On Monday, January 21, 2002 at 13:30, John A. Lind
<olympus@xxxxxxxxxxxxxxx>
wrote re "Re: [OM] The SC MC Debate again" saying:
> At 07:36 1/21/02, Tom Trottier wrote:
> >On Wednesday, December 05, 2001 at 16:39, ll.clark@xxxxxxxxxxx
> >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)?
> :-)
Catching up (by accident).
> 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.
True.
> 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%).
Again true. But what matters is not whether the film gets 90 or 990f the
light coming in, but rather where does the reflected light go?
If you have 100f the light in a photo smeared all over it, that's just 3
stops down. Instead of shadow detail, you now have fog if it's well
spread, artifacts if not. If 1%, about 6.5 stops down, if spread, weaker
artifacts if not. If 0.1%, about 8 stops down if spread, very weak or
invisible artifacts if not. Of course, if a light source like the sun is in
the photo, 8 stops down from its exposure may be middle gray!
And what matters is the light reflected within the objective or camera that
reaches the film. Coatings on the front of a lens mostly improve
transmission, especially on the very front. Coatings on the back of each
lens are the most important. Reflections from them go towards the film.
(Front=facing subject, back=facing film)
Movie lenses are calibrated in T-stops (True-stops), which measure actual
light transmission to match shots better in a single scene, but that's not
a concern in still photography.
> 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.
A great treat for lenses using high-index glasses.
> 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
Nice page.
> -- 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?
Agreed. 3 groups, 4 lenses, minimal reflections (only 4 air-glass surfaces
inside the lens). Within its limitations (coverage, aperture), a good
design still being used today in high end lenses e.g. f/9 Nikkor-M 300mm
for view camera, Hasselblad Tessar CB f/4.8 160mm. .
And minimal glare with coatings!
tOM
------------------------ http://groups.yahoo.com/group/ottawa-photo-clubs
tOM A. Trottier, ICQ:57647974 http://abacurial.com
758 Albert St, Ottawa ON Canada K1R 7V8
+1 613 860-6633 fax:231-6115 N45.412 W75.714
"The moment one gives close attention to anything,
even a blade of grass, it becomes a mysterious,
awesome, indescribably magnificent world in itself --
Henry Miller, 1891-1980
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