Page 1 of 2 12 LastLast
Results 1 to 10 of 16

Thread: The Wratten Mystery, or, What's My Factor?

  1. #1

    Join Date
    Apr 2015
    Location
    Purcellville, VA
    Posts
    1,342

    The Wratten Mystery, or, What's My Factor?

    This is one probably of the older-timers.

    Having recently purchased a set of Wratten gels, I took the #38 blue out to see what it might do for our clematis. I gave it a factor of 2 just looking by eye, then came back in and tried to find the actual factor. I cannot find one listed. The old Adams Natural Light Photography lists a C5 blue with a factor that "may be" 6 with panchromatic film. Any correspondence there? Yes, I'll see the negs when developed, but I do wonder why no factor is (apparently) given. I don't have a copy of the old Kodak guide to Wratten filters.
    Philip Ulanowsky

    Sine scientia ars nihil est. (Without science/knowledge, art is nothing.)
    https://www.flickr.com/photos/156933346@N07/

  2. #2

    Join Date
    Sep 1998
    Location
    Buford, GA
    Posts
    14,257

    Re: The Wratten Mystery, or, What's My Factor?

    You do know that hose gels fade with time, heat and light, don’t you?

  3. #3
    Drew Wiley
    Join Date
    Sep 2008
    Location
    SF Bay area, CA
    Posts
    16,820

    Re: The Wratten Mystery, or, What's My Factor?

    The Wratten handbook gives transmission charts and percents at various wavelengths. For daylight 5500K or close to that, it's around 48% transmission for light blue no. 38; so a single stop of correction. In other words, a 2X factor just like you guessed. But you might need to test a bit, and give a bit more exposure since pan films are not fully sensitive to green. Depends on your subject matter too. Then there is what Bob said - these can fade over time, depending on how they were used and stored.

  4. #4

    Join Date
    Apr 2015
    Location
    Purcellville, VA
    Posts
    1,342

    Re: The Wratten Mystery, or, What's My Factor?

    Thank you, gentlemen both. Yes, I know that filters fade. These have been well kept and handled, and while they may not be full-intensity, they look close by eye and, as I indicated, the negatives will tell the story. I could test them all but see no need, since I don't expect to be using them often. My concern is primarily portraiture, B&W only, and nothing flashy; these will be for occasional landscape, still life, etc. Kelvin, light direction, and subject matter are factors to consider, but that goes with any contrast filter.

    Thanks again.
    Philip Ulanowsky

    Sine scientia ars nihil est. (Without science/knowledge, art is nothing.)
    https://www.flickr.com/photos/156933346@N07/

  5. #5

    Join Date
    Mar 2002
    Location
    now in Tucson, AZ
    Posts
    3,240

    Re: The Wratten Mystery, or, What's My Factor?

    The publication "Kodak Filters" has all the data you're ever likely to need about Wratten gels. There were several editions; of course it's out of print now, but I'm sure you can find a used copy. I used it extensively in my Kodak days... in my department there we had a 6' high stationery cabinet filled to the brim with (probably) every filter Kodak ever made, in every size up to 12" square. And my work there did call for some unusual filters on occasion.

    Although I doubt that the book offers exposure factors for most of the specialist filters (like your #38), Drew's advice about reading transmission levels should provide a starting point. "Some testing required for critical work."

    It's true, per Bob Salomon. that gel filters will fade. This is mostly an issue when used for printing or copying, where the filters are exposed to high-intensity light for long periods of time. Occasional use over a camera lens to make (relatively) short exposures should not cause fading for a long time. But who can prove that, either way? Make your tests and keep notes. best of luck!

    (addendum) I still have my copy, contact me off-list if you need help.

  6. #6

    Join Date
    Dec 2005
    Location
    El Pueblo de Nuestra Señora la Reina de los Ángeles de Porciúncula
    Posts
    5,378

    Re: The Wratten Mystery, or, What's My Factor?

    About +2/3 stop

  7. #7
    http://www.spiritsofsilver.com tgtaylor's Avatar
    Join Date
    May 2006
    Posts
    4,667

    Re: The Wratten Mystery, or, What's My Factor?

    Go outside with your spot meter and filter. Take a spot reading of the blue sky opposite the sun. Then place your filter flush with the meter and take a reading. That's the filters f-stop factor.

  8. #8

    Join Date
    Sep 1998
    Location
    Oregon now (formerly Austria)
    Posts
    3,084

    Re: The Wratten Mystery, or, What's My Factor?

    Quote Originally Posted by tgtaylor View Post
    Go outside with your spot meter and filter. Take a spot reading of the blue sky opposite the sun. Then place your filter flush with the meter and take a reading. That's the filters f-stop factor.
    Actually, the test should be done with a grey card illuminated by nice, neutral 5600K daylight. Note that the color of the object you read with and without the filter will skew the results. Read the blue sky with and without the filter and you'll get a rather low "factor" (the meter's just seeing blue the whole time - the factor may be close to zero). Try that same test with a red-painted house and the "factor" will be much higher.

    The laboratory method of finding a filter factor (described in the my copy of "Wratten Filters" pub. by Kodak Ltd, London, 1953, pp. 9-10) was to expose the film through the filter with a calibrated light sources using an intensity-scale densitometer. Curves are plotted and speed point found. The factor is based on the exposure through the filter that results in the same density as the exposure made without a filter.

    The method given for finding one's own filter factor is as follows:

    "Select a scene containing a neutral-grey area and make one exposure without the filter. Then, with the filter in position, make a series of exposures of the same scene. Start the series with a lens opening one-half stop larger than the one used without the filter. Progress by half-stops through a range of 2-3 stops. Match the density of the neutral area in the unfiltered negative with the density of the same area in one of the filtered negatives. The factor for that particular filter can then be calculated from the difference in exposure between the two negatives.

    It is important that, when matching the density of the neutral area, the gamma values of the two negatives must be roughly equal ; a 25 per cent, increase in development is recommended for negatives made through a dark blue filter to compensate for the loss of contrast.

    For very critical work, tests should be carried out using shutter speeds similar to those which are to be used in practice."


    My later Kodak Photographic Filters Handbook, pub. Eastman Kodak Co. 1990, has a more abbreviated section on filter factors. The method for determining filter factors given there is:

    "Choose a subject having a neutral-gray area, or place a KODAK Gray Card or a photographic gray scale in the scene to be photographed. (In photomicrography, use the illuminated field without a slide or with a clear area of slide.) Make one exposure without a filter. Then, with the filter in place, make a series of exposures, increasing by half-stop increments, through 2 to 4 stops greater exposure, depending on the filter. Match the density of the unfiltered shot with the comparable density on one of the filter series; do this either visually or with a densitometer. Determine the filter factor from the difference in exposure between the two exposures producing equal densities."

    Note that there could still be discrepancies due to the difference in spectral response between meter and film.

    All that said, instead of determining and applying factors, you can just meter your subject through the filter and generally get good results. Keep notes and add "fudge factors" for specific film/meter/filter combinations (development adjustments might need to be made as well).

    Personally, I find spot-metering through the filter after having tested for the above to be more accurate than applying factors.

    Interestingly, neither of the two references gives factors for the larger majority of available filters; only for CC filters, the color-conversion filters and the light-balancing filters. There is a ton of transmittance data and graphs, however, from which one could extrapolate starting points for factors.

    Best,

    Doremus

  9. #9
    http://www.spiritsofsilver.com tgtaylor's Avatar
    Join Date
    May 2006
    Posts
    4,667

    Re: The Wratten Mystery, or, What's My Factor?

    Quote Originally Posted by Doremus Scudder View Post
    Actually, the test should be done with a grey card illuminated by nice, neutral 5600K daylight. Note that the color of the object you read with and without the filter will skew the results. Read the blue sky with and without the filter and you'll get a rather low "factor" (the meter's just seeing blue the whole time - the factor may be close to zero). Try that same test with a red-painted house and the "factor" will be much higher.

    The laboratory method of finding a filter factor (described in the my copy of "Wratten Filters" pub. by Kodak Ltd, London, 1953, pp. 9-10) was to expose the film through the filter with a calibrated light sources using an intensity-scale densitometer. Curves are plotted and speed point found. The factor is based on the exposure through the filter that results in the same density as the exposure made without a filter.

    The method given for finding one's own filter factor is as follows:

    "Select a scene containing a neutral-grey area and make one exposure without the filter. Then, with the filter in position, make a series of exposures of the same scene. Start the series with a lens opening one-half stop larger than the one used without the filter. Progress by half-stops through a range of 2-3 stops. Match the density of the neutral area in the unfiltered negative with the density of the same area in one of the filtered negatives. The factor for that particular filter can then be calculated from the difference in exposure between the two negatives.

    It is important that, when matching the density of the neutral area, the gamma values of the two negatives must be roughly equal ; a 25 per cent, increase in development is recommended for negatives made through a dark blue filter to compensate for the loss of contrast.

    For very critical work, tests should be carried out using shutter speeds similar to those which are to be used in practice."


    My later Kodak Photographic Filters Handbook, pub. Eastman Kodak Co. 1990, has a more abbreviated section on filter factors. The method for determining filter factors given there is:

    "Choose a subject having a neutral-gray area, or place a KODAK Gray Card or a photographic gray scale in the scene to be photographed. (In photomicrography, use the illuminated field without a slide or with a clear area of slide.) Make one exposure without a filter. Then, with the filter in place, make a series of exposures, increasing by half-stop increments, through 2 to 4 stops greater exposure, depending on the filter. Match the density of the unfiltered shot with the comparable density on one of the filter series; do this either visually or with a densitometer. Determine the filter factor from the difference in exposure between the two exposures producing equal densities."

    Note that there could still be discrepancies due to the difference in spectral response between meter and film.

    All that said, instead of determining and applying factors, you can just meter your subject through the filter and generally get good results. Keep notes and add "fudge factors" for specific film/meter/filter combinations (development adjustments might need to be made as well).

    Personally, I find spot-metering through the filter after having tested for the above to be more accurate than applying factors.

    Interestingly, neither of the two references gives factors for the larger majority of available filters; only for CC filters, the color-conversion filters and the light-balancing filters. There is a ton of transmittance data and graphs, however, from which one could extrapolate starting points for factors.

    Best,

    Doremus
    I'll stick with measuring the blue of the sky opposite the sun - simpler that way and the results agree with the manufactures (quality manufactures such as B+W, Tiffen, etc) published f-stop factor. Moreover the blue sky is generally considered to be the same tone as neutral grey.

    Thomas

  10. #10

    Join Date
    Sep 1998
    Location
    Oregon now (formerly Austria)
    Posts
    3,084

    Re: The Wratten Mystery, or, What's My Factor?

    Quote Originally Posted by tgtaylor View Post
    I'll stick with measuring the blue of the sky opposite the sun - simpler that way and the results agree with the manufactures (quality manufactures such as B+W, Tiffen, etc) published f-stop factor. Moreover the blue sky is generally considered to be the same tone as neutral grey.

    Thomas
    But it's blue...

    Using the blue sky instead of a neutral source will almost certainly result in lower factors for blue filters (I know, not used that much) and higher factors for any filters that block blue (not that big of a deal, since you'd just end up overexposing a bit). Using something neutral gray would be more even across the board, which is why Kodak and others recommend doing that.

    But, if you're getting good results, what you're doing is close enough. Our medium has lots of wiggle room

    Doremus

Similar Threads

  1. Replies: 19
    Last Post: 28-Feb-2018, 02:08
  2. Q-Factor
    By Claude Sapp in forum Digital Processing
    Replies: 3
    Last Post: 29-Jan-2009, 11:23
  3. Which filter factor to use?
    By PViapiano in forum Lenses & Lens Accessories
    Replies: 6
    Last Post: 13-Oct-2006, 16:01
  4. Filter Factor
    By Jerry Cunningham in forum Lenses & Lens Accessories
    Replies: 4
    Last Post: 5-Oct-2006, 14:17
  5. Which filter factor to use
    By Lawrence Floyd, Jr. in forum Lenses & Lens Accessories
    Replies: 3
    Last Post: 25-Mar-2004, 19:30

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •