Re: How to test a Polarizer's Quality (was - Re: Bad Kenko filter)

Discussion in 'Digital Photography' started by John Doe, Aug 24, 2004.

  1. John Doe

    John Doe Guest

    Hmmm ...... Err .. uhmmm .... Me a newbie, you see. My equipment is:
    - Olympus C-750
    - A cheap Kenko UV filter (which I highly suspect is any good except
    producing flares)
    - A Marumi Circular polarizer

    I took pictures with and without the UV filter, noticed the flare and
    concluded (maybe not correctly) that the UV filter I bought is evil.
    So I was looking for some similar experiment I could run for the
    polarizer. You know, like keep an certain object (something very
    colourful or a light source), take photographs with and without the
    C-PL and then draw some conclusions. Unfortunately, the sun hasn't come
    out in Bombay for a good month now, so I can't take pictures in bright
    sunlight where C-PLs are supposed to make the difference.
    Anyways, thanks for all the info.

    Siddhartha

    JustPassinThru wrote:
    > On Tue, 24 Aug 2004 09:43:13 -0400, Bob Salomon
    > <> wrote:
    >
    > >In article <>,
    > > JustPassinThru <> wrote:
    > >
    > >> would appreciate if someone could suggest a method to check the
    > >> >quality of the Marumi C-PL I just bought.

    > >
    > >Use a high quality light box that is daylight balanced with a CRI of

    95
    > >to 98+.
    > >Put any polarizer you want to test on it and photograph the

    polarizer on
    > >the box with daylight slide film.
    > >
    > >The slide should show that a really good polarizer is neutral grey

    in
    > >color.

    >
    > And this will show you ... what? The evenness of dye layers in your
    > slide film and its ability or inability to accurately reproduce
    > colors? Or the quality of color and evenness of lighting in your
    > light-box, that you paid dearly for and are trying to find uses for
    > it to try to justify the cost? Or that you are now trying to find
    > any use at all for your outdated film equipment that is now rapidly
    > fading in value and use? Or is this just red-herring hype from
    > someone that's trying to dissuade people from an inexpensive and
    > easy way to find flaws in their overpriced polarizing filters that
    > they can now test right in the store before purchase (and then throw
    > them back down on the counter when they fail to pass the previously
    > posted, easy, and quick-to-perform test).
    >
    > >
    > >Do bear in mind that there are many grades of polarizing foils of

    which
    > >very few are designed to be placed in front of a lens for

    photography.
    > >Anyone that recommends sheet foils to be placed in front of a lens

    is
    > >recommending a method to degrade your results. Sheet polarizers are
    > >intended for lighting purposes or other, non-photographic, uses.
    > >Optically they will degrade the image as they are impossible to hold


    > >absolutely flat and parallel to the lens.
    > >
    > >Also, as they are not coated, they easily can create flare which

    will
    > >reduce contrast and color saturation and, as contrast effects
    > >resolution, the apparent sharpness of your result. Lastly they are

    very
    > >difficult to rotate continuously as any mounted polarizer can do.

    >
    > While some of what you claim may be true for some poorly made "sheet
    > foils", it is not always true. Just what do you think is mounted on
    > the glass of those high quality polarizers? Answer: polarizing film
    > (or in your words, "foil"). Any photographic filter you have ever
    > used started out its lowly life as a plastic sheet film first,
    > (unless it is the old style gelatins; or was built up from
    > molecule-thin layers in evaporative-deposition processes, found only
    > in the most expensive of laboratory-grade filters). You might like
    > to know that the best filters for use in astrophotography techniques
    > are UNMOUNTED FOILS/FILMS because the parallelism between the
    > front/back surfaces is hard to match in rigid glass filters, even in
    > optical flats. The thinness of them will also impart no distortions
    > from diffraction. It is also never true (with camera lenses) that
    > optically-flat filters are best. It is even better to use a curved
    > (convex/concave) filter in front of a lens, or else the perimeter
    > will impart chromatic aberrations into the image, as always happens
    > when using any planar filter in a convergent light path.
    >
    > You sound like you don't know much about physics but do know an
    > awful lot about old-school, lighting-school marketing hype. The kind
    > that comes from 3rd rate commercial schools or ragazines that
    > advertise on late-night TV shows, and employ desperate inept
    > educators who "can't, so they teach". I suggest you try some
    > hands-on tests of these materials yourself lest you be outted again
    > the next time you are passing on misinformation to others from your
    > inexperienced and misinformed life.
    John Doe, Aug 24, 2004
    #1
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