“I’m not a photographer, so I didn’t get into f-stops or ND filters or background, foreground, cross-light, all that stuff.” — Keanu Reeves
Camera filters are available in lots of different different shapes, sizes, and materials including delicate gelatin filters that drop into holders or lens hoods, glass filters to screw onto your lenses and rectangular filters that fit modular holders. For more, see my riff on Wratten Filters.
- Gelatin filters are made by dissolving organic dye in liquid gelatin, which after drying is cut into square or rectangular shapes. The most popular size is 3×3 inches, although they’re available up to 14×18. Gel filters are only .1mm thick, and offer excellent optical quality but are fragile. You can tape gel filters to the front of a lens, but they’ll last longer if you use them in a holder. Gels used to be inexpensive but now you can expect to pay up to $25. True filter hounds always check the junk boxes of photo swap meets when gel filters can often be found for a buck or less.
- Some glass filters are constructed by sandwiching a gelatin filter between two sheets of glass. Over time, these materials separate, causing bubbling and peeling. An alternative is to dye the glass when it’s in a molten state, which means there’s no color shifts as the filter ages. All polarizers are laminated because they use polarizing film to make the filter do what it’s supposed to do.
- How well a filter ring is made should be obvious from just picking it up, twirling it around in your fingers, and screwing it onto a lens. Heliopan and B+W mount their filters in brass because it l won’t bind or cross thread. Hoya uses aluminum because it absorbs shock in case of accidental impact.
In Jurassic times camera manufacturers kept the filter thread sizes on their lenses consistent between different focal length lenses but nowadays they don’t seem to let that get in the way of a new lens design. What this means is that you might need several sizes of the same filter to fit your different lenses. With modular filter systems you only need one filter along with a holder and adapters for different lens threads. Filters for Cokin’s “A” size measures 75x75mm while the larger “P” size is 84x84mm. An adapter lets you use “A” filters in “P” holders. The super-sized X-Pro filter measures 6.7×5.1 inches. Cokin may have popularized the modular concept but similar systems are available from lots of other companies.
Some cameras, such as my beloved Olympus E-P3 that was used to make the photo at right, have internal filters that are built into their firmware.