Pål Jensen wrote:
> There is indeed a difference between SMC coatings from different eras. Lately,
>however, Pentax seems to use different coating for different lenses. Eg, FA-series
>lenses may vary widely in coating properties. An example: The FA* 85/1.4 and 77
>Limited have a dark violet color while the FA* 200/4 ED IF Macro show a greenish
>color tint with no hint of violet. Its apparent to me that Pentax uses coatings with
>different properties for these lenses.
Dark violet reflections mean coatings to optimize transparency for yellow-green
central part of the visible spectrum, while green reflections are due to maximum
transparency at the red and blue ends of the spectrum. Looking at others primes it's
easy to generalize Paal's observations.
Then why would Pentax use different light transmission patterns in long tele
lenses as opposed to normal - short teles? It must relate somehow to the glass
composition rather than the optical scheme. My guess is this is a subtle filtering
technique to compensate for specific light absorption in elements made of different
glass. To further support this, if we look at wide to tele zooms it's easy to notice
these have 2-3 reflections of
different colour. As each reflection comes from a different element, than some
elements must be differently coated than the others. As these zooms employ very
different glass (plastic!?) to achieve both wide and tele good performance, it makes
sense to suppose these have different light absorption properties, so they have to be
compensated individually. How can this be achieved better than by coatings? It's still
SMC, it's still 11 layers or
more, but balanced for a different light transmission pattern to match light
absorption of that particular element.
Yours wildly speculating, Alin
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