I was working to get experience with filters. The subject was the west wall of an old, weathered, white wood building with dark amber windows. The time was 3:00 pm with broken clouds. My intention was to get bright sun on the building with broken clouds to the north above the church. I was using Tri-X Pan at 320, developed in HC 110. By the time I got everything set, the sun was behind clouds and the church was no longer bright white. The sky visible above the church was heavily clouded but I still had clouds and sky evident. Rather than go home empty handed, I shot one with no filter with the white church side set for Zone V (perhaps a bad decision, but I'm new to this). Then I shot with a Red 25 filter +3 stops.
The unfiltered shot has more snap, for lack of a better word, than the filtered one. There is more contrast between the lighted wall and shadowed areas of the wall. The rusted metal roof has mose contrast between the shiny portions and the rusted areas. It is cleaner and crisper. Unfortunately, the sky is white with no distinction between cloud and sky. Very uninteresting. The filtered sky shows some cloud and sky at the same exposure as the building, but needs twice the exposure to develope the rather nice, ominous sky. A higher contrast paper doesn't help the snap.
Quoting Ansel Adams in The Negative, "...red filters tend to ... produce strong contrast effects..." For some reason here, the contrast effect of the filter is oppposite what I would expect and want.
Today looks encouraging to try again. The sky is bright blue with scattered clouds. If that will hold untill this afternoon, I will go back out. Fortunately it's only about 10 minutes away.
I think it is mistake to think of filters as increasing contrast. It is perhaps more correct to describe them as changing the tonal relationships so that objects of similar colors as the filter appear lighter and those with different colors appear darker. The most dramatic example of this is perhaps the effects on the sky where a red filter will make a blue sky appear darker and the clouds lighter. Shadows cast by bright sunlight may also appear darker with a red filter because they are lit by the blue sky. The effects on foliage can vary considerably because different plants reflect different amounts of red, greens and blues. All of this may give the false impression of greater contrast, but I have read that many "contrast" filters actually decrease contrast. Gray objects, for example, may seem to have less contast with a filter than they would without.
I read something about a year or so ago about using a Wratten 25 to reduce contrast on interior shots with bright windows. Can't recall where right now, but the results were initially unexpected, then exploited to reduce contrast in similar circumstances where it was needed. The source I read could easily have been older.
You got exactly what you would expect for a red filter: increased contrast between the blue sky and white clouds.
You need to think in terms of what effect the filter will have on colours in the subject. A red filter will allow colours from the red end of the spectrum to pass and block colours from the blue end. The closer to red, the more will pass, the closer to blue, the more will be blocked. With a deep-red filter, even green will be heavily blocked as can often be seen in many landscape shots taken with a red filter.
The blue of the sky was heavily blocked by the filter, reducing its exposure on the film and hence reproducing darker on the final print, giving the contrast between cloud and sky. The red/brown rust will have reproduced lighter with the red filter, hence reducing the contrast between it and adjacent highlights (assuming they did not have a high red component too).
Cheers, Bob.
P.S. If you placed a white sunlit building on zone V you are at least 2, possibly 3 stops underexposed which will not help shadow detail and separation as you have pushed the shadows down into the toe of the film where contrast is lower. If you pushed them below the toe then they will print as featureless black.
You need to think in terms of what effect the filter will have on colours in the subject.
I find it impressively educational to just hold up various filters and *look* through them at prospective subjects. You'd think this would be obvious, but it took me forever to realise that I should be doing it habitually and that it would save some of the "error" part of the trial-and-error process with actual exposures.
Quoting Ansel Adams in The Negative, "...red filters tend to ... produce strong contrast effects..." For some reason here, the contrast effect of the filter is oppposite what I would expect and want.
I think Adams was referring here to scenic photography with lots of sky, where the red filter will have the effect of darkening the blue sky, to bring out more contrast in the sky between the sky and the clouds.
To increase overall contrast of the image, regardless of color, you could increase development time, although this only works to a certain extent before the negative is way too dense. You could also choose slower film, which is intrinsically more contrasty.
After reading all this and thinking more about the situation, I think the basic problem was that the light had changed so much that it was no longer what I "pre-visualized". The sky was cloudier than I was looking for. The sun was behind clouds and were not highlighting the building. I shot the building a V because that was was what it looked like at that time.
I tried a Yellow 12 in addition to the Red 25 and the cloud/sky was no better than the unfiltered.
I went back out yesterday about 3:00 and the exact thing happened with the lighting. I came back home. Today looks promising so I'll try again a little earlier before the heavy clouds move in. Make sure the sun is shining on the building and shoot it at VII. Also make sure there is plenty of good blue sky and white clouds in the sky.
JOhn Bond was on it. But it more like lower light intensity being the progenator of the increased contrast. There is more contrast between lower exposure zones than there is between higher ones, hence expose for the shadows . . .
So, there are several things that affect what a filter will actually deliver. Yes, you can indeed hold the filter over your eye and get a rough idea, but bear in mind that what is actually captured is a convolution of the filter transmission and the film sensitivity as a function of wavelength. The deal is that if the film sensitivity has a sharp derivative (change) over the wavelength range of the filter, then you can get one result.... and if the sensitivity is flat over that window then you get an entirely different effect. Another factor that enters is the actual spectral distribution of the light that you're shooting with, which can of course change quite rapidly (as John noted). So there are at least three factors that determine what a filter will do with a particular scene with a particular film.
(This is why filter factors need to be taken with a grain of salt. An extreme example of this is using the near IR films with an IR filter that is right on the edge of their sensitivity. You can get some very high edge contrast, and wildly variable filter factors, among other things.)
On top of all this, of course, the main reason why people associate red filters with more contrast is that the Rayleigh scattering associated with haze goes as 1/(wavelength)^4, so in other words longer wavelengths scatter much less in the atmosphere and a red filter will thus "see" less haze. Use a blue filter and you get lots of haze. People associate haze with low contrast....
Unexpected result with Red 25 filter 
Linear Mode
