>excess blue protection....

Optical glass does not transmit all visible wavelengths of light
equally, and the effects can be additive with multiple elements.  Lens
coatings can reflect certain portions of the spectrum more than others
too.  These factors can also contribute to coloration.  Designers can
compensate by balancing the effects of different types of optical glass
and coatings to achieve good overall neutrality, but I wonder how often
this is done.

I check lenses for coloration independent of film by examining an
evenly illuminated sheet of white paper through the lens.  If the lens
is neutral there should be no color cast; the paper viewed through the
lens will appear less bright, but if the lens is neutral it will not
appear slightly yellowish or bluish.  Unless the coloration is
pronounced it may be necessary to examine several lenses side-by-side
to see the differences and then decide which are neutral.  Polarizers
often exhibit coloration that is easy to see using this method.  The
paper can be lighted with any continuous spectrum light source; I've
been able to see differences between lenses using an ordinary
incandescent bulb or sunlight.

Interestingly, two lenses which appear to have identical neutrality may 
give slides having differing color casts, especially at high elevations.
I suspect that UV, in greater abundance in higher mountain areas, will
be transmitted by some lenses more than others, and film may interpret
this as blue.  Of course a tiny difference in exposure with two lenses
may result in a density difference in the slides which may be interpreted
as a coloration when the difference is actually due to exposure.

Lenses, film and viewing system each contribute to the appearance of the
image.  Using cool lenses with film favoring a warm palette may result
in pleasingly neutral chromes, and vice-versa.  Using cool lenses and neutral
or cool film may require a viewer on the warm side to give pleasing
results (crank down the rheostat).  Film preferences may result from lenses
and viewers in use as much as the inherent characteristics of the films
themselves.  Experiment and see what looks best to you, and watch for
changes when you change equipment.

John Weiler
Columbus OH 


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