jowilcox@xxxxxxxxxxxxxxxxxxx wrote:
>
> I just don't think the Super Dooper coated filters are
> worth the money. The more simply coated ones from Hoya,
> which are less expensive (though not inexpensive), do a
> good job.
--------------------------------------------
The difference in light transmission/reflection between a single element
being single or multicoated is not that great. I think it would be
pretty hard to tell the difference between a single and multicoated
filter. Furthermore, although the front side of the filter may cause a
small amount of light loss, only the back side of the filter is going to
be able to contribute anything to internal flare
Where it makes a big difference is in a lens with multiple elements and
especially a zoom with lots of elements. See:
<http://www.cartage.org.lb/en/themes/Arts/photography/photproces/photogralens/carlzeiss/carlzeiss.htm>
which says that single coating results in light loss of 2 to 4 percent
per surface. Multicoating cuts the loss to 1/2 to 1 percent. If you
have a 13 element zoom with the best multicoatings passsing 99.5 percent
of the light on each surface you're still down to about 88 percent
transmission after passing through 26 surfaces. Or, looked at another
way, 12 percent of the light is scattering about inside trying to cause
flare and reduced contrast. Adding one more surface at 2-4% reflection
shouldn't amount to much.
------------------------------------
* Coating - Before coating, each transmission surface resulted in
about a 4% to 8% loss of light to reflection depending on the refractive
index of the glass. So an uncoated Dagor or Protar with four
transmission surfaces looses 15% to 29% of the light to flare. An
uncoated Tessar looses 22% to 40% of the light to flare. An uncoated
Planar with eight surfaces looses 28% to 49% of light to flare. The
flare would exhibit itself on the film as unfocused non-image forming
light which reduced the contrast of the picture.
o Single Coating - After single coating, this dropped to
about 2% to 4% loss of light per transmission surface. Applying the
coating at quarter wavelength thickness could greatly increase the
effectiveness of the coat, but it could completely block some
wavelengths of light and partially block others. Typically blue-green
wavelengths were suppressed with an amber coat, or green wavelengths
with a purple coat.
o Multi-coating - Multicoating was first done as two separate
coats at different wavelength thickness on different transmission
surfaces to balance the color of the light transmitted to the film.
Later, multi-coating as we know it, one coat stacked on another (first
used on a production lens by Leitz) reduced the light lost to
diffraction further to about 1/2% to 1% per transmission surface. The
classic second coat was bismuth oxide again applied at quarter
wavelength thickness for a different wavelength, typically orange-yellow
for the second coat and green-blue for the first coat giving a faint
green reflection. A multi-coated Planar could now only loose about 4% to
8% of the light to flare, quite a difference.
Coating and multicoating allowed designers to use more complex designs
with more air spaces which allowed easier design for correction of
spherical aberrations. The difference between uncoated lenses and coated
lenses are great, the difference between single coating and
multi-coating is visible, but not nearly as great as the first leap from
uncoated to coated. Coating and multicoating opened the way for many
otherwise unfeasible modern lens designs, such as complex wide-angle
lenses, big multi-element zooms, and lots of marketing hype. Coating
still won't save you from nasty flare in certain lighting conditions,
such as shooting into the sun, so make sure to use those lens shades!
Chuck Norcutt
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