True, all other things equal, greater transmission is better.
It is confusing, then when none of those other "things" are compared
here at the same time.
And...if this is your first priority, it can easily be "gamed".
Don mentioned how you have to consider different wavelengths. Otherwise,
someone would design and promote the lowest green loss at the expense of
red and violet. For electromagnetically similar reasons, losses on center
can be optimized whilst even more quickly inducing poor transmission
at the edges of the field. I remember looking at some ProMaster binoculars
that claimed stunningly low losses. I was confused when I saw a noticeable
dimming at the field edges. Then the "aha!" moment: they described a 20-layer
coating process. That's inevitably a great thickness, and a narrow spectral
optimum, and an even narrower optimum across the field, in the eyepiece elements.
All else is usually...not equal.
I have a target with fading densities printed out on the paper.
The faintest printed line has RGB values of 225-each..
Now, the 18mm Paradigm Dual-ED is acknowledged generally
as great for the money, and it is fully multi-coated. I love it myself.
And...the 18mm Kellner from the Zeiss Silvarem 6x30s of ~1915
can see more letters as they fade in and out from irregular printing
(awesome accidental feature) than the Paradigm.
This is not to specifically knock the Paradigm, but the differences
are extensive when it comes to coating. The Zeiss...has none.
It has 4 glass-air surfaces, of course.
Life was not made easy on the Zeiss Kellner, btw:
both eyepieces were checked at F5.
And in the sky, at least a few times, more subtle detail
was available with the ancient Zeiss on M16 (Eagle Nebula),
which is easy to see, but has different appearances depending
on your brightness and contrast......
What isn't equal? Mainly the number of elements.
Oddly enough (or not), Zeiss cameras and microscopes and eyepieces
have around 7 layers coating, and less super-transmission
the others. They stick with it, though, because racing for
'my transmission is better than yours' leads to trouble.
I would strive for low losses, but avoid the lowest transmission losses
like the Plague.
One thing that is quite probably true is that eyepieces with many elements
wouldn't be very good without multi-coating. But it's also true that
multi-coating is a Hi-Q filter, basically, and because it is Hi-Q ,
it is extremely sensitive to surface contamination. You can't avoid
that at the first and final air surfaces. Another problem is that, for example,
an Abbe naturally and easily tosses off out-of-field scattering, whereas
this is a lot more difficult to design with many elements and wider angles.
So...thus we have the roots of the debate over
"simpler is better contrast, contrast is the key"
and , "lowest lost is best....transmission is key".
An Abbe, now with muli-coating is almost impossible to defeat for fine detail....
but under 50 degrees. I wish they made one with 2-layer outer coatings, though.
You can get great contrast "considering"
with a widefield EP, but some look and see the proof in the galactic pudding.
Edited by MartinPond, 05 June 2020 - 08:16 AM.