Filters: Glass, Gel, Acrylic, Latex and Electronic
Not all that long ago, if you mentioned photographic filters to the amateur photographer it was assumed you were talking about traditional glass filters, specifically the round type that screws into the front of the lens. Professional photographers might ask if you were referring to round screw-in glass filters, rectangular glass drop-in filters, or perhaps gelatin or acrylic filters for color correction, neutral density (ND), graduated density and light-balancing. Today you can add digital (a.k.a. photo-imaging software) filters to the list. Though there are many shooters who believe these electronic filters have made traditional glass, acrylic and gelatin filters obsolete, there are a number of applications for which only glass, acrylic and gelatin filters will do.
Glass Filters: Glass filters, be they screw-in or square/rectangular filters are available for almost every photographic application imaginable. While a strong argument can be made that many conventional filter applications can now be emulated post-capture electronically using stand-alone and plug-in software applications, there are a few applications that will always be the domain of glass, including Polarizing, neutral density (ND), UV (ultra violet) and infrared (IR) blocking filters.
What makes Polarizing, UV, IR blocking filters, and to a lesser degree ND filters unique is that each of these filter types can only perform its designated tasks when you’re taking the picture, be it film or digital. While there are a few manufacturers who include digital “Polarizing” filters in their electronic-filter catalogs, what these filters really do is pump up the saturation levels of the individual color channels to emulate the look of a Polarized sky, vegetation and water.
What digital Polarizing filters cannot do (at least not yet) is eliminate glare and reflections, which is what saturates the color levels of Polarized photographs in the first place. The same holds true for UV and IR-blocking filters. Ultraviolet and infrared light can only be effectively blocked at the time of capture.
Note: In addition to being available as glass screw-in and drop-in style filters, Polarizing filters are also available in varying degrees of warm tones (81a, 81b, 81ef, 85, 85b) as well as with Skylight and heavy UV-blocking coatings. It should also be noted that while Polarization is only possible at the time of capture, warm tone adjustments can be made post-capture in Photoshop to JPEGs, TIFFs and most effectively, with RAW files.
Acrylic and Gelatin Filters: Acrylic and gelatin filters (also known as “gels”) are predominantly used for neutral density (ND), color and white-balance correction purposes, and to a lesser degree, IR, UV and graduated-filter applications. Lighter and less expensive than their glass counterparts, acrylic and gelatin filters require filter holders that attach to or screw into the front of the camera lens. As for image quality, the better gel filters such as those available from Kodak in 3 x 3”, 4 x 5”, and 4 x 12” sizes, are optically purer than the finest glass filters. The down side is that they are extremely susceptible to scratches and fingerprints, which are all but impossible to remove.
Larger sheets of heat-resistant acrylic and gelatin filters are commonly used in the motion picture industry to “gel” lights, and single-sheet Polarizing filters are most commonly used for Polarizing the light sources for copy work, and for photographing artwork and objects with reflective surfaces.
Digital Filters: Unlike conventional glass, acrylic, and gelatin filters, digital, or electronic filters, can be used for enhancing and/or correcting the shortcomings of digital image files long after the time of capture. And in many cases, digital filters offer far more flexibility in terms of tweaking the degree of change and/or image corrections compared to the more static effects of conventional filters. Digital graduated filters are particularly more pliable than their conventional counterparts in that you can often control the precise levels, shape and starting points of where the tonal gradations occur based on the dynamics of the photo being altered.
As mentioned above, there are several filter effects that, despite manufacturer’s claims, cannot replicate the anti-glare, anti-reflection qualities of Polarizing filters, nor can they replicate infrared (IR) or ultraviolet (UV) filters. For Polarizing, IR, and UV filtration stick to the originals—glass.
Digital filter kits are available from a variety of manufacturers including Tiffen, nik, and onOne. Additionally, there are a number of digital-specific software applications for performing advanced sharpening, tonal and smoothing effects from nik, Alien Skin, DXO and Auto FX Software.