Color Filters and BW Film
If you shoot black and white film, you'll notice that some colors come out darker while others come out brighter even if the subjects are similar in brightness to your eye. This is because photographic film sees these wavelengths in varying intensity. By using color tinted filters, you can manipulate this attribute to create effects on the image your film produces. Let me explain.
Here is a spectral sensitivity graph from Kodak for Tri-x 400. What this graph shows is how sensitive the film is to particular wavelengths of visible and invisible light. I added the color so you can see where it lines up.
Property of Kodak
As you can see, the sensitivity goes up and down depending on the wavelength and color. Tri-x has lighter blues, slightly darker greens, bright yellows, with oranges and reds becoming dark as it heads toward the infrared wavelength for an average exposure. This means that if you put blue and red next to each other, red will appear darker than blue. This also means that if you are taking a landscape, the sky will come out much brighter than the green foliage, both due to the sky's luminance, and the film's lower sensitivity to green.
What color filters allow you to do is remove particular wavelengths from the image allowing you to selectively darken and lighten areas in the scene by color. Specifically, it will appear to brighten the colors the same color as the filter, but darken the other colors by filtering out those wavelengths. The effect will vary depending on the strength of the filter, and the saturation of what is being photographed. For a quick example, I took two photos on a slightly hazy day using a red filter, and then a blue filter. They are cheap filters with what I would say is a medium strength.
Red Filter
Blue Filter
As you can see the filters had nearly an opposite effect. The red filter made the mountain, trees, and sky darker, but the yellowish orangish grass stayed a bright "golden" shade. It also darkened the gravel path that had a blue hue to it. Because the sky was hazy, it didn't darken the sky as much as it could've, but the sage green tress became almost black making the mountain stand out through the haze. The blue filter however, made the haze appear brighter, removing detail from the mountain and some of the hills. The grass was given more contrast however, but the gravel path lightened up to blend in with the surrounding area. The sage trees also appear brighter, giving them more texture, but as it gets hazier, this subtle detail goes away.
You can use a single filter or combine filters to create different effects using this same principle. Nearly all filters will have a contrast effect, depending on their strength. The key to remember with this is that it will contrast based on the color of filter. The most popular color filters are red, magenta, orange and yellow because they have an overall contrast effect. Yellow being the least strong, and deep reds being the strongest. Ansel Adams is know for using deep red filters to nearly black out a sky.
Let me know in the comments what filters you use. I personally will use a red on occasion but I generally shoot without. Thanks for reading!
I am a photographer and content creator from the Bay Area of California.
