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

[Ulophot]

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.

[Bob Salomon]

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

[Drew Wiley]

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.

[Ulophot]

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.

[Mark Sampson]

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.

[BrianShaw]

About +2/3 stop

[tgtaylor]

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.

[Doremus Scudder]

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

[tgtaylor]

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

[Doremus Scudder]

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

[tgtaylor]

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

Doremus,

Take your filters and meter outside and hold them up to the blue sky opposite the sun and see what you get. I did that this afternoon after my bike ride and this is what I got:

Deep Yellow 15 (Tiffen glass filter): 15.4 EV sky, 13.6 EV sky and filter. (Tiffen 1-2/3/stops).
Yellow Z001 (Cokin resin filter) 15.4 sky, 14.1 sky and filter (Cokin 1 stop).
Green Z004 (Cokin resin filter): 15.4 sky, 13.4 sky and filter. (Cokin 2-stops).
Blue Z020 (80A) (Cokin resin filter): 15.4 sky, 14.0 sky and filter. (Cokin 1-1/3 stops).
Blue Z021 (80B) (Cokin resin filter): 15.4 sky, 14.3 sky and filter. (Cokin 1-stop).
Red Z003 (Cokin resin filter): 15.4 sky, 11.4 sky and filter. (Cokin 3-stops). 1 stop off.
Orange Z002 (Cokin resin filter): 15.4 sky, 12.8 sky and filter. (Cokin 1-1/3 stops). 2-2/3 stops off.
Red Z007 (IR) (Cokin resin filter): 15.4 sky, 7.5 sky and filter. (Cokin ? - Amazon says 6 stops).
Light yellow-warming (81B): 15.4 sky, 14.8 sky and filter. (Cokin 1/3 stop). 1/3 stop off.

Sekonic L-758 DR light meter.

Thomas

[Doremus Scudder]

Ha!

If you think I'm going to see any sun here in Eugene for a few weeks, you're an optimist. Much less a nice clear patch of blue sky opposite the sun. And then there are those pesky mountains and trees blocking the horizon too :)

I don't know where you are, but "blue" sky here in the Willamette Valley is often closer to white than the blue skies in the mountains or down in the SW deserts. I guess your meter reading would depend greatly on the quality of the blue sky you're metering.

Best,

Doremus

[Mark Sampson]

Haha. I read Doremus' last comment... and was reminded of the old joke (popular among RIT students) that Kodak invented the 18% gray card because that was the color of the sky in Rochester. Having been born and raised in Rochester, and spent the first 55 years of my life there, I'll say there's at least a grain of truth in that.

[Drew Wiley]

Tom - metering through filters doesn't work so well for two key reasons. First, the peak sensitivity of the meter is probably different from the film; second, pan films differ somewhat from one another in this respect, including in degree of blue sensitivity. To determine an accurate filter factor, one must make parallel gray card shots under identical conditions, with and without a filter, and then develop them together and read both on a densitometer afterwards. This is easier, or course, if a bracket test is done on roll film for the filtered version at slightly different settings, provided such a roll film has the same emulsion characteristics as the same label of sheet film. But this is such a light blue filter to begin with that it's not going to present a big problem.

[Ulophot]

[QUOTE= I'll say there's at least a grain of truth in that.[/QUOTE]

A grain -- I get it.

[tgtaylor]

I'll stick with my method which, except for the orange filter, turned out pretty accurate with the Mfg's ratings. Once the rating is verified it is not necessary to do it again unless the condition of the filter changes. I think that they slight differences recorded were due to my trying to hand holding the filter flat against the meter is somewhat difficult due to the thinness of the resin filters and my innate reluctance to get fingerprints on it, and that I didn't have the meter pointed at the exact section of the sky with each reading. But then the value of the sky is consistent until the sun nears the horizon. Years and years ago I metered a composition thru the filter as Doremus suggested and ended-up with a bad exposure and never did that again.

Instead of making"...parallel gray card shots under identical conditions, with and without a filter, and then develop them together and read both on a densitometer afterwards. This is easier, or course, if a bracket test is done on roll film..." I'll just stick with my method.

Thomas