What some folk don't seem to comprehend is that a colour enhancing filter will indeed enhance specific colours while desaturating, neutralizing, or simply stated, muddying up other colours, A filter that is designed to enhance yellows and reds may gray down the blue sky. This is because the filter colour-biases the entire scene. The leaves on a birch tree may be enhanced in an autumn scene but the gray bark will record as yellow/red.

I have some experience with didymium filters. In my industrial photography, I oftentimes find myself shootg in factories, warehouses and other industrial installations that are lighted with sodium vapour lamps. Prior to the advent of high output LED lamps, sodium vapour lamps were extremely popular in the industrial environment in that they produce more lumens per watt. Much of this light is still in use. These lamps have discontinues spectrums so they played havoc with colour films so many layers of gel filters, on the lens, were employed to somewhat normalize the colour rendition. Without the filtration, the film would produce a significant green, cyan, or cyan/gree shift. Sometimes a FLD or FLB filter usually used to correct for some fluorescent lighting would facilitate enough correction to enable tweaking in the printing of colour negatives. Transparency fils were still difficult to correct with massive filter packs. Enter didymium filters! Some corrections were notable but backgrounds, people, and other elements in the scene aside from the subject were seriously muddied in colour rendition. The extent of the effect of these filters is determined by their thickness. The thicker filter had more artifacts. I am sure these filers have an important function in scientific photography, medical imaging, photomicrography, and astrological photoghay and some practical usage in landscape work but there are caveats.

Oftentimes, with all kinds of filters, the transmission data, as to which segment of the spectrum are transmitted or blocked, look good on paper but because of variations on film, digital sensors and all kinds of odd and mixed light sources, the data do not always translate into the good colour correctness. Usually, some experimentation is required to zero in on the exact method.

Not surprisingly, in digital photography, I have been able to solve most colour balance issues with a custom white balance or even an automated white balance adjustment, even under what was previously, with film, a massive headache. As for colour intensification via reflection control and negation, the CPL filter is my favourite tool- a good filter has no artifacts, all colours remain true and saturated, and minor tweaks can be easily applied in post-processing.

If you don't like the effect of a CPL filter, or it is not necessary under certain lighting conditions, simply do not use it. In my own work, there are many occasions where I have to shoot a particular out-of-doors scene, an architectural exterior, or something on an industrial or construction site, and I have no choice as to the time of day. Even in the studio, under controlled conditions, there are many items that requre precise reflection control so suffice it to say that my collection of CPL filters are well used. I even had to replace a few that were just worn out, had delaminated, or just fell apart from hard usage on hazardous industrial sites.

Attached is a shot of a commemorative postage stamp collection and accompanying book. This is not a montage shot- the stamps were carefully mounted in small blocks of Foam-Cor and suspend on a makeshift rack. The stamp is printed in a caly-coated stock that is very reflective. The CPL filter saved the day and a hold lot of extensive retouching.

Not totally true about no longer useful in the digital age. It can be still used for regular autumn photography even though the effect can be simulated in PP. As I said, I am a fan of doing everything possible in camera before any post processing.

The biggest use of didymium filters now is used in glass making and astrophotography The didymium selectively blocks the yellowish light at 589 nm emitted by hot sodium (sodium is used in glass making). In astrophotography, it helps to cut out the sodium vapor street lights during photography in light polluted areas.