I guess I've been drawn into this thread... so here goes.
First, this may or may not help the OP, who is likely just beginning to wrestle with filter factors. Hopefully, it will dispel some of the misconception many have about metering through the filter vs applying filter factors.
Before getting into anything, let me say I'm an advocate of metering through filters, with appropriate compensations.
Filter factors are guidelines given by either the filter manufacturer or the film manufacturer. The latter are the more accurate. However, filter factors can be off by a great deal depending on the characteristics of the scene being photographed.
The idea behind a filter factor for black-and-white photography is that grey card, which should contain an even distribution of all colors, will be rendered exactly the same density when taken without a filter and with a filter with the appropriate filter factor applied. This varies with film and lighting and, in an ideal world, would be determined by testing for each film/lighting combination using a grey card. Manufacturers just do their best and give an intermediate value with error on the "safe side."
This is great for taking pictures of grey cards, but doesn't work very precisely in the real world where colors are more randomly distributed. Filters for black-and-white negative film block light of certain colors and hence render those colors darker in the final print. That's why we use them; yellow darkens the blue sky (but also the blue-lit shadows...), orange removes blue and a little green, etc. When we choose a filter and apply a factor, we are basically ensuring that a grey card (if there were one in the scene) will be "faithfully" reproduced. And, although we know which color we are trying to darken, we have no quantifiable way of determining just
how much the filter will affect, say, that blue sky, or those blue shadows, not to mention that yellow sign with white lettering (oops, that'll turn out an unreadable blank with a yellow filter...). We instead use our knowledge and experience to make a best guess. In the end, considering that filter factors are approximate anyway, and that we really cannot determine how much a given filter will change colors in a scene, we're just making an educated guess when choosing an exposure based on a filter factor.
Besides, using a filter is subjective thing and the effect it will have will be very dependent on the colour distribution in your subject. It won't just darken the sky, it will darken your shadows quite a lot and some subjective choices need to be made. The filter factor provided should be taken as a guide only and adjusted to taste with experience.
I couldn't agree more, which is why I advocate metering through the filters.
There is an inherent problem with this however, as RobC notes:
metering through a filter works sometimes and not others; because meters are sensitive to a certain spectrum of light it is unreliable. Darker filters seem to cause a more unreliable result.
Let me elaborate on this: metering through a filter doesn't work when the spectral sensitivity of the meter is significantly different from the spectral sensitivity of the film for the colors being transmitted by the filter. This seems to be more the case at the extremes, i.e., for pure colors with a narrow spectrum, than for weaker filters that transmit a broader spectrum and several colors.
Still, considering the guesswork we are doing anyway when using factors, metering through the filter is really not that much different in terms of final results. Many small and medium format users with in-camera meters just toss a filter on over the lens and snap happily away and get good results. In short, metering through the filter works probably as well as applying a factor. Still, it is not perfect. But, we can make it better!
Using a spot meter and compensating for differences in spectral and emulsion response
Many of you have remarked on what happens with different colors when you spot meter through a filter:
... If the filter has any effect, a spot meter will give different readings when the spot meter is pointed at an Orange vs Blue target.
The filter factors are for average scenes. If you look at an orange subject through an orange filter 100% (or at least a lot) of the light will be transmitted. And therefor no increase in exposure is required. If you look at a blue subject much less light is transmitted as that is a colour that the orange filter blocks. ... A spot metre is so selective it may get you in this situation, however if you are spot metering a grey object the concept does not apply.
If you point the meter at a yellow object and make a reading, then place a yellow filter over the meter, there should be no change in the reading. Unless the filter is not all that good. If you point the meter at a blue object and place the yellow filter, no light will reach the meter.
This fact, that a filter transmits close to 100% of its color and blocks most of the light from the complementary color is exactly what makes applying a factor so hit-and-miss, and exactly what makes metering through the filter, in theory at least, a much better method.
As a Zone System user, my approach is based on spot metering different areas of my scene and using that information to visualize the final print, determining exposure and development along the way. Using a filter and just applying a factor defeats this. Therefore, my best argument for wanting to spot meter through the filter is simply this. It is the only way to meter and compare the filtered reflectances of a scene. With my meter and a yellow filter mounted, I can see that the lettering on that yellow sign won't have any separation in the final print. Likewise, I don't have to guess at how much the shadows will dump when using a red filter; I can meter them and I can place them.
In an ideal situation in which the spectral response of meter and film were identical (exactly the idea behind the Zone VI modified meters some years ago) and emulsion response to different colors yielded identical densities for the same exposure, metering through the meter would be the only logical choice.
In practice, however, the mismatch between meter and film sensitivity (even the Zone VI meters...) and, to a greater extent, the reaction of the emulsion itself in terms of photosensitivity and contrast when exposed to certain colors renders metering through the filter less than ideal.
As alluded to above, not only are the spectral sensitivities of meters and films not aligned, there is an even greater problem when panchromatic sensitized emulsions are exposed to pure colors (red, blue, green). Anyone who has learned about making color-separation negatives knows that you can't give the same exposure and development for those three colors; the emulsion reacts differently in terms of sensitivity (final density) and contrast (i.e., the red-sensitive component develops differently than the blue and green). Keep in mind that this is a general problem and is there when applying factors as well. These characteristics vary with the particular film. For an in-depth discussion of this, see David Kachel's article here:
http://www.davidkachel.com/assets/zsfilter.htm . Although he comes to the opposite conclusion as I do, i.e., not metering through the filter, the information there is applicable to that as well.
In contrast to Mr. Kachel, I believe we can refine our "metering through the filter" technique to compensate (somewhat) for much of this and get even better results than just applying factors. Bill notes Gordon Hutchings' factors for metering through filters:
There is a chart called Gordon Hutchings Filter Factors
Meter through filter then increase exposure by...
#8 no increase - Medium Yellow
#11 one stop - Lt. Yellow-Green
#12 no increase - Dark Yellow (AKA Minus Blue)
#16 one stop - Medium Orange
#21 one stop - Light Red
#25 two stops - Medium Red
#29 two stops - Deep Red
#44A one stop - Cyan (AKA Minus Red)
I have expanded Mr. Hutchings approach and include development changes as part of my factors for different filter/film combinations (these are applied after determining exposure by metering through the filter). For example, with 320Tri-X and a #25 red filter I add 1.6 stops of exposure and subtract "1" from my N-number (e.g., N+1 becomes N; N becomes N-1, etc.). I have tested such factors for the two films I use most and for the more extreme filters (#25, #58, #44). Testing is not difficult and similar to the usual Zone System calibration tests that many of us do. I don't even own a densitometer. Visual evaluations are good enough and a high degree of accuracy is not really needed in order to make this system work better than just applying regular filter factors. Meter and expose a scene with sun and shadows lit by skylight through your two or three strongest filters and then with no filter. Develop and proper proof and you'll see which way you have to tweak exposure and development for a film/filter combination. Make an educated guess and go out and shoot. Keep good notes and adjust as needed. I only test the extremes. For weaker filters, like the #11 and #15 I'll just add 2/3 stop after reading through the meter. Despite being a bit slapdash about this, I still feel that this is more accurate than just applying a regular filter factor.
Most importantly, it allows me to use my meter and filters as a visualization tool, something that just applying a factor can never do.
Best,
Doremus
