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[OM] Flare and Filters

Subject: [OM] Flare and Filters
From: "R. Lee Hawkins" <lhawkins@xxxxxxxxxxxxxxxxxxx>
Date: Tue, 28 Jul 1998 11:47:55 -0400
Cc: lhawkins@xxxxxxxxxxxxxxxxxxx
Hi all:
I was off at Stellafane (an amateur telescope maker's convention in
Springfield, VT) for the weekend, so am just catching up on mail.  Joel

>A while ago I asked whether MC filters might not be a solution to flare in
>SC lenses. Lee Hawkins responded without explanation that it would make
>flare worse. Since I am an erstwhile academic, I accept statements of
>authority readily. Taking the hint, I assumed that the MC coating passes
>more unflared light, and this aggregate of light causes the SC lens to go
>into a worse state of flare.  Of course, as an academic the only thing I
>really know about light is that Book Three of _Paradise Lost_ begins "Hail,
>holy light, offspring of heaven first-born."  A nice definition of "Zuiko,"
>but I need a physicist!
>My attempt to explain this to myself takes as a given a state or situation
>in which a MC lens would not flare "*because* it is MC but a SC lens would
>flare because it *is* SC.  But SC lenses most of the time are not in a
>state of flare.  Would an MC filter *induce* flare or just exacerbate it
>when flare going to happen anyway?

Some comments:
        1) Any filter will pass less light than a filterless lens, MC or

        2) Adding an MC filter to an SC lens introduces one more element
           in the flare equation.  The MC filter will pass perhaps
           98-99 0f the incident light, but the SC element behind it
           will reflect at least 2-3%, which will the be reflected again
           off the back side of the MC filter.

        3) If the filter is used in a situation when the SC lens is
           is not in a flare condition, it certainly won't cause 
           flare.  MC filters can't "make" extra light.

        4) If the filter is used in a situation where the lens will 
           flare, the filter will certainly not reduce flare, and 
           might make it worse because of 2) above.

>Are there any circumstances in which a MC filter would be either neutral or
>perhaps an improvement over a conventional uncoated filter with an SC lens?

A multicoated filter is always preferred to an SC filter.  It just can't
magically make a lens that will flare not flare.


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