Metering through filters

Discussion in 'Large Format' started by kevin_kemner, Sep 21, 1999.

  1. My question is about metering through filters. I recently returned from a photo trip where I decided to try metering through the filter rather than adjusting the exposure according to the filter factor. (as I normally do) Now that I'm developing, the shots where I did this are fairly underexposed. I'm guessing about one stop to two stops depending on the filter. I use a Gossen Luna-Pro F with the variable angle attachment and Bergger film developed in Pyro. I haven't had problems with this combination before so it must be the change in technique. Does anyone have any thoughts?

    <p>

    Thanks

    <p>

    Kevin
     
  2. The spectral response of film often differs from the spectral response
    of the meter cell. Which means readings from brightly coloured areas
    or through a filter must be interpreted with care. Typically, metering
    through a red filter will underexpose by about 2/3 stop with most
    panchromatic film, depending on the meter cell. Filter factors are
    based on the reproduction of a middle grey. Personally, I don't even
    know if filter factors are necessarily the best compromise either
    because in many shooting situations, I find the effects of filters on
    local contrast extremely difficult to predict with any precision.
    For e.g., I know that a red filter will darken a blue sky but I'm
    never able to predict by how many zones it will darken the sky. I
    sort of makes stabs at it by metering with and without the filter.
    That, I think, is the information metering through a filter can
    perhaps give you. However, for exposure, the filter factor is
    perhaps a better compromise to make, at least for most field
    shooting situations. If I have a very complicated (for me) subject, I
    take the lazy way out and shoot a couple of sheets and develop them
    one by one. DJ
     
  3. I also find that metering through the filter doesnt give me the
    proper compensation factor. When I first started using a polarizer, I
    metered through the filter and it gave me a 1 1/3 stop factor, but
    using 1 1/3 stop still gave me an underexposed chrome. By trial and
    error, I finally came to a 2 stop compensation factor, which seems
    about right (for my polarizer, anyway). So... good idea, it just
    doesnt work!
     
  4. And so the spectre of Zone VI modified meters rears it head....

    <p>

    I believe it was Gordon Hutchings that developed a filter factor
    technique that has been published in Steve Simmons book, Using the
    View Camera and also in View Camera magazine, It may be on the V.C
    website. If interested I can look it up for you so e-mail me.
     
  5. Not much to add, except to say that I too have found metering
    through filters to be not as reliable as I would have thought, esp. for
    polarizers. Could be due to spectral sensitivity of the film, meter,
    or whatever. Now I shoot according to the data book (colour
    compensation filters only, no black and white work) and it seems to
    work.
     
  6. At what are you aiming the meter when you take a reading through the
    filter? If you were to aim the meter at a target that was the same
    color as the filter, the filter would transmit most of the light
    striking it. On the other hand if you were aiming at a complimentary
    colored target, the filter would block far more light. A good
    illustration is the technique commonly used for shooting sand dunes at
    dawn. Yellow filter, no compensation, place highlight of dunes in
    Zone VI, give N+1 development. The lightstruck areas of the dunes are
    very near in color to the yellow filter. The filter passes pretty
    much all of that light. The blue light in the shadow area is
    partially blocked by the yellow filter, creating a thinner negative in
    that area. This, plus the extended development, which pushes areas
    placed in Zone VI into Zone VII, enhances the contrast of the
    negative, thus transforming what, in color is somewhat flat, into a
    contrasty, dynamic black and white image. Perhaps aiming the meter at
    a grey card would yield exposure offsets that would more accurately
    reflect the specified filter factor. One other note: Very often a
    filter's factor is a suggested range as opposed to a single number.
    Depending on the spectral response of the film, the color temperature
    of the light source or some of the other variables (like the color of
    the subject) can influence the practical factor for that situation.
     
  7. For more than you ever probably wanted to know on this subject, see
    http://members.aol.com:/workshops5/zsfilter.htm.
     
  8. Hint! I have a Zone VI modified spot meter (Soligor) which give me
    very accurate readings in all light situations, with and w/o filters.
    Pat
     
  9. Hate to say it but the modified Zone VI meter really does let you
    meter accurately through the filter....
     
  10. I have to agree with the Zone VI meter faction here. I own two modified Zone VI meters, a Soligor and a Pentax digital, and routinely meter through them with good results. There are, however, some other factors which affect film speed, etc. to be considered when using filters besides matching the spectral sensitivity of the meter to that of the film one is using. A look at the spectral response curve for any B&W film will show you that the film is more sensitive, i.e. faster at certain wavelenghts and less sensitive, or slower at others. A sharp cutting red filter (that means, photographing with red light only) can slow some traditional B&W films by up to three stops! Other colors have different effects. This is also the reason why B&W films have different speeds in daylight and tungsten light. One could get very scientific about the whole thing and compare spectral curves of filters and films and different phot cells and probably earn a Doctorate at RIT. Empirical testing, which takes much less time, shoud get you in the ballpark so that you can use your meter to read through filters (whatever it is, but I recomment the Zone VI since it approximates the sensitivity of B&W films more closely) and then apply the appropriate film speed "fudge factor" which has been determined from tests. Contrast can also be affected, so test that too, you may need a development factor as well.
    Using filters is unpredictable and inexact, but one can reduce the inconsistencies significantly and have a practical working method that produces consistently printable negs. For more info see David Kachel's article on exactly this subject. You can find it and many other interesting articles at: http://members.aol.com/workshops5
    Hope this helps. ;^D)
     
  11. Just a thought.

    <p>

    Metering through the filter is exactly what several million SLR users
    do, apparently with OK results. So what difference can a hand-held
    meter with the filter make?

    The only one I can think of is if there is airspace between the meter
    and the filter, which is allowing off-image light (can we call this
    flare?) to reflect off the filter and be read. Maybe if the filter
    was hard against the meter cell things would be different?
     
  12. I have used filters extensively in my photography and have learned how
    much exposure compensation to assign to each filter. There are a lot
    of factors involved in determining your exposure compensation for each
    filter. What film type, sun angle and intensity, color of the light,
    subject color and reflectivity, ect. All of these should be taken into
    consideration and learned. And the easy way to get the correct
    compensation is to meter through the filter you will be using. That
    pretty much takes care of the majority of problems. But now you have
    to consider and then meter for contrast range within the scene. Then
    all things will be good. I'm not familiar with all the different types
    of Gossen meters but I would like to know if the meter you were using
    was an incident or spot meter? If it was a spot meter then there must
    have been something else going on to give you that much underexposure.
    even 1 stop is a lot of under exposure for any filter mistake. But if
    you were using an incident meter then there is your problem. It
    doesn't take into account the contrast ratio of the scene itself in
    regards to the reflectances of the different components in the scene.
    If you are using an incident meter, discontinue using the filter in
    conjuction with it. Just learn to compensate with the info you get
    from youir prints. james
     
  13. A 1-stop difference wouldn't surprise me. Spectral responses of films and meters differ. See also B&W contrast filters and TTL sensitivities in photo.net Q&A.
    Kevin: On the Gossen meter, I assume you are using the correct mark for the 'spot' attachment, i.e. the red or green circle, rather than the yellow triangle?
     
  14. Just to clarify. There's normally at least two problems with metering
    through filters. One is what has been alluded to already i.e., film
    sensitivities and meter cell sensitivities can differ (the
    Zone VI modified spot meters are supposed to help combat this
    problem). The second problem is perceptual and depends on what color
    and tone the area you're metering is and how well you're able to
    perceive these. If you use a yellow filter (something like #8 which
    passes about 90% of yellow light) and meter off a yellow object, your
    meter will obviously not suggest a ton of compensation. However, if
    you meter off a blue object, your meter sees a lot less light and will
    suggest a large increase in exposure which can block up the yellow
    parts of the image. You could try metering off grey cards but why
    bother when that is what the filter factor is meant to do - reproduce
    a middle gray. Add to this the fact that filters are said to change
    the contrast index of the negative (which can again affect local
    contrast which is what we're trying to control with the filter).

    <p>

    I've come to the conclusion that working with filters has all the
    perceptual problems involved with metering. When you meter a subject,
    there are always perceptual problems involved. What area do I meter?
    How do I want it reproduced i.e., which zone (its worth remembering
    that subjects don't occupy zones, we assign them to zones)? How does
    this relate to the reproduction of this other area? And so on. Given
    all of this, looking for this level of precision (one overall factor
    for all situations or one working method like metering through the
    filter) is a bit of a wild goose chase. I think you're much better off
    thinking about your picture and how the filter affects various parts
    of the picture with different filter factors. In other words, I
    suspect that better results are obtainable by understanding the
    components of the scene and what the filter does to each of these
    components. Good luck.

    <p>

    DJ
     

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