At 2:27 AM +0000 7/7/03, olympus-digest wrote:
>Date: Sun, 06 Jul 2003 21:02:18 +0800
>From: Albert <olympus@xxxxxxxxxxx>
>Subject: Re: [OM] True, or just BS?
>
>Moose,
>
>I've read that coating off the front is not that big of a deal, but
>coating off the rear element is. I don't even claim to know the math
>behind it or how to calculate it..
There are long answers, but here's the short answer.
Front surface: Light never enters the lens, so cannot cause flare or ghosts.
Only a slight loss of light, 4% at worst. This 4 0s the per-surface reflection
from uncoated glass. This is log2(1-0.04)= 0.06 stop, which is a negligable
loss.
Rear surface: Up to 40f the light coming out of the lens (on its way to the
film) is instead reflected back into the lens, where it bounces around causing
flare and ghosts. However, most modern lenses have very good internal baffles,
and the coatings of internal len surfaces will have survived, so most of this
reflected light is simply absorbed.
So, I bet that loss of either front or back or both coatings makes little
difference in practice in that one will get ghosts in situations where one
would already get ghosts, and flare isn't likely to be much worse.
Joe Gwinn
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