Filters vs Gelled light

[Pieter12]

I am going to be setting up a table-top shot that I will shoot in B&W--probably on FP4+. I want to darken the yellow and red tones in the image. Normally, I would use a blue or green filter on the lens, but I don't have one big enough for the lens I will be using.
Two questions:
1. If I gel the lights blue or green, will I get the same effect as using a filter on the lens?
2. When using a filter, I would apply the filter factor to the meter reading. With gelled lights, I assume the light will be diminished and I would not have to make any other compensation, so just go with the reading as is.

I think my assumptions are correct, but I thought I'd take advantage of the more knowledgable of you out there to verify or correct me.

 

[AgX]

It makes no difference where the light passes the filter (before or beyond the subject).

(Only exception would be luminescence at the subject.)

 

[John Koehrer]

#2 is correct. reading direct from meter is good.

 

[Sirius Glass]

#2 is correct. reading direct from meter is good.

NO, ever compared reading through a filter to the filter factor? Because light meters do not react to the spectrum the way film does, the results are different. I have compared both and for other than sky light filters and polarizers, stick with the filter factors.

 

[Pieter12]

Even though filter factors are supplied with the gels, tit would seem the factor would vary with the distance and intensity of the light source. I guess I could take a reading of the ungelled light and reduce that by the corresponding gel factor, It just gets complicated with multiple lights.

 

[Doremus Scudder]

NO, ever compared reading through a filter to the filter factor? Because light meters do not react to the spectrum the way film does, the results are different. I have compared both and for other than sky light filters and polarizers, stick with the filter factors.

Even though filter factors are supplied with the gels, it would seem the factor would vary with the distance and intensity of the light source. I guess I could take a reading of the ungelled light and reduce that by the corresponding gel factor, It just gets complicated with multiple lights.

I've got to chime in here...

Yes, meters respond differently to various colors than black-and-white film does (with the possible exception of the Zone VI modified meters). However, filter factors are similarly imprecise; they are not calibrated to a specific film and are figured using a specific color temperature of light (usually average daylight), which may or may not be what the photographer has available.

As far as using hot lights... Filter manufacturers used to give "tungsten factors," since the filter factors for many colored filters is radically different under tungsten light than daylight (the usual calibration standard). Factors for red filters are much less under tungsten light; green and blue filter factors are much greater. Unless you have the tungsten factors for the filters you are using and know the color temperature of your lighting (2400k - 3400k for different types of photofloods), just applying the filter factor may end up with an exposure that is way off.

I've been reading through filters for years with success; better exposure accuracy than just applying factors. The trick is to do a bit of calibration and come up with an exposure compensation factor for each filter/film combination. Note that the factor will be film-specific, since different films have different spectral responses (that's why factors are inaccurate...). I've found that the weaker filters, like #8 yellow and even a light yellow-green filter (XO) don't need much compensation, so one could start calibrating with the strongest filters first to save a bit of time. A #25 red filter needs both exposure and development changes from "normal" for me; less development with TXP, but more with TMY. It seems the way the film is color sensitized affects the contrast when developing as well.

The procedure is simple. Take a reading through the filter, make an exposure using that and bracket on either side by a stop. Make two of each neg. Develop one set as you usually would for the subject-brightness range of the scene and make some prints. If you find you need more or less development for your best exposure, make the appropriate change for the subsequent neg.

One could just take a reading, make a negative and see how it prints and then make whatever extrapolated adjustments one thinks are necessary for the next photograph. By keeping good notes, one could zero in on the right compensation in just a few sessions.

Since gelled lights work on the same principle as colored contrast filters, the same method works when using them. A couple of tests with the filter/film set up and one should be able to simply read with the meter and apply the compensation factor.

[Note: light-to-subject distance changes the intensity of the light reaching the subject according to the inverse square law, but the color temperature of the light remains the same. Whatever factor works at one distance will work at another.]

Best,

Doremus

 

[DREW WILEY]

To understand true filter factors under a specific light source, using a specific film, one has to run a test, then compare the developed film results using a densitometer. This is most easily done using bracketed exposures on roll film. Different brands of even the same nominal filter color can sometimes differ, as do lighting gels from lens filters. As already noted, very strong contrast filters can sometimes affect contrast gamma, not just exposure density, and possibly require different development times. Metering through strong filters often gives a deceptive reading, as does TTL metering. Avoid discontinuous light sources. Both photofloods and daylight are continuous spectrum, but will require different filter factors. Whenever I purchase an unfamiliar filter, I run a test using my favorite films. But don't get intimidated. The FP4 spec sheet will give you fairly approximate recommendations for the most common filters under standardized lighting.