Folks,
Two issues with filters & coatings.
1. The issue is not the transmission rate, but rather the reflection rate. The
difference between 92.7 and 91.3% transmission is not a meagre 1.4%, but rather
an increase in stray light bouncing around in the lens from 7.3% to 8.70f the
total light input. In other words, the increase in glare from 7.3 to 8.7 is
19%.
2. If the film has a 9-stop range, the lowest lite capturable in an exposure is
2**9, or 512 times, weaker than the strongest lite, or about 0.190f the
strongest lite. If the overall glare adds just 10f the incoming lite across
the field, most of the shadow detail will be obscured.
Now, not all of the glare makes it onto the film. Some goes back out the front,
some is absorbed by the black lining of the lens or camera. Some is intensified
by optical effects, giving the pretty patterns when a light source is included.
But any reflections will add fog & reduce contrast all over the film.
I can do without a 19 0ncrease in glare. Especially in low light situations
with mucho shadow.
Tom
> Bob Broder wrote and asked a question about the merits of single vs
> multi-coated filters.
>
> Brian Huber replied:
and then Chuck Norcott:
...
> When it comes to multicoating the first thing we need to keep in mind is
> that single coating is already about 97 0.000000e+00ffective in eliminating
> reflections from a single surface. The first glass surface of the lens
> is going to pass 970f the light and 3 0s going to reflect off. Off
> the first element it's only light loss. Off the interior elements that
> 3 0s trying to bounce around and go other than where you want it to.
>
> Multicoating gets you into the 98-99% range. Doesn't sound like much of
> a difference until you consider a multi-element lens. Some of the
> elements don't count because they're cemented and there is no air/glass
> interface to cause a reflection. But, assume there are 4 surfaces we
> have to contend with (the front of the lens and 3 air gaps between
> elements). At 97 0.000000e+00fficiency the total light transmission will be
> .97 x
> .97 x .97 x .97 = 88.5%. If the multicoating is 98.5 0.000000e+00fficient on
> a
> single surface then the total transmission will be .985 x .985 x .985 x
> .985 = 94.1%. So we get 6.6% more of the light through the lens and we
> reduce the pesky reflections.
> Pretty good but you have to care a LOT to call it dramatic.
>
> Now consider adding one more piece of glass and another air gap... the
> filter! Whether the lens is single or multicoated we're only going to
> add one other multiplier. If the lens is multicoated and the filter is
> single coated we'll end up with .941 x .97 = 91.3% transmission. If the
> filter is multi-coated we'll get .941 x .985 = 92.7%.
>
> As you can see, the effect is VERY small. Far less than the difference,
> say, between a single and multicoated lens... and I haven't heard of
> anybody throwing away their single coated OLY lenses.
...
----
From: Tom Trottier, President, ACT Productions Inc.
infoanim@xxxxxx http://www.act.ca
+1 613 594-4829 fax +1 613 594-8944
199 Holmwood Ave, Ottawa, Ontario, Canada K1S 2P3
"Make it as simple as possible, but no simpler" - Einstein
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