You are hinting that you think it is not so simple to just have a 'filter factor' and you are correct.
If you really want to be in control, in terms of AA's premise of 'visualization,' you need to know not only the film's spectral sensitivity, but also you need a light meter that would match the film's spectral sensitvity to meter through the filter to find the actual zone placements. (I'm not going to get into the issue if any meter was ever made that actually does this )
In general, though, the above information is not routinely available at the time of exposure. So, exposure compensation for filters is something you can gain from experience; guided by the filter factor.
Here we go again... I apologize in advance for this being too long.
In some ways, understanding filter factors is quite simple, in others, it takes a bit more mental gymnastics. Let's start with the basics:
If you photograph a gray card and a white card (i.e., absolutely even balances of colors) through a filter, you will find that you have to increase exposure to equal the negative density achieved without the filter. This is the "down-and-dirty" way filter factors are calculated. Other colors get changed depending on the color of the filter: "like colors are rendered lighter, unlike colors are rendered darker." That's all many of us need when using filters. (I'm assuming "white" light for all of this; warmer or colder light changes all this as well.)
Now however,, let's stretch the mental ligaments a bit: Let's use the OP's original orange filter. It transmits "orange" light, more than other colors. "Orange," however, consists of a range of red, orange, yellow and some green light, with greatest transmission centered on orange in the spectrum and diminishing in transmission in either direction. On one end of the spectrum, red is diminished a little. On the other side, dark green only barley gets through. Blue, indigo and violet (to use Newton's original designations) don't get through at all practically speaking.
Now, let's add some fruit to our gray and white cards; an orange, a dark red apple, a tomato, a green pepper, and an eggplant (aubergine for you Brits...) and take an exposures with and without the filter, using the appropriate filter factor. If all was done correctly, the cards will be the same density in both pictures. The fruit, however, should differ greatly. The orange, tomato, and apple will all be rendered lighter in the shot with the filter than they are in the unfiltered exposure (in decreasing amounts more than likely). The green pepper, depending on how light a green it is, will be marginally to quite a bit darker in the filtered shot. The eggplant will be significantly darker, if not black.
The same thing happens with the factors for filters of other colors/transmission spectra, i.e., the unsaturated colors are rendered the same as the no-filter exposure, but the more colorful parts of the scene being rendered lighter or darker depending where they fall in the filter's transmission spectrum.
To summarize: when one meters a scene and simply applies a (correct) filter factor, one is ensuring that objects with balanced color or low color saturation are rendered the same tone as without a filter. What is not quantified, however, is how brightly colored objects will be rendered. You do not know by how much (or if at all) a color will be changed. Yes, if I use a red filter, it will darken blue skies and green foliage, and lighten that Ferrari over there, but, by how much? Often it's a crap shoot. Experience and study help, but a usable quantification is lacking. Ya shoots thru the filter and ya takes yer chances.
Advocating spot metering through the filter.
If one had a spot meter that reacted to light in exactly the same way as your film did, you could simply meter through the filter and know to very great degree of accuracy which tone a given color would be rendered as. This was the idea behind the development of the Zone VI modified meters (how successful they were is debatable...).
Unfortunately, these are no longer produced (as far as I know) and are getting rarer on the used market. However, even with discrepancies in the spectral responses of different films and spot meters, the technique of metering through the filter and placing the values still has merit. One simply has to do a few tests to arrive at a set of "fudge factors" for using different filter/film combinations. These serve to "smooth out the bumps" in the mismatch between film and meter sensitivities. Some tests with a gray card and standard color swatches and you'll be ready to go. Just see if your results match your meter and, if not, adjust exposure and development accordingly.
For example, I use Tri-X a lot, and find that it needs an extra stop of exposure and about 20% less development than my Pentax spot meter would ordinarily call for when I use a #25 red filter. Dark green filters (e.g., #58) go the other way. Different films and meters react differently, so this has to be done for every film/meter combination. Fortunately, if find that weaker filters, like #8 and #11, can be used without adjustments and be well within "operating parameters." I keep a list of my filter fudge-factors with me and simply apply them when using strong filters.
The advantage to spot-metering through the filter (not incident, TTL or wide-angle metering) is that one can measure much more precisely where saturated objects will fall on the exposure scale. ("That blue sky still isn't dark enough with an orange filter... Maybe I'll use the #25, or add a polarizer.") This method is also helpful for recognizing and avoiding mergers ("Oops, with that orange filter, the evergreens and the blue sky behind them will be rendered the same shade of gray..."). Without a spot meter, of course, you just have to guess and hope.
Thanks for bearing with me...
Last edited by Doremus Scudder; 03-13-2010 at 02:52 AM. Click to view previous post history.
Filters always do three things:
Originally Posted by Shaggysk8
1) they transmit the color of the wavelength that is the color of the filter itself--red filter transmits more red wavelengths (thus more negative density for any object reflecting red wavelengths, thus ultimately lighter in the final print)
2) they absorb colors of the wavelength that are complementary to their own color--a red filter absorbs varying amounts blue wavelengths (thus less negative denisty for anything reflecting blue wavelengths such as the sky and thus ultimately darker in the final print)
3) they reduce the intensity of light reaching the film; to compensate for this reduction, a filter factor is employed. The basis of the factor is to maintain a middle gray negative density for any filter that is used (paraphrased from The Negative). Meaning, either extra time with the shutter or extra light with the aperture is needed to maintain the negative density for a middle gray subject, such as the gray card.
I just tried to put a different twist on some of the explanations. The attachment shows the Kodak colorwheel as found in The Negative with the complementary color opposite or somehwat opposite the color of the filter you are considering. Factors are not rock solid as they are relative to the film being used and what the manufacturer may state the filter factor may be for their film----------like anything, filters require practice with some experimentation to determine how best to use them.
And the light being used.
Originally Posted by CPorter
Factors for daylight and artificial light may differ quite a bit, depending on what colour filter is used.
Originally Posted by Doremus Scudder
Confusion where there should be lux;
For panic, you can't blame us.
Yet Newton strides upon the scene--
Photonic sage, Doremus.
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Thank you guys, I think I get the general idea I do have a pentax spotmeter 1/21 so I could check the correction I had to make, I have been out shooting and the pics have seemed to of come out ok I will print them tomorrow to get the real answer.
So spotmeters mine works but its big and old, also 2 stops off in low light. What are my options?
It's rare that my ramblings inspire someone to turn to versification! Thanks for the compliment and the fine rhyme; I'm flattered!
Last edited by Galah; 03-15-2010 at 12:13 AM. Click to view previous post history.
this is great !
Originally Posted by MikeSeb
There are other factors to consider even with TTL metering because the actual metering cells have a different spectral response to film, and tend to be over-sensitive to the red end of the spectrum,although the modern silicon photo diodes are better than the earlier Cadmium Sulphide cells were an 8X red filter can still confuse a TTL meter.
Originally Posted by Galah