Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site decwrl.UUCP
Path: utzoo!linus!decvax!decwrl!dec-rhea!dec-friday!hagerman
From: hagerman@friday.DEC (what is hip?)
Newsgroups: net.rec.photo
Subject: don't use filters to protect lens
Message-ID: <3466@decwrl.UUCP>
Date: Wed, 29-Aug-84 12:42:00 EDT
Article-I.D.: decwrl.3466
Posted: Wed Aug 29 12:42:00 1984
Date-Received: Thu, 30-Aug-84 10:30:41 EDT
Sender: daemon@decwrl.UUCP
Organization: DEC Engineering Network
Lines: 70

x
This is in response to a recent question about coated filters.

First I want to apologize in advance for being a Leica fanatic.

The basic point was that some filters are coated and others
aren't.  The reason for coating an optical glass surface
is to reduce reflection at that surface by using destructive
interference.  But on a filter that is going to have a 100% to
400% change in exposure anyway, saving another 8% by coating
is pretty insignificant.

But another comment was that a UV filter was being used to
protect the front element of the lens.  I'm not sure that
that is a good idea.  For one thing, the front (and rear)
elements of all camera lenses are made of glass that is tough,
and the coatings are selected specifically so that they
can be cleaned;
abiding by this limitation is part of the lens design process.
Unless you are a real klutz and put your thumb on the lens
every time you pick up the camera, I think you're better off
avoiding the extra two surfaces to get dirty.

There are also optical effects.  For example, the curvature of
long focal length lens elements is small relative to that of
short lenses.  Clearly a non-flat filter will have a greater
detrimental effect on a long focus lens than on a shorter lens.
Can you justify spending a couple of hundred bucks on a telephoto
lens and then putting a $6 filter in front of it?

Also it's questionable whether a UV filter is desireable.  This
is a quote from an informational (read:  sales) booklet on 
Leica Reflex Lenses:

	"...UV rays are invisible to the eye, but affect the film...
	Ultraviolet light must therefore be filtered out.  In the past,
	this was done with a UVa filter on LEICA lenses.  LEICA lenses
	of the current generation (from about 1965 onwards) absorb
	ultraviolet rays throught the use of certain types of glass
	and above all through a certain method of cementing the lenses,
	so that basically a separate UVa filter is unnecessary."

	"...slightly tinted UVa filters were in the past recommended for 
	colour photographs of subjects with exceptionally high UV
	and blue content of light, such as subjects in the shade
	or distant views through a slightly blue haze.  Their use
	with the current LEICA-R lenses is discouraged, because
	they will make the colour rendering unnaturally warm."

	"...It must, however, be mentioned here that even high-quality
	filters may create problems in certain situations.  At high
	contrast, for instance during sunsets, in night shots including
	powerful light sources in the picture and when bright objects
	are photographed through a dark arch, the risk of reflections
	even from optically flat and coated filters is very great.
	Double images or a general degradation of the contrast or partial
	lightening through stray light are relatively frequent.  In
	such photographic situations all filters, including the UVa
	filter, should be removed.  With ultra-wide angle lenses, too,
	filters may lead to inferior results.  Owing to the wider angle
	of field the marginal light rays must traverse a slightly longer
	path through the filter in front of such lenses than the light
	rays in the center.  This may often adversely affect the picure
	quality, and is one reason why, for instance, no filter is
	offered for the 19mm ELMARIT-R f/2.8 lens."

So I only use a filter when there is a specific need for it, and
buy big-buck glass filters, or gelatine filters.

DHagerman