Polariser filter

Discussion in 'Beginner Questions' started by peterlove, Jun 23, 2022.

  1. Hi all I understand that it is not ok to use an analogue polariser on a digital camera but is it ok to use a circular polariser on film ?.
    Peter.
     
  2. There's no law against it. The problem is in the metering not in some monstrous effect on the image.
    Polarizer-2002-09-PP-i.jpg
    clip from Popular Photography 2002-09

    There are also some rules about stacking circular and linear polarizers -- just try things and see for yourself what happens.
    WIth digital you don't have to wait for the film to come back..
     
  3. Thank you for the reply, I am using a Mamiya RB67 pro SD and my metering is by handheld meter, I also use the Cokin filter system I already have a circular polariser and I have just found a linear polariser on ebay, so, I will be making some comparisons.
    Thanks again. Peter.
     
  4. Any polarising filter will be fine with your camera, but it is difficult to measure correctly with a handheld meter - because the light transmission depends on the angle of the filter.
     
  5. A polarizing filter is most beneficial in that it mitigates reflections, increases saturation and contrast without impairing a color cast, makes clouds more vibrant, acts as a neutral density filter. It cuts haze because it filters UV. Early scientists trying to comprehend the double image they saw through a transparent Islandic Spar filter, wrongly assumed light had a north and south pole -- somewhat like a magnet. They labeled this action polarization; too late to change this name now.

    Light photons travel following a wave motion pattern. Unlike water waves that ungulate up then down, light waves vibrate in all possible planes (unpolarized). These waves can become polarized (vibrating in restricted planes) after being reflected from some surfaces and/or traversing some transparent materials.

    The polarizing filter acts like prison bars in that light waves can only navigate through, if the direction of vibration matches up with alignment of the bars. We adjust the polarizer for a desired effect by rotating it prior to snapping the shot.

    Sorry to report that polarized rays can interfere with some of the automation of a modern camera (both digital and film). This interference is on a case-by-case basis; better to err on the side of caution. We mount a circular polarizer in lieu of a simple linear polarizer.

    The circular polarizer is actually two filters sandwiched together. The upfront filter is a standard linear, the behind filter is called a “retarder”. The retarder undoes the polarization so that the exposing light has no impact on the camera’s automation. However, the upfront linear has done its deed, so we get the full influence of the polarization filter.
     
    Ricochetrider, JDMvW, kmac and 2 others like this.
  6. A polariser has a fixed correction factor, that provides correction for the light loss due to the material it is made of.
    The polarising effect, selective (!) darkening of parts of the scene, is why you use the filter in the first place, and you do not need to correct that.

    Some people do compensate for the overall effect of using a ploraiser (dulling of the scene) by adding a bit more brightness to it. But that is not required, and if you do not like it you should not use a polariser to begin with.
    TTL meters do it automatically, because they do not understand that selectively losing light is what filters are used for. But that is a fault. Not something you should emulate when not using TTL-metering.

    So it doesn't matter that it can be difficult (is it?) to measure with a handheld meter, because you do not need to. Just apply the filter factor that is on the rim of the filter.
     
    Last edited: Jun 23, 2022
  7. Doesn't the rotation angle setting affect the stops?
     
  8. Using a circular polarizer with film is no problem.

    AFAIK, TTL metering through a polarizing filter is no problem as well, regardless of the angle of the polarizer EXCEPT for Canon F-1, new F-1, FT, FTb, and FTbn. Because of their beam-splitter light meters, TTL metering is a PITA with a regular polarizer. The owner's manuals for these Canons recommend using a circular polarizer.
     
  9. No. It affects the effect. Which is blocking light you want to have blocked.
    And yes, if you, selectively,block more light,you get less light. But that's the entire purpose of the thing.

    When you compensate for a greater effect of the polariser, you will overexpose the bits in the scene not affected by the filter. You do not want that.
    The bits that aren't touched by the polariser filter do not need compensation for anything except the constant, and non-selective absorption of light by the filter material. Apply that and you will be fine. But stop there.
     
  10. It can (!) be a problem in all metering systems that have a reflecting surface in the path the light has to take to reach the sensor. And not just those Canon cameras direct light to the sensor using reflecting surfaces. Whether that is a problem depends on the nature of the reflecing surface.

    A CPL filter, however, is always a safe choice. Whether needed or not, it does the job.
     
  11. The main difference is that modern CPL filters tend to be multicoated and absorb slightly less light than old linear polarisers, which AFAIK haven't been in production for about 20 years.

    I've used linear polarisers - simply because I had 'em - on several DSLRs and to be quite honest I could see no adverse affect on either the metering or AF. So don't believe all the advertising BS put about!
    Nobody will die and nothing will catch fire from using a linear pol on a digital camera. :rolleyes:

    Same goes for using a CPL on film - it'll work identically to a linear polariser, except maybe giving you +1/3rd stop more light, thanks to more efficient polariser foils and better AR coating.
     
  12. For DSLRs with anti-aliasing filters, those are made with birefringent material, and so are affected by linear polarized light.

    For most scenes, you might not notice, or might only notice looking very carefully.

    Simpler digital cameras depend on the lens not to be able to resolve enough detail for aliasing,
    and so don't have anti-aliasing filters. (They might also not have a way to hold filters.)
     
  13. Any polarising filter only has an effect on already polarised, or partially polarised light. Therefore, if an anti-aliasing filter was noticeably affected by polarised light, then that effect would also be noticeable with light that was naturally polarised; e.g. blue sky, reflections off water, glass, foliage etc.

    Additionally, a Circular Polarising filter uses a bi-refringent 1/4 wave plate sandwiched with a linear polarising membrane. The nett result being almost identical to that of linear polarised light passing through a bi-refringent material in close proximity to the sensor.

    In short, there should be no difference at the sensor between naturally polarised light, artificially (linear) polarised light, and that from a circular polarising filter consisting of a sandwich of linear polariser followed by a bi-refringent retardation plate. All will tend to nullify, or partially nullify, the beam-splitting effect of a singly bi-refingent anti-aliasing filter.

    FWIW, Nikon's explanation of the action of their anti-aliasing filter is that it consists of two layers of bi-refringent material with their axes at right-angles to each other. This arrangement should result in a total 180 degree polarisation phase shift that's largely immune to incident polarisation effects, while simply producing four spatially shifted images.
     
    Last edited: Jul 3, 2022
  14. Your post is very confusing to me. First you sayu NO than you say yes.

    The instruction for a polarizing filter says to adjust let's say between 1 1/4 to 1 3/4 stops. How do you do that in the real world? What parameters do you use?
     
  15. I use the fixed correction factor that is usually printed on the rim, and nothing else.
    And the answer is no. Do not compensate for the variable effect of the filter. See my earlier reply.
     
  16. Usually the effect of the filter is small enough that you don't need to allow for it, but if a significant amount of the incoming light is all polarised in the same direction you may need to compensate if not using TTL metering (self compensating)
    On occasion I've had reflections from water that provided far more than half of the light. Arrange the polariser one way & expose for the reflection, twist it by 90 degrees & you have to expose for the dim pond floor...
     
  17. As far as I know there is no such thing as a digital polariser. Newer filters sold for digital may have better coatings but they still polarise light using real light before any digital to analogue conversion. Polarisers themselves can be high extinction or high transmission types, then there is a linear/circular choice depending on if a quarter wave plate is added. It doesn't work well combining two circular polarisers (at least unless you switch one round - two CPLs back to back give very weird effects, front to front they can work like a variable ND).

    You can get perfectly good results using a 50 year old linear polariser on a digital camera, my grandfathers old 'pile of plates' polariser (probably over 100 years old) would also work if it was a bit wider - less than 1/2" diameter is just too small for my lenses.

    FWIW circular polarisers came out long before digital photography was practical, yes they work fine with film as that's what they were originally made for! I think my first CPL was brought in the mid 1980's
     
  18. And that is what you want the polarizer to do. If not, the choice is not to add exposure (and overexpose the unaffected part), but use less of the polarizing action. Or none of it.
     
  19. Indeed it's what you what you use it for, but it's effect does require the effect to be compensated, or the pond floor will be invisible.
     
  20. No. Only the general/overall absorbance, unrelated to effect, needs compensation.
    That different parts of a scene need different exposure to end up in the desired tone/brightness is a diffetent matter, completely separated from selective filtering.
     

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