Re: [Leica] Fighting windmills?

["Tom Schofield" <tdschofield@email.msn.com>]

Erwin,

I have always wondered whether use of filters is more problematic with
extreme wide angle lenses.  It seems to me that, while at the center, the
light path is perpendicular to the filter, at the edges, the light path is
more oblique.  It therefore strikes the filter at an oblique angle, and
travels through more glass than at the center of the image.  Therefore, I
would expect degredation, or at least differences, at the outer areas of the
frame.  What are your thoughts on this?

Tom

- ----- Original Message -----
From: "imx" <imxputs@knoware.nl>
To: "LUG" <leica-users@mejac.palo-alto.ca.us>
Sent: Monday, February 26, 2001 10:44 AM
Subject: [Leica] Fighting windmills?


> The question of the possible image dgradation by the use of filters will
> never end, nor be settled by whatever MTF measurement results. The reason
> for my statement is the fact that the question is falsely framed.
> The real question should be this:
> what optical properties and optical quality does a filter need in order
NOT
> to degrade the image quality?
> First a few facts:
> Photographic optical systems consist of several lens elements that work
> together to provide the designed image quality and optical properties,
> Some basic reflection on the nature of ray bending in such a system will
> tell you that (theoretically) adding a lens-element (a filter that is) to
> such a system will change the optical properties and thus the image
quality.
> This is known by all optical designers and also by practitioners in the
> field of cinematography.
> When we use a close focus lens as an attachment we just do this: alter the
> optical properties.
> A filter in front of a lens does the same.
> The amount of change does depend on only one factor: the quality of the
> filter itself. Why should Leica add a built-in filter to their best ever
> lenses (Apo-Summicron 2/180 and 4/280), if they were convinced that a
filter
> inherently would degrade the image quality. It does not, what can be
proven
> by using a 2/180 with and without the filter.
> IF a filter has truly plane surfaces and is made from first grade glass,
> there is NO difference in optical performance. I conducted test with MTF
> apparatus to study this effect. BUT a filter with a heavy amount of dust
and
> fingerprints will reduce the MTF values significantly. As will do a front
> lens surface that is full of dust and fingerprints.
> The focal length of a lens can be defined in diopters. If a filter is
> without any defects its own diopter value will be zero, not giving a
change
> in diopter value or focal length. If the filter is tilted in its mount, or
> not plane or has other defects, there will be a diopter value to add to te
> lens.
> So testing 'a' filter in front of a lens has no value at all, unless you
> first measure the optical properties of the filter itself.
> to summarize:
> any filter can generate flare in some circumstances, the amount of
> parameters involved is not known.
> Any filter might change the focal length of the system, depending on
quality
> of the filter, focal length used and distance setting. Te larger the
> diameter of the filter, the more likely that a defect will turn up.
> Any filter might degrarde the image quality, depending on the quality of
the
> filter. Leica filters are known to be fully plane.
>
> To prove that 'a' filter might degrade image quality is futile: it has
been
> done already and very scientifically and practically.
> To deny that a filter might degrade image quality is futile too: there
will
> be a drop in image quality when using filters with defects.
> To assume that the degradation caused by a filter is more important than
> defoccusing errors, exposure errors or erors of movement, is simply not
> having a clue of the relative importance of the components of the imaging
> chain.
>
> Erwin
>
>