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  1. #1

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    Filter factors with 25A, ND, Polarizer

    Hey all, this is my first post here and I was told to come here because you were very knowledgable regarding this stuff.

    In a little over a week I'm going to be headed out to the Grand Tetons and Yellowstone for a week of camping and photography. For the trip, I bought a Hoya 25A filter to help enhance the look of the sky in my shots. While looking at the Kodak site, I found that Tri-X has a filter factor of 8 (3 stops) for that particular filter. It looks to be the same for Tmax films, and 2 stops for the Pan-X film. Has anyone here verified this? Also...how does the filter affect the meter usually? (ie - it might only meter that I need 1 stop more of light, but in reality I'd have to set the exposure compensation to +2 to make up for the total difference of 3 stops). I guess this may be something I'd have to play around with. I'll have to get real good at doing exposure math in my head :-)

    I also have a circular polarizer that I may use in addition to the 25A. The polarizer I believe adds 1.5-2 stops to the exposure time, correct? Would the filter additions be additive? (ie - would it be 2 + 3 = 5 stops, or 2 x 3 = 6 stops?) I'm guessing additive but I just wanted to verify :-)

    Also, looking through the the Tri-X and Tmax 100 technical publications, it seems to say that the filter factor for using a polarizer is 2.5 (1 2/3 stops). Is this in ADDITION to the effect that it'll already have on my meter or will this exposure change pretty much line up with my metered value?

    My goodness...if I used my polarizer + 25A + ND 3x, I'd have about 8 stops exposure difference (and maybe some vignetting if I'm lucky, lol).

    Thanks a bunch!

  2. #2
    Paul Sorensen's Avatar
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    The polarizer will vary depending on its orientation. I have had good luck metering through the polarizer when using a camera with TTL metering. I have not tried one without, so I can't answer the question about specific factors. With a red filter I do not trust the meter. The TTL meter on my camera will tend to show about 2 stops and I adjust by another stop to make sure. As for the differences between films, they do have differing levels of red sensitivity, so I would be inclined to follow the tech specs, I have just never looked it up myself.

    As for the additive nature of the filters, logic tells me that you just add the numbers of stops of correction. If you are using a wide angle lens, you should have plenty of vignetting, depending on the lens. I get vignetting with my Mamiya 45mm 645 lens with two flters, three would be a huge amount.

    Paul.

  3. #3

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    Paul,

    Thanks for the help! I'll have to do some tests on my camera to see what the red filter will show normally. With most b&w film I tend to overexpose about a stop anyways (a professor I once had gave the suggestion and it does seem to help, especially on tmax 100), so I'll have to keep all those factors in consideration. I always forget about the reciprocity factor too, which could be an issue if i'm dealing with 8 stops worth of difference in exposure.

    I'm going to be using my 17-35mm lens and a 28-135mm lens. On film I'll most likely use mostly 24-28mm, kind of a standard landscape focal length for 35mm. My 3 stop ND filter is a slim filter so hopefully that'll cut down on vignetting a little. Oh well, I suppose I don't mine a little bit of it.

    I'll also be shooting with a Hasselblad 80mm lens with a Bayonet 50-62 adapter, and then a 62-77mm step-up ring (heh, yea...I really don't feel like buying more filters than I have to). I'm thinking this setup most likely won't give too much in the way of vignetting but I can't be sure.

  4. #4
    David A. Goldfarb's Avatar
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    In general, yes, filter factors are additive.

    Can you trust your meter, when you meter through the filter? It depends. The spectral sensitivity of the meter may be different from the film, and then if you've got a lot of filtration, you may reduce the light getting to the meter sufficiently, so that the response of the meter is no longer linear.

    With a circular polarizer, you should be able to meter accurately through the filter, unless the light is very low to begin with. One thing to watch out for with the wide lens and a polarizer is uneven polarization. This is particularly visible with landcapes that have a lot of open sky. I usually don't use a polarizer with anything wider than 28mm on a 35mm camera or equivalent, unless it's a scene that lends itself to that where the polarization effect is needed only in one area.

    An ND 3x filter is 1.5 stops (not 3 stops). Is this a solid ND or an ND grad? If it's an ND grad, meter for the land, and then adjust the grad filter to bring down the sky, unless you have matrix metering, where you can probably get a good result by adjusting the filter and then just metering normally.

    In general a 25A filter has a factor of 8x or 3 stops, but this can vary with some films. Careful with a 25A. It's a filter that beginners tend to overuse (I certainly did!). It usually is a bit overdramatic. Now I'm much more likely to use a medium yellow or orange filter to improve cloud separation. Another neat trick is to use one of those color grads, like a tobacco grad, designed for cheesy fake sunsets with color, but use it for B&W. It will increase contrast in the sky and bring the exposure closer to the land, but won't affect the land.
    flickr--http://www.flickr.com/photos/davidagoldfarb/
    Photography (not as up to date as the flickr site)--http://www.davidagoldfarb.com/photo
    Academic (Slavic and Comparative Literature)--http://www.davidagoldfarb.com

  5. #5

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    Actually the ND filter is 3 stops (8x), and it's a solid one. I shouldn't have put 3x in my first post, sorry for the confusion. What I don't quite understand is the notation that some places use which are .3 for 1 stop, .6 for 2 stops, and .9 for 3 stops. I can understand the 8x math (2^3). Hmm, weird :-) Anyways, thanks for the tip on metering. I may go with grad NDs down the line at some point once I get more comfortable shooting landscapes in b&w.

    Good tip on overusing the 25A. I will certainly watch for that. Have you ever used the 25A in portrait work? It seems like it would be a good filter for that, given skin tones. In my digital conversions of color->bw, I almost pick a method that emphasizes the red tones in portrait work.

  6. #6
    David A. Goldfarb's Avatar
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    A red filter (usually you would use a light red for portraits, if at all) can lighten skin blemishes for portraits, but it can also make the lips unnaturally light. In the old days they used to use green lipstick to make the lips darker.

    The density units .3, .6, etc. are logarithmic units (LogD), and they are the same units that are used on a transmission densitometer for measuring density on a negative. So when Adams suggests that exposure should be adjusted so that Zone I density is .1 and for typical enlargements development time should be adjusted so that Zone VIII density is around 1.2 (the actual value will depend on the paper, enlarger, and print developer, but 1.2 is not a bad starting point), these are the same units as the units used to describe ND filters.
    flickr--http://www.flickr.com/photos/davidagoldfarb/
    Photography (not as up to date as the flickr site)--http://www.davidagoldfarb.com/photo
    Academic (Slavic and Comparative Literature)--http://www.davidagoldfarb.com

  7. #7

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    Very interesting info, especially about the lipstick :-P. I'll have to remember that if taking portraits of my sister or girlfriend.

    The density information is great as well. Thanks for the info! It's always good to have an understanding of all of this stuff :-)

  8. #8

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    log(2) = 0.3
    log(4) = 0.6
    log(8) = 0.9
    etc.



 

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