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[OM] Multi coated filters

Subject: [OM] Multi coated filters
From: Chris Barrett <cpbarrett@xxxxxxxxxxx>
Date: Tue, 19 Jan 1999 06:06:03 -0800
Alexander wrote:

"What happens, when you put a multicoated filter (i.e. B+W) on a single coated 
lens?
Are the results multi coated or single coated?"

I remember the barrage of mail on this subject.

Any glass element will reflect light to a small extent due to the mismatch 
between its refractive index and that of air. Coating an element  (a lens or a 
filter) will reduce this. A single coating will be designed to give the minimum 
reflectivity at one wave length (usually green) and the reflectivity will 
increase towards the blue and the red. The results are still pretty good,  
however. Multilayer coating allows the designer more 'poles' ie wavelengths at 
which he can place reflectivity minima. He can therefore design a coating which 
has a flatter reflectivity over the visible wavelength range.

As adding a filter will tend to introduce reflections, which will tend to 
reduce the contrast of a lens a little, I think its a marginal advantage to use 
a multi-coated filter, even on a single coated lens. I would rate the best 
contrast as:

no filter>multicoated filter>single coated filter> uncoated filter.

Putting a multicoated filter on a single coated lens does not make a 
mulitcoated lens, as to be truly multicoated all the elements in the lens 
should be multi-coated on both surfaces.

I believe from previous debate some lenses may be multicoated only on their 
front surface. This would not give a great advantage over a single coated lens.

I'm sure the next question is: "Is a multi-coated lens with a single coated 
filter better than a single coated lens with a multicoated filter?"

I think the former is better since it has a greater number of multicoated 
surfaces.

Bear in mind that the reflectivities we are talking about are small 
percentages, and mulit-coating is really squeezing the very best from only the 
best lens designs.

Hoping this doesn't stir up another controversial thread.

Chris


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