Dear Folks,
Just to confuse things even more: A few months ago we all listed ours lenses
and whether they were MC or SC. When the list of 50mm F/3.5 Macros was compiled
mine stuck out like a sore thumb. I had checked its reflection against my SC
35mm and decided it was MC. But the serial number sequence in the list
suggested it was too old to be MC.
I then compared it with my MC 50mm f/1.8 - it looked SC!
So, I then compared it side by side with both the 50mm and the 35mm. It nows
seems to me to look more like the SC than the MC.
I'm not too surprised if the formulation of the multicoating (and even the
single coating has varied over the years). Both depend on the coating of the
glass with thin films of (often) metal oxides - just one for an SC lens, and
several for an MC (ie more than 2). The films are thin, around 1/4 of a
wavelength.
For an SC lens, it should be exactly 1/4 wavelength, usually of green light,
and be of a refractive index which is the square root of the glass refractive
index. This places a single reflectance minimum at the middle of the visible
spectrum.
For an MC lens the design is more complex, you can vary the number of layers,
the thickness and the indices. The aim is to get a flatter minimum reflectance
over the visible wavelengths. So many designs are possible depending on the
availability of coating materials of given indices. The index is the biggest
constraint as only certain indices are available to the coating designer. As
new materials are developed, the designer is better able to optimise his design.
The point is:
1) even for an SC lens the overall effect of lens refractive index and coating
index and thickness can be varied. It may be that later SC lenses had a
different coating which gave a slightly different 'look' to the reflection.
2) for MC lenses the variation can be even greater, both between lenses of
different types, and maybe over production runs as different coating designs
are used. The coating materials are still being developed, so it wouldn't be
surprising if a new coating design was applied to a new production run.
3) However, as a general rule of thumb, the reflection colour is a pretty good
measure, but I would try to compare any unknown with both a known SC and a
known MC lens.
Chris Barrett
Malvern
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