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Circular or linear filter ???

Discussion in 'Sony Chat' started by navychap, Jan 13, 2002.

  1. navychap

    navychap Active Member

    Having recently purchased a Minolta Dynax 7 with a 24-105mm D lens, could anyone tell me the correct type of polarising filter to use. I know most AF lenses use Circular polarisers, but I did hear something that because of internal focusing it should be a linear polariser. Please help.
     
  2. RonM

    RonM Alpha Napper

    Colin,

    The internal focusing is the Lens focussing mechanism, which is a boon for using the Circular Polarising Filter you need to get for using with your Dynax 7, or anyother AF camera.
    Can I suggest that you invest in a good quality polariser for use on your 24-105 as 'cheaper' filters can tend to cause vignetting at the wider angle end of your zoom, something like the Hoya SHMC Pro-1 polarising filter or the Cokin P system polarising filter.

    Hope this Helps


    /img/wwwthreads/smile.gif<font color=blue>RonM FRIPN
     
  3. navychap

    navychap Active Member

    RonM, thanks for the quick reply to my question. I think I will invest in a Hoya circular polariser as you suggested. At £40 they aint cheap, but I suppose if I want a bit of quality, I am going to have to pay for it.

    Regards

    Navychap
     
  4. BigWill

    BigWill Gorgeous oversensitive Nikon-loving cream puff

    Indeed Colin, having just spent a small fortune on a spanking new Dynax7, why "spoil the ship for a hapeworth of tar" to use an old Navy expression. Thought you would appreciate the Navy bit! /img/wwwthreads/smile.gif

    BigWill
     
  5. idv

    idv Member

    From what I've heard a linear poliriser could be used with an AF camera only if the front element of the lens doesn't rotate while focusing. As most of them do a circular one is needed.

    Also I totally agree with RonM that you need a good quality polariser to avoid vignetting. I had a cheap one (bought in Singapore ;) for my previous camera which wasn't too bad but did get some shots vignetted. The higher filter quality is the thiner it is =&gt; less chances of vingnetting. Also less chances it gets stuck...

    Hope it's useful.

    Igor
     
  6. Benchista

    Benchista Which Tyler

    No, that's wrong. I think there's a confusion of terms here - "circular" doesn't relate to the shape of the filter, but to the way the polarising effect is achieved.
    Linear polarisers don't work with beam splitters, which all AF SLRs use for their AF systems, and a fair number of other cameras use one for the metering system. (A beam splitter here is a semi-silvered portion of the reflex mirror with a smaller mirror at an angle to it, reflecting light to the AF or metering sensor. A linear polariser will prevent a portion of light getting through the mirror arrangement, thus not providing enough light for AF and/or corrupting the meter reading.)


    Nick NRIPN
    Never attribute to malice what can be adequately explained by stupidity.
     
  7. idv

    idv Member

    Nick,

    Thanks for the explanation. I'm aware that "circular" doesn't relate to the shape of the filter as I've seen "linear" filters that are in circular shape ;)

    I'm still a bit confused as you say "A linear polariser will prevent a portion of light getting through the mirror arrangement, thus not providing enough light for AF and/or corrupting the meter reading.)"

    As far as I understand a circular polariser prevents a portion of light getting through as well, so it's not about prevention of light, but about HOW they do it.

    As it seems to me the reason why a linear polariser won't work that if the front element of the lens is rotating while focusing it affects the amount of light getting through (the filter's effect is changing with this rotation), thus confuses AF.

    With a circular polariser once you set it in some position (by rotating the filter's adjustment ring) it then doesn't change its effect regardless whether the front element rotates or not.

    This my understanding that could be wrong but I'd appreciate if you could explain why ;)

    Thanks,

    Igor
     
  8. Benchista

    Benchista Which Tyler

    No, no, no, forget rotation of the front element, that only effects the polarisation effect overall and is a red herring on this topic. Both circular and linear polarisers are affected by rotation of the front element and have to be adjusted after focus.
    The point about a linear polarisier is that it prevents light passing through the mirrors in addition to that through the lens, because it polarises it in such a way that it can't pass through the beam splitter. Normally, the light that passes through the beam splitter is in a fixed proportion to that reaching the film. With a linear polariser, that relationship is broken and so less light reaches the sensors than would reach the film. With a circular polariser, that relationship is maintained, so correct exposure and AF can be maintained.

    Nick NRIPN
    Never attribute to malice what can be adequately explained by stupidity.
     
  9. idv

    idv Member

    Nick,

    Thanks a lot for explaining this in details, it cleared up my understanding ;)

    I've also found a post in other forum that goes into even further details on how it all works, so I thought it might be worth reposting to finish up our discussion ;).

    Once again, thanks for your explanations!

    Igor
    -------------------------------------------------------------------------------
    Date: Thu, 15 Oct 1998 10:27:56 -0400
    From: Brian Reynolds &lt;reynolds@panix.com&gt;
    To: photo-3d@calcite.rocky.edu
    Subject: P3D Re: Circular or Round?
    Message-ID: &lt;19981015102756.B12910@panix.com&gt;

    [skip]

    This is another of my pet peeves, so I'm going to go into some detail
    about it. Further disscusion (beyond correcting any stupid mistakes I
    make) should probably go to tech-3d. Note that it's been a long time
    since I took electromagnetic theory, so I might have gotten the
    "handedness" of the diagram wrong.

    Light can be thought of as an electromagentic wave (it is also thought
    of as a particle (or photon), but that's not important for this
    problem). One component is an electric field, the other is a magnetic
    field. They are always at right angles to each other and the light
    wave travels perpendicular to both fields. WARNING: ugly ASCII art
    follows.

    E
    |_M
    \
    d

    fig. 1

    In figure 1 the vertical electric field E is perpendicular to the
    horizontal magnetic field M and they are travelling along direction d
    (which should be perpendicular to the surface of the screen).
    Normally all of the E fields in a beam of light will be at random
    orientations with respect to each other.

    If you have a beam of light where all the E fields are arranged the
    same (e.g., all pointing up, or all pointing down and to the left, or
    whatever) the beam is said to be linearly polarized. Because the M
    field is always perpendicular to the E field, the M fields will also
    be aligned with each other.

    A light beam reflecting off certain materials (e.g., water, metal)
    will become linearly polarized. A polarizing filter (which contains a
    single polarizing surface) is oriented so that it is 45 degrees from
    the angle of polarization of the incoming light. This will cut the
    intensity of the linearly polarized line in half, thereby reducing
    glare.

    The EM fields can also rotate about the d direction. If all the E
    fields in a beam rotate in the same direction (clockwise or
    counter-clockwise) the beam is said to be circularly polarized.

    Many modern cameras use off the film (OTF) metering. The camera's
    electronics measure the light reflected off the film during the
    exposure in order to determine the exposure for the photo. This light
    becomes linearly polarized when it reflects off the film. Using a
    linear polarizing filter with such a metering system will give
    inaccurate results because the combined polarizing results of the
    filter and the reflection will lower the amount of light reaching the
    metering system, but not the light reaching the film. To correct for
    this camera companies created a special polarizing filter. The first
    part of the filter is a linear polarizing filter, the second part
    takes the polarized light (from which the glare has been reduced) and
    randomizes it again so that it is no longer linearly polarized. This
    light can then reflect off the film and be read correctly by the
    camera's metering system. Unfortunately even thought the light from
    this special polarizing filter is not circularly polarized, the camera
    companies decided to call it a circular polarizing filter.

    --
    Brian Reynolds | "Dee Dee! Don't touch that button!"
    reynolds@panix.com | "Oooh!"
    NAR# 54438 | -- Dexter and Dee Dee
    | "Dexter's Laboratory"
    -------------------------------------------------------------------------------
     
  10. dogbyte

    dogbyte In the Stop Bath

    Total cost of ownership gets interesting doesn't it when you get a new lens and then come to find you want the hood, the polariser, the skylight, the softener, the 80b, the 81b & so on
    NAVYCHAP: if you're thinking of another lens as well, consider its filter size. If say the shorter lens takes 52mm filters and a longer lens takes 58mm you could buy all 58mm filters and a step ring to enable you to screw the 58mm filters onto the 52mm lens so avoiding the cost and carrying of 2 lots of filters. But make sure they don't restrict the movement of the lens as you zoom from one end of the zoom range to the other. Obviously you'll want a skylight on both lenses to protect the glass.

    Pete
    Not waving but drowning, gulp
     
  11. TimF

    TimF With as stony a stare as ever Lord Reith could hav

    Or choose system filters (Cokin, Lee, Hitech etc)

    "Shut up Eccles!!"
     
  12. dogbyte

    dogbyte In the Stop Bath

    Too inconvenient for general use? Essential for graduating a landscape. Again I found with my vintage 1989 Canon lenses that I cannot travel the whole zoom range unless I fit an extender ring to the system filter, which in turn can cause vignetting.

    Pete
    Not waving but drowning, gulp
     
  13. TimF

    TimF With as stony a stare as ever Lord Reith could hav

    Obviously you'll want a skylight on both lenses to protect the glass.

    I'll take an 81A over a skylight myself. /img/wwwthreads/smile.gif


    "Shut up Eccles!!"
     
  14. TimF

    TimF With as stony a stare as ever Lord Reith could hav

    Is that down to the lens design, or just focal lengths wider than 28mm? Lee and Hitech both do wide angle adapters for this kind of thing. I don't know any reason they can't be used for longer lenses.

    "Shut up Eccles!!"
     
  15. dogbyte

    dogbyte In the Stop Bath

    Dunno, my comment is based on my humble jessops and Cokin filter holders. I've just checked - the 70-210 is not affected, but the 28-70 is. So unless I use an extender ring I need to remove and refit the filter holder if I want to go from one zoom extreme to the other

    Pete
    Not waving but drowning, gulp
     
  16. Benchista

    Benchista Which Tyler

    But it's very personal, and depends on film choice - I use a Skylight 1B.
    Those purists who remove the protecting filter before exposure can get away with anything.

    Nick NRIPN
    Never attribute to malice what can be adequately explained by stupidity.
     
  17. TimF

    TimF With as stony a stare as ever Lord Reith could hav

    This is due to the construction of the lens? Certainly from what you say it sounds like the front moves in and out when you zoom? In which case the WA adapters probably wouldn't make a difference.

    "Shut up Eccles!!"
     
  18. TimF

    TimF With as stony a stare as ever Lord Reith could hav

    Agreed.

    "Shut up Eccles!!"
     
  19. Benchista

    Benchista Which Tyler

    That's exactly it - or at least it is with the 28-80 USM MkI. The filter thread retracts within the barrel at about 45mm.

    Nick NRIPN
    Never attribute to malice what can be adequately explained by stupidity.
     
  20. Larry Shone

    Larry Shone Well-Known Member

    &lt;i&gt; a linear poliriser could be used with an AF camera only if the front element of the lens doesn't rotate while focusing. As most of them do a circular one is needed.&lt;/i&gt;

    not a problem tho if you use manual focusing on an AF camera,i presume.

    click,whirr,i got a free roll of HP5 and im not afraid to use it &lt;img src="/img/wwwthreads/wink.gif"&gt;
     

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