It was not that long ago when I, or anyone who photographed commercially, stacked quite a few filters in front of the lens to guarantee control over color and exposure in our photography. Well, we were shooting on…film! There was some remedial balancing with a color negative film-to-print capture, but there was no post-production correction at all for color transparency film. If you were shooting on Kodachrome in a 3400˚ K tungsten light environment, out came the 80A filter to make things right. CC (color correction filters) of every color in 10cc increments were used for specialized color environments. In cinema or TV, there are CCB/CCO (color correction blue/orange) to address the CCT (correlated color temperature).
Most everyone needed a UV filter over their lens for distance shooting. Ultra Violet scattered in the atmosphere and film was extremely sensitive to UV. For daylight shadows, it was the very common “daylight” filter; basically, a pale pink to warm up the blue in daylight shadows. Fortunately for us, in the digital age, silicon is rather insensitive to UV; and our 99% + of our cameras use a mosaic color filter set such as the Bayer Color Array. It filters almost all of the light energy into a trichromatic array of red, green, and blue. This means that because our captured information is already color encoded and filtered, only in the most unusual situations would any additional filtration need be required to obtain correct color balance. Today, digital color correction is a white balancing act set on the camera then refined in post.
Filters for digital photography
There are still some VERY valuable filters to keep in our kit because what they accomplish cannot really be duplicated with any post-production manipulation. So, what filters still remain relevant and valuable in the realm of digital cameras?
- Polarizing Filters
- Neutral Density Filters
- IR Filters
- Soft Effect Filter
Let’s examine the Polarizer. The others will be explained in future posts. While some reading this already use polarizers, an understanding of how they work explains why what their effect is really difficult if not impossible to do in digital post-production.
Light propagates in both particles and waves. Those waves vibrate in all directions. Imagine light as a beam traveling from point A to B. Those waves could be described as deviating from a straight line in a sine wave oscillating in all directions from that line.
If the oscillations were blocked from all but one plane, that light would be “polarized”. The easiest way to make this happen is with a wire grid Polarizer, where multiple parallel thin wires create a light “strainer”. It reflects light from all but one wave direction. Think of a picket fence where a clothesline is threaded through the middle. If one line holder moves the rope up and down, the motion travels unimpeded. Any side-to-side motion is absorbed by the pickets.
Dr. Land and the plastic polarizer
In the 1930s, Dr. Edwin Land devised an inexpensive method of making a thin film of plastic to accomplish the same thing. His method stretched polyvinyl alcohol doped with iodine to create microscopic bands duplicating the effect of a wire grid. While his initial goal was to use this material to shield night drivers from the blindness of oncoming car lights, this material ended up in everything from sunglasses to LCDs, and of course, photographic filters. Dr. Land went on to invent “Polaroid” instant photography.
By using this “light strainer” to alternatively pass light with polarization in one given direction but block that which is 90˚ out of phase, it becomes possible to select part of a light source and block the other. As it turns out (all puns intended) many shiny substances polarize the light as it reflects from their surface, and by selecting the orientation phase of a polarizer, we can alternatively pass or block that reflection.
So, if reflections are alternately blocked or passed from the surface of a body of water, we can either see that reflection or see beneath the surface! This effect cannot be duplicated in post-production because if the reflection from the surface blocks our view of a fish in the water, the fish won’t be in the photo. No amount of algorithm or post-production is going to resurrect this fish.
Images taken below are with a Rodenstock MC Circular Polarizer.
While we can darken selected parts of a sky in post-production, if we don’t wish to darken (and thus not have to mask) objects not part of the atmosphere itself, like trees or wildlife, we can avoid that extra task by using the Polarizer.
In practice, when using a polarizer to darken skies, it pays to be mindful that the sky is polarizing light pending the direction of travel of the sun’s rays. It’s most effective at a 90˚ angle from the source. Also keep in mind that with a wide enough angle lens, you might capture the full graduating darkening of part of the sky, with the rest at full illumination, as you might be well past the optimal polarization angle in much of the image!
In the next chapter, we’ll explore neutral density filtration and alternate spectrum photography.