Using colored filters was a thing back in the days of film ... and primarily for shooting black & white film because it changes the contrast. The filters allow things to pass through if they match the color of the filter, but block things that don't match.
It was sometimes even used for color film becuase you could do things like enhance sunsets, etc.
But this is now so very easy to do with photo editing software that the need to use physical color filters is virtually eliminated with few exceptions.
In astrophotography, we use broadband light pollution filters and sometimes also use narrowband filters to enhance details from deep-sky objects (mostly emission nebulae).
For typical photography (non-astrophotography), the filters still commonly used are:
(1) circular polarizing filter (aka "CPL") because this will help reduce unwanted reflections and can help make colors pop.
(2) neutral density filters (aka "ND" filters). These change the shooting circumstances. They are commonly used to allow for longer exposures to deliberately create blur in moving subjects (waterfalls) but there are many uses for them.
Some people will still use "gradient neutral density" (aka "GND") filters. These are typically rectangular filters that are clear on one half and darked on the other half. They let you do things like darken the sky without darkening the foreground. You can technically do this in post processing. But if you're losing dynamic range in a landscape shot, it may be easier to use a physical neutral density filter rather than rely on post-processing. So this is a third category where some photographers still use physical filters. Since these filters are retangular, they require a filter-holder. The holder threads onto the lens and has slots to let you slide in the filters.
Most other uses for a physical filter has probably been replaced by digital manipulation in post processing software.
If you have multiple lenses with different filter thread diameters, you can get "step up rings". These are rings that have a smaller thread diameter for the camera side ... and a larger thread diameter for the filter side.
For example, most of my lenses use 77mm diameter filters. But my 100mm macro has a 67mm diameter. I own a 67 to 77mm "step up ring" which lets me attach the 77mm filters to my 67mm diamter threads. The "step up rings" are cheap (typically $15) and avoids the need to buy another expense set of filters in a different diameter.
Tim Campbell
5D III, 5D IV, 60Da
