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Thread: exposure with filter and spotmeter

  1. #1

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    exposure with filter and spotmeter

    I use a Pentax spot meter. I am wondering if rather than calculating filter factors, I can just place the filter over the spot lens and take a reading. Does that work?

    Thanks!

  2. #2

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    Re: exposure with filter and spotmeter

    It won't deliver a linear result across the range of colors, so the theory is that you shouldn't do that. For example, if the meter's sensor is overly blue sensitive and you meter on something blue, you might get inaccurate exposures. That said, I've been doing it for years and get pretty consistent results.
    Michael W. Graves
    Michael's Pub

    If it ain't broke....don't fix it!

  3. #3

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    Re: exposure with filter and spotmeter

    The Zone VI modified meters are supposed to be accurate when you take a reading through the filters. The regular non-modified meters are not calibrated for that use.

  4. #4

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    Re: exposure with filter and spotmeter

    Or you can dial in a new ISO setting with the filter factor (Don't forget to change it back, later!!!)

    But even with my fairly modern Leica R6 TTL meter, B/W filters won't jibe with the factors... Then there's that spectral sensitivity curve on the film thing...

    Steve K

  5. #5

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    Re: exposure with filter and spotmeter

    Quote Originally Posted by fralexis View Post
    I use a Pentax spot meter. I am wondering if rather than calculating filter factors, I can just place the filter over the spot lens and take a reading. Does that work?

    Thanks!
    If you use a Green, Yellow, Orange filters over the spot lens it should give a more consistent result. With Blue and deep Red filters reading should be less consistent.


    If you look at the SPD spectorscopic sensitivity chart of the Pentax:

    http://www.pacificrimcamera.com/rl/00042/00042.pdf

    you see that spectral sensitivity for blue is 50% than with green.

    So with a deep blue filter over the meter you will block a lot of long waves, and as the SPD of the film is different that the meter the reading will tend to overexpose (I guess is like this, not the counter), also the same will happen with deep red, but perhaps with a lower extend.

    I'd suggest you to try it, just look what exposure indicates the meter with the filter, and what is calculated without the filter and applying the correction factor. Then just write in a paper the difference you have with each filter, and form then you know if the yellow match or if with the deep red you have to stop down 1/2 from what the Pentax says with that filter over it.

    All also depends on the WB of the light source and the color of your subject...

    Also the filter correction factor is orientative, as it do not account for SDP of the light, the color of the subjects and the film SPD.


    Regards.

  6. #6
    Drew Wiley
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    Re: exposure with filter and spotmeter

    All my meters are precisely calibrated and matched Pentax digital spotmeters. I find it far more predictable to bracket-test for filter factors relative to SPECIFIC films and your representative subject matter and lighting conditions, then use the filter factors, rather than metering through a filter over the meter itself, which just opens a writhing can of worms. After awhile, filter factors get downright second-nature, and with most black and white applications you can round a bit, to the nearest full stop. Generally the filter factors published by the specific filter manufacturer are a perfectly usable starting point for typical panchromatic films in
    daylight. Just beware of films with peculiar spectral sensitivity like ACROS, which is actually orthopan.

  7. #7

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    Re: exposure with filter and spotmeter

    I have a Zone VI modified one. It has worked fine with yellow, orange and green filters. Film (Tri-X, usually) is very underexposed with a red filter. So I use the filter factor instead. Would an unmodified meter give me nearly the same results? Could be, I haven't tested.

  8. #8
    Drew Wiley
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    Re: exposure with filter and spotmeter

    No, they won't give the same result. I don't want to get in a debate over whether Fred Picker improved the meter or ruined it with those modifications. About all I can say is that he "standardized" it to Tri-X, and hence "unstandardized" it to anything significantly different. Hollywood cameramen also have their own debates over what this does to color film metering. I am accustomed to its out-of-the-box peak sensitivity to green, just like our own human vision, and know that panchromatic films are slightly depressed in green sensitivity, but otherwise differ from one another in the degree of blue and red sensitivity.

  9. #9

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    Re: exposure with filter and spotmeter

    I've written a bit here and on APUG about how I use my Pentax spotmeters to read directly through filters. I'll elaborate a bit later, but first, here's a quote from the manual that comes with the Pentax digital spotmeter:

    "By mounting different filters in front of the objective lens of the Pentax Spotmeter and comparing the readings taken of the various colored areas with each filter, it will be easy to determine which on provides the best contrast for black and white film."

    So, obviously the manufacturer thinks metering through the filter a viable option.

    However, there are a couple of problems with metering through filters. First is the mismatch of sensitivity spectra that has already been mentioned. This can be overcome by doing a bit of testing with the most extreme filters you use on a given film and arriving at some exposure factors.

    The second, and less discussed, problem is that extreme filters that cut all but one wavelength of light (e.g., #25 red or #58 green, etc.) can affect the contrast of the film. This is not a property of the filter, rather a property of the film and the way it is sensitized and will occur whether reading through the filter or applying factors. For example, with TXP, a #25 red filter increases contrast for me the equivalent of about N+1. However, with TMY, the same filter reduces contrast somewhat. Again, a bit of testing with the films you use will give you a development factor if you wish.

    There is a distinct advantage to metering through filters instead of applying factors that makes it a no-brainer for me. Only by metering different areas through the filter can you really see and approximately quantify the difference in exposure for areas of different color. This will show you if there is enough tonal separation between these areas or not. It's really the only way you can visualize a scene when using filters and a great tool for us Zone System users. I can easily spot a convergence between say a blue sky and a green roof and change filters if needed. Just applying factors doesn't allow this.

    So, what I have done is made a chart for selected filters and the films I use (as Pere mentions, only for the more extreme filters) that includes an exposure factor and a development factor. Example: when metering through a #25 filter and using TXP I'll give 2/3-stop more exposure and develop N-1 from the normal indication.

    Search on the topic and my name here and on APUG to turn up more discussions on the subject if you're interested.

    Best,

    Doremus

  10. #10

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    Re: exposure with filter and spotmeter

    Quote Originally Posted by Doremus Scudder View Post

    "By mounting different filters in front of the objective lens of the Pentax Spotmeter and comparing the readings taken of the various colored areas with each filter, it will be easy to determine which on provides the best contrast for black and white film."
    I've got some great ideas from your comment...

    When one masters the spot metering with the filter on the spot lens... he can know with precission how the sky is darkened by the filter, for example, in order to determine a N-2 development or not... !!!

    This is a powerful tool.

    Anyway one has to know how the meter reacts to colors. Spectral sensitivity of film is not as critical as it is mostly flat, a 20% more sensitivity in a band it is a tinny fraction of an stop.


    For consistency for now I'm using a Nikon F5 to meter LF in spot mode, if has a RGB photometer and things are pretty consistent.

    With an F5 you can meter with any filter on the lens and results are ultra precise. For LF I spot meter with a prime lens (50mm) i the F5 that has same or close optical transmision than an LF lens. I tested it by placing the F5 i the back of the view camera.

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