question about stacking square filters

[TimHGuitar]

Yes. if you are using Wratten filters... don't stack them. The filters are very strong. But if you are using CC filters then you can stack them. CC filters come in different densities... .025, .05, .10, .20, .30, .40, and .50. None of these are as strong as a Wratten filter. CC filters are color correction filters and meant to be stacked.

[Mark Woods]

Having taught color theory at the American Film Institute Conservatory, I can accurately say that green and red are not complimentary colors. Green's complimentary is magenta, and red's complimentary is cyan. With that in mind, I would take the advice about stacking filters the same way I would about the statement of red and green being complimentary. All filters in the optical path will affect the quality of the light passing through them.

[Doremus Scudder]

Having taught color theory at the American Film Institute Conservatory, I can accurately say that green and red are not complimentary colors. Green's complementary is magenta, and red's complementary is cyan. With that in mind, I would take the advice about stacking filters the same way I would about the statement of red and green being complementary. All filters in the optical path will affect the quality of the light passing through them.

Mark,

Pardon my oversimplification (and resulting inaccuracy) for the purposes of simplicity and clarity; I didn't want to go into color theory in any depth since the OP needs to walk before he can run.

However, maybe we should give the OP (and anyone else interested) the basics:

Here's an easy way to think about it. The primary (single component) colors red, green and blue have complementary colors that consist of the other two components. For example, the complementary of red is blue-green or cyan. Extending this, we get the complementary of green, which is red-blue, or magenta and the complementary of blue, which is red-green, or yellow. (Note the use of the word, "complementary," not "complimentary," which means to augment or complete in this context. A primary and its complementary comprise all three of the primary colors.)

Seeing how yellow and blue are complementary is a bit counter-intuitive, since it's not so readily apparent that red and green together make yellow. They don't at all if you mix, say red and green paint (that's just an ugly brown...), but they do if you mix red and green light together. As a child I had a top that was colored with red and green stripes. When you spun it up, it was... yellow! I was fascinated by this phenomenon when I was young. Later, in optics and physics classes, we combined light of specific wavelengths by projection to see how the eye responded to them (the eye has rather wide-band receptors for red, green and blue that overlap in sensitivity). Red and green light projected together made a most vibrant yellow.

Back to filters: colored taking filters for black-and-white transmit their own color(s) and block the others. A yellow filter (we know from the above) transmits red and green and blocks blue. (This is an over-simplification, but will do for getting the general principles down.) That's why blue skies and other blue objects end up darker in a photo made with a yellow filter. A blue filter would do the opposite; passing blue and blocking red and green; blue skies would be lighter and red and green objects darker with a blue filter. Note that this filter pair is a primary color (blue) and its complementary, yellow (red-green). If we stacked a blue and a yellow filter together, the result would be the removal of all three primary colors, i.e., red, green and blue would all be removed... That's no light making it through to the film at all. Stacking filters of complementary colors always ends up removing all three primary colors, giving no light at all.

We can do the same with the other complementary pairs: A red filter (primary color) blocks cyan (blue-green). A photo made with a red filter darkens blue and green objects, such as the sky, green leaves, etc. and lightens red objects. Imagine a photo of a red rose make with a red filter; the flower itself would be rendered quite light, almost white, while the green leaves would be rendered almost black. The complementary filter to red, i.e., a cyan filter is less-often used, but is useful for blocking red and lightening blue and green objects. Interestingly, a cyan filter, when used with panchromatic film, will approximate the rendering of orthochromatic film, which is only sensitive to blue and green, not red.

On to green. A green filter blocks the other two primaries, red and blue, which together make the hue magenta. So when we use a green filter, red and blue objects will be rendered darker and green objects rendered lighter. Try the photo of the red rose with a green filter. Now you get lighter foliage and a very dark flower. Magenta filters are rarely, if ever used in black-and-white photography, but if they were, they would darken green and lighten the other two primary colors.

Thinking in terms of the three primary colors and their complementary colors helps us understand how filters work in black-and-white photography, but it, too, is an oversimplification for the purposes of understanding more easily. Keep in mind that their are a whole rainbow of colors between red and blue on the spectrum and that the human eye is a very complex organ in terms of spectral sensitivity, but with limits as well. In color photography, knowing more about saturation, hue, transmission spectra, crossover, etc., etc. becomes more important. Thankfully, we don't need to get into any of that to use black-and-white contrast filters effectively.

A quick note about CC filters. These are filters used primarily for color printing and balancing and come in different strengths. These filters can be stacked, as long as one stacks the same color, to get a stronger degree of filtration. For example, stacking two cyan filters of different strengths results in a filtration with the combined strengths of the two filters. Sometimes, we need to remove a bit of two of the primary colors from the mix in different proportions, so we can even stack two filters of different colors and strengths to do this. This type of filtration is useful in color printing and in color photography to get the right white balance to the print or the film. It is rarely necessary to be so subtle with black-and-white film.

Hope this helps,

Doremus