Shawn and/or Janis writes:
85/2
Primary reflections: Pale green, purple, yellow.
Secondary reflections: Purple, yellow <- from SC element?
50/1.8
Primary and secondary: Blue, Purple, Yellow
35/2.8 and 135/3.5 both SC:
Purple, yellow.
================================================
the 85/2 is multicoated if there is green. the colors in the sequence
correspond to layers not elements. it is doubtful that some elements
are single coated in a multicoated lens. the 50/1.8 sounds like it
is multicoated, but have to be sure the blue isn't purple. actually,
I can't substantiate but tend to believe that the early multicoating
of olympus had blue, purple and amber, and green indicates a later
coating formula.
the way single-coatings work is that there are substances that pass
certain wavelengths and reflect others. Two such substances are known
to pass through about 950f the light across the entire visible spectrum.
these are fluorites I believe but I'm not sure. but one of them is
purple, the other amber, which is why those are the colors you see in
a single coated lens. there are different variations on single coating.
some SC formulas use purple on some elements and amber on others to
get proper color rendition. some more advanced single-coatings are
actually a 2-layer process where the purple and amber might be combined
on the same element. this is still single-coating. the late Mamiya TLr
lenses do this. there was even a formula developed by Zeiss in the 30's that
had 3 single-coating layers on the same element, but it was still just
using the purple and amber single-coating substances. what makes these
single-coating is that a single substances passesmost of hte light
through across the visible spectrum. combining them in multiple layers
on the same element is done to get proper color rendition, but they still
are single-coated lenses.
so it is not multiple layers of coating on a single element that makes
a lens multicoated. then, what is multicoating? there are other substances
that are more efficient at passing light through, passing about 990f
all light through, but they don't do this throughout the visible
spectrum. instead, they might pass 990f the light through but only
in a narrow band of the visible spectrum, whereas they might only pass
50% through elsewhere in the visible spectrum. what Pentax discovered
in the 70's was that these substances, sometimes in combination with the
coatings used for single-coating, could be layered together in very
thin layers in a very special way to cause interference between the layers.
this interference would enable the coating to pass about almost all (99.5%)
of light through across the visible spectrum. But it takes a very special
formulation and isn't just an arbitrary adding a bunch of layers. Pentax
had a patent on the original SMC process in the late 70's, just before they
switched from screw mount to K-mount. the screw mount SMC Takumars will
give any current computer designed lens a good run for its money in terms
of quality. Nikon, Leica, Zeiss, didn't have anything comparable, nor
did Minolta or Olympus.
It is my opinion that Pentax fumbled this advantage away. for about 8-10 years
they had the best optics of anyoen because of the SMC process. If Pentax
had nikon's marketing savvy, they probably could have superceded nikon
as the primary pro 35mm system. photographers are picky about having the
best quality lenses. Pentax licensed the patent to Zeiss to use as the
original T* coating, but the other companies just developed their own
multicoating formulas.
The reason multicoatings have changed is because of hte rise of computer
designed optics. This has enabled the optical companies to optimize
their multicoating formulas. There are at least two different multicoating
formulas used by Olympus. Some people claim that Pentax still uses the
original SMC formula, but this seems unlikely to me.
However, one big advantage of Pentax manual focus gear is that it is
trivial to establish teh coating status of a lens-- if is is marked
SMC it is multicoated, and if not, it isn't, but most Pentax lenses
have been SMC since K-mount came out.
j. albert
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