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Re: [OM] The SC MC Debate again

Subject: Re: [OM] The SC MC Debate again
From: Damon Wood <deewhy_au@xxxxxxxxxxx>
Date: Wed, 5 Dec 2001 17:00:27 +1100 (EST)
John wrote:

-------------------

Damon,
Don't think of AR optical coatings as "filters."  They're not.  They 
work 
by creating an interference with the light waves passing through them.  
The 
single-coating model on the front objective is probably the easiest to 
explain.  The single coating has an index of refraction between the 
glass 
it coats and air.  As light strikes the coating, some of the light is 
reflected from the coating.  The remainder passes through to the next 
interface of coating and glass.  Some of the light reflects from this 
interface also and passes back out past the first air-coating 
interface.  The coating index of refraction and its thickness is 
selected 
so that the light waves from the two reflections are now 180 degrees 
out of 
phase and "cancel" each other.  "Conservation of energy" states that it 
cannot simply disappear!  Nor is it absorbed.  Where does it go?  Past 
the 
second coating-glass interface and into the glass.  This is how an AR 
coating improves light transmission through the optics it coats.  
Because 
various ray paths are at different angles, and a coating is only good 
over 
a portion of the visible spectrum, it's not 100 0.000000e+00ffective.  Thus, 
light 
transmission into the glass is not 100%, but for a single coating it's 
improved quite significantly compared to no AR coating.

For a single coated lens, the coating works optimally in the 
yellow-green 
center portion of the spectrum.  Multi-coatings have the centers of 
each 
coating spread across the spectrum (e.g., for three coatings, one would 
be 
in the upper red, another in the green and the third in the lower 
blue).  This improves transmission better across the entire spectrum, 
and 
this is why a multi-coating makes a slight improvement over a single 
coating, but not nearly as much as a single coating does compared to no 
coating.  It also improves contrast slightly.

I suppose you might be able to measure the slight difference in 
spectral 
transmission between single and multi-coated lenses of the same 
formulation 
using sophisticated optical lab equipment.  However, in practical 
application I have yet to notice the difference except in very slightly 
better flare control and then it's only under very difficult lighting 
conditions prone to extreme flare.  The one SC lens I have the most 
experience with under high risk of flare is the 35/2.8 Zuiko Shift 
which 
I've used for night photography in urban areas with many, very bright 
pinpoint sources of light.  No objectionable flare has ever been 
observed.  Refraction around the corners where the aperture blades meet 
causes a much more noticeable star effect, the points of which are 
still 
very, very small compared to the star filter I have.

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.

-- John

---------------

Well, I cant argue with that can I. Tis almost an essay!

Thanks very much again John. 

PS TO ALL: Never feel stupid to ask what might seem stupid questions - look 
what happens! :)

Thanks to those who participated in my questions as of late.

Damo


Damon Wood
Dip. Technology (Applied Science) 
Advanced Manufacturing Technology Centre (AMTC)
Sustainable Development (BSc) 
Undergraduate Student
Murdoch University
Secretary M.C.C. WASTCA


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