Compensating for Filter Factor

Discussion in 'Exposure Discussion' started by S.larsson, May 4, 2012.

  1. S.larsson

    S.larsson Member

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    Hi,

    I am currently using a Nikon FM which doesn't meter through the lens and I've just mounted a Circular Polarizer which has a filter factor of between 2.3 to 2.8. To simplify things lets average this number to 2.5. The film in my camera is 400 ISO, so in theory I can directly compensate by changing the ISO so the filter factor which equates to about 1 1/3 stops. Can somebody tell me if I'm on the right path? What would I have to change my ISO setting to on my camera for it to meter correctly?

    Regards,

    Stephan

    P.S hope I didn't write that in a overly confusing way :tongue:
     
  2. Steve Smith

    Steve Smith Member

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    It does meter through the lens so you shouldn't need to alter the ISO setting.

    I would usually use two stops of compensation for a polariser so take it outside and meter the same scene with and without the polariser and see what the difference is.


    Steve.
     
    Last edited by a moderator: May 4, 2012
  3. S.larsson

    S.larsson Member

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    Really? I thought the light meter took light from the little hole between the bottom of the prism and the lens mount?
     
  4. S.larsson

    S.larsson Member

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    Aha, perhaps thats just for the aperture reading...
     
  5. Steve Smith

    Steve Smith Member

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    I think that's so you can see the aperture reading in the viewfinder (I'm guessing as I don't own an FM).


    Steve.
     
  6. S.larsson

    S.larsson Member

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    Yes, thats what it is, Thanks Steve!
     
  7. ROL

    ROL Member

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    This is only a complete answer to your question from someone who both owns an FM and has verified the question both with the original manual and empirically, so of course it is only opinion :whistling::


    • The Nikon FM meters through the lens (TTL). The FM will meter anything you put in front of the lens, including light transmitting filters.
    • Additional exposure compensations, as stated, will underexpose your film if relying on the FM's on board meter. Your strategy will work, though I would prefer to make exposure adjustments with speed or aperture, if either using the camera and lens to meter a scene w/o filter, or using a separate light meter – though one may also meter through the correctly rotated polarizer off camera (a subject of much debate in itself), as we do with cameras without meters (e.g., LF)
    • The FM shows the user all exposure information directly in the viewfinder, one reason, along with its all mechanical workings, this magnificent little camera was so popular. The aperture, in particular is mirrored from an overhang on the viewfinder prism housing up from the lens barrel to the top of the viewfinder.
    • Polarizers in general will require an additional 1.5 to 2.5 stops, depending on the light in any composition. They are only useful in polarizing light when directed at right angles to the direction of the light, or in the case of reducing specular highlights. They may also be used to reduce exposure generally, in lieu of neutral density filters.
     
  8. andrew.roos

    andrew.roos Member

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    If the FM didn't have TTL metering, then you would need to set the camera ISO to 400 / 2.5 = 160 (where 400 is the film speed and 2.5 is the filter factor).
     
  9. Poisson Du Jour

    Poisson Du Jour Member

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    I am very surprised by the suggestion of more than 2 stops compensation for a circular polariser at the full setting. The maximum should not be any more than +1.5 or +1.6.
    Hand metering with a baseline calibration of the filter factor is the most reliable and takes away the known hit-and-miss of TTL meters. A hand meter will also allow you to rise above the undesired effect of completely flattening illumination (resulting in a dark, flat and overall unsatisfactory exposure) by analysing the light either with spot- or incident metering.
     
  10. S.larsson

    S.larsson Member

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  11. andrew.roos

    andrew.roos Member

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    Yes.
     
  12. MattKing

    MattKing Subscriber

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    Maybe :smile:.

    (don't you hate it when answers are unclear, or contradictory)?

    The uncertainty comes from the fact that the information you gain from the meter reading will need to be interpreted with the effect of a polarizing filter in mind.

    To understand this, I'd suggest an experiment. With the filter in place on the camera, take a number of meter readings, each with the polarizer set at a different position on its ring. You should see variation in the readings, because the polarizer tends to filter out the polarized light reflecting from the scene. You have to consider, however, whether it is the polarized light portions of your scene that you want to key into when you are choosing your exposure. In many cases, you are most interested in the other parts of the scene.

    So which of the various meter readings do you choose?

    If you meter separately, without the polarizer in place, and then apply the standard filter factor, you will get consistent exposure of the parts of the scene unaffected by the setting of the ring on the filter. That may lead to better results.
     
  13. ROL

    ROL Member

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    Well, apparently mine will, but yours is the subject of opinion, just to clear it up.
     
  14. Poisson Du Jour

    Poisson Du Jour Member

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    If it is a TTL meter, yes.
    But if you are unsure, run a roll of film through the camera with the polariser in place, at midline and maximum effect. Then you will be in no doubt as to what works and does not. Be aware that a polariser set to maximum effect on overcast days e.g. in forests, can make the scene very drab, dark and generally underexposed, which is where you have to be on guard and leverage the exposure with exposure compensation. It all returns to the suggestion of running a roll of film through the camera and actively experimenting. That is the only way you will be sure.