67/77 = .87
67 is 87% of 77.
100% - 87% = 13%
I forgot about that other formula...
Tom C.
From: John Francis <[EMAIL PROTECTED]>
Reply-To: [email protected]
To: [email protected]
Subject: Re: InfraRed Filters
Date: Wed, 8 Mar 2006 20:06:31 -0500
On Wed, Mar 08, 2006 at 05:01:30PM -0700, Tom C wrote:
> >From: Joseph Tainter <[EMAIL PROTECTED]>
> >At B&H the Hoya RM 72 costs as follows:
> >
> >67 mm: $ 80.95
> >77 mm: $283.50
> >
> >Hard to figure why the 77 would be so much more. A 77 would be logical
for
> >me. It would fit my DA 14, and with step-up rings, every other lens. It
> >would also fit the forthcoming DA 16-50 F2.8, which I would guess will
> >have a 77 mm. filter thread. But, oh, the price!
> >
> >Joe
> >
>
> That's easy.
> The 77mm has to block 13% more infrared photons as the 67mm version.
<NITPICK>
I think you mean 32% more photons?
(I don't know where you got 13% from - even using linear scaling,
77mm comes to 15% more than 67mm. Area, of course, is proportional
to the square of the diameter, so a 77mm filter has a 32% greater
area than a 67mm filter).
</NITPICK>
