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Re: [OM] here is an interesting question...

Subject: Re: [OM] here is an interesting question...
From: "Giles" <cnocbui@xxxxxxxxxxxxx>
Date: Tue, 19 Jan 1999 11:39:43 +0000
The idea behind multi coatings (MC) is not to magically transform the light in 
some 
way as with a filter.  For MCs to have the intended effect they must all be 
combined together on the same surface.  Who is Mr Carrington?  Is Alexander 
perhaps 
referring to Michael Covington?

The main purpose of MC is to reduce the amount of light reflected off a lens 
elements surface.  The light reflected off elements surfaces is a problem 
because 
most lenses, especially zooms, contain numerous elements and therefore numerous 
surfaces - ignoring cemented elements.

If you can imagine light entering a lens through the front element, after 
passing 
through it then encounters the front surface of the second element.  Most of 
the 
light passes through, however a small portion is reflected back at the 
underside of 
the the first element. Some will pass through, but again some will be reflected 
back 
into the lens.  Some of this may well pass through all subsequent elements and 
reach 
your eye or the film and contribute to the final image in the form of lens 
flare.

This process can happen for all the elements.  So the main purpose behind MC is 
to 
reduce, as much as possible, the light reflected off an elements surface.  
Multiple 
coatings are employed because they can reduce reflections more than single 
coatings 
(SC).

A beneficial side effect of MC is that the total transmission of light through 
the 
lens is maximized.

Now sticking an MC filter on the front of a lens is not going to  magically 
transform 
the coating on every element in the lens to which it is attached.  In fact, it 
could 
be argued that it might increase flare compared with an SC filter because 
slightly 
more light passes through into the lens ;-)

Giles

c.e. packard wrote:

> I agree with Mr. Carrington. If the light is travelling through more
> than one lens coating , albeit on the lense or filter, couldn't one
> agrue that the light is passing through "multiple" coatings?
> 

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