At 07:36 1/21/02, Tom Trottier wrote:
On Wednesday, December 05, 2001 at 16:39, ll.clark@xxxxxxxxxxx
<olympus@xxxxxxxxxxxxxxx>
wrote re "Re: [OM] The SC MC Debate again" saying:
> In <4.3.2.7.2.20011204230130.05629b60@xxxxxxxxxxxxxxxxx>, on 12/04/01 at
> 11:27 PM,
> "John A. Lind" <jlind@xxxxxxxxxxxx> said:
>
> >IMHO, much more ado about SC versus MC is made than the benefit
> >provided by an MC lens. There are aspects of the lens design that
> >have a much greater effect on contrast and flare resistance than going
> >from SC to MC.
>
> Listen, oh brothers, to the clear voice of reason...
Hah!
Yeah, just stick to small-aperture non-retro Tessars like the days before
coating. Forget big (> 3.5) apertures or zooms. Always have the light at
your back, and shoot in the dark. Have a bellows lens hood. That's the way
to avoid flare.
tOM
Tom,
Are you "harvesting" the archives (look at the date of what you quoted)? :-)
If you were comparing no AR coating to single- or multi-coating I would
agree, but not in going from a single-coating to multi-coatings. A single
coating does absolute wonders for flare resistance compared to none; the
improvement is dramatic. By comparison, changing from a single to a
multi-coating formulation only makes a small incremental
improvement. Reason? All multi-coatings do is make the effect of AR
coating more "broadband" and even across the entire visible spectrum. A
properly selected and applied single coating is already fairly
broadbanded. I will freely admit this is "per surface" and the reflection
(loss) is cumulative. Very complex zoom lenses with fifteen or more
elements will suffer transmission loss and flare much more than
primes. The loss is multiplicative; i.e. 99% transmission per surface
through one surface is 99%, but through ten surfaces it's about 90% (99%^10
= ~90%).
The efficacy of AR coatings (single or multiple) is very dependent on
coating selection for its index of refraction compared to that of its glass
substrate (the lens element) and its deposition thickness (based on light
wavelength). I don't doubt some lens makers have done a better job of AR
coating formulation for the specific optical glasses being used. More than
one type of glass is used in a lens. Lens design is not just shape and
position relative to the other elements/groups, it's also its index of
refraction. Furthermore, multi-coatings gain less over single coatings
when used on higher refractive index optical glass. A properly chosen
single coating increases efficacy across the bandwidth of visual spectrum
as refractive index of the glass substrate is increased.
For those interested in exactly how AR coatings work and the "why" behind
some of the above, go here and scroll down to the section on "Optical
Coatings" (about 2/3 of the way down the long page). Clue: AR coatings
are NOT "filters:"
http://utopia.cord.org/cm/leot/course06_mod08/mod06_08.htm
-- John
P.S.
Don't poke too much fun at the Tessar. It's one of the 20th Century's
finest lens designs (almost 19th Century) with elegant simplicity, and is
likely the most copied lens formulation ever (after Carl Zeiss' patents ran
out at about the end of WWI). Unfortunately it doesn't hold up well in a
formulation faster than f/2.8 (aberration), and I probably would not use
one for macro "copy" work which requires a very flat field, but how many
are using Zuiko's faster than f/2.8 in other than "standard" 50mm lengths?
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