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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