No trade secrets, just practical facts. Ira Tiffen explains how camera filters are made and why those differences matter when you choose a filter.
There is often more than one path toward a goal. Choosing among them can be difficult unless you know the important differences.
Filters are similar in this regard. They can be made in several ways using a variety of materials and techniques. Here’s what you need to know.
We’re going to cover the key differentiators listed below. As we do, we’ll find that they are connected in many ways – select one and you often determine another.
• Material • Colorant and Effect • Optical Quality • Durability • Coatings
Filters long played an important role in making better images. George Eastman, knowing that his new film would require some assistance in managing the infinite variables of the real world, made a special effort to have filters produced that would tailor reality to his films’ characteristics. Recording media of all types have become more capable since then, yet there is still the need for this ‘last line of defense’ to manipulate the light just before it enters the camera.
The first important applications for filters were to make adjustments to the color spectrum. For black and white imagery, you still needed to alter the light to complement, and not detract from, the color sensitivity of the recording media, which tended to be stronger for some colors than for others. This allowed more pleasing contrast and tonal rendition.
Subsequently, filters have taken on new roles as diffusers, polarizers, makers of mist and magic and more. We’ll get to some of those as well as the color filters. First, let’s look at the materials used to make filters.
A filter is essentially ‘just’ a flat optical element. When I was getting started making them, using ‘optically clear’ stone tablets, I was told that making optically flat elements with both sides suitably parallel was more difficult than making lenses. When making a lens, one side might come out slightly more curved than it should have and you could often compensate for this when making the other side. But a flat surface can only be flat and both sides have to be parallel. Otherwise, you will have all sorts of problems.
If both sides are flat, but are at even a slight angle to each other, the resultant ‘wedge’ will cause a shift in the light that will make the image appear distorted and/or out-of-focus. Using traditional lens-making methods to make filters as many did back then, machining one side at a time, made it difficult to guarantee both sides would be parallel. My father, Nat Tiffen, was a pioneer in the use of ‘double-disc’ grinding and polishing equipment which machined both sides simultaneously to consistently produce critically flat and parallel optics. This subsequently become standard industry procedure for producing glass optical filters.
But camera filters weren’t originally made from glass. Back in Eastman’s day, another method had to be developed, since coloring glass with the necessary precision and range of spectral transmission characteristics was not possible then. Even today, there are some limitations. This new method (for its time) involved making organic gelatin sheets (no doubt an offshoot of making gelatin substrates for film) that contained a precise mix of dyes to produce the desired effect. Eastman had to develop a comprehensive, world-class dye system in order to make the materials his customers depended on. That facilitated the production of an extensive range of dyes and colorants for filters. It resulted in what are still known today as the Wratten series of gel filters, named after the color chemist at Kodak who originated them.
I’ll add here that what are also known as ‘gels’ for lighting are not the same thing. Gel filters for cameras have to be optically flat; lighting gels do not. Lighting gels have to withstand high temperatures and light intensity; camera gels do not. Lighting gels are generally made from polyester or polycarbonate resins that have these capabilities, as well as the ability to be precisely dyed in a wide array of colors. They are also used in much larger sizes that require a less expensive cost of manufacture. Just getting camera gels to be sufficiently optically flat is an arduous process with exacting requirements that necessitates handling in relatively small sizes and run quantities, with the attendant higher cost that usually entails.
To give you an idea, camera gels are cast in small batches on optically flat surfaces and hardened over time; lighting gels are extruded on machines with wide web widths and speeds described in ‘feet per minute.’ This is not to imply that lighting gels are not precision-made, just that the requirements are different. Nothing beats the color accuracy of properly made photographic gels.
So gels could produce the necessary color control and optical clarity. But they are also flexible; their use on a camera lens requires a frame for rigidity. And they are relatively fragile, susceptible to scratches and other damage. Something more durable was necessary.
Gels were for many years cemented between two layers of glass. This was an expensive process that gave the gels greater durability, but still was less than ideal. The refractive index of the gel was substantially different from that of the glass, rendering it somewhat less than optically ideal, and it was still susceptible to damage from heat and humidity.
Glass itself, while much more durable and capable of fine optical finishing, was not able to be colored with the same degree of control as gels. Typical basic glass mixes of silica, sodium carbonate, and various mineral oxides produce a generally clear result. Colors come from the addition of certain minerals and metals to the ‘melt’ that produce the beautiful array of color most often seen in stained glass windows and dinnerware.
The difficulty with this method of coloring is that fine-tuning spectral transmission of solid-colored glass to the degree and in the range needed to compare with gels was not practical. However, glass as a filter medium took a major step forward with the introduction of a process known as ‘lamination.’
If you could replace the ‘gel cemented in glass’ with a cement that itself had the correct colorant properties of the gel plus a similar refractive index as glass plus the ability to conform to the internal glass surfaces, you would have the best of both worlds, glass and gel. This, in effect, is what lamination largely accomplishes. The internal color layer optically blends seamlessly with the external layers of clear glass, forming a durable color-accurate element capable of fine optical finishing.
An early version of this process was offered by Harrison and Harrison, beginning back before WWII, which made important contributions to motion picture production for many years. In the 1960’s, my father introduced a method of glass lamination that subsequently came to prominence due to advances in features and capabilities. Schneider and others have since introduced their own variations on this theme. The key to lamination is color control approaching that of gels, yet with the durability and optical functionality of glass.
In addition to still photography, many filters for television and motion picture production are of laminated construction, due, in part, to the fact that it can economically produce many types of filters not having huge market demand that would not be cost-effective in modest quantities using solid-glass-melt methods.
Coloring raw molten glass producing solid optical filters come a long way from Eastman’s day. Many colorants are available, some with unique spectral-attenuation characteristics, like didymium filters for enhancing red hues. There are many filters produced using this method (think Hoya, Heliopan and B+W) which have great color durability and can readily be heated to the temperatures needed for traditional optical anti-reflection surface coating. They can be very cost-effective, especially in the more popular colors.
Another important material used for camera filters is acrylic. Think Cokin, here, although Lee and others have made them for many years as well. This is typically cast resin, but thicker and more rigid than gels. It has the ability to be surface-dyed or cast-in-color. The former is a way of taking clear acrylic and coloring it by causing dye to be absorbed into the surface. This provides an excellent way of making gradients, for instance. The latter, much less in use, involves adding dyes to the uncured resin prior to casting. It results in color that is throughout the resin thickness, but is in many ways functionally similar to surface-dyeing.
Acrylic filters can be less expensive than glass, may be less prone to breakage, but are more likely to scratch. They also generally do not exhibit the same level of optical resolution as is possible with glass. This is why most camera lenses are still made from glass. They function well in many situations; as with anything, it’s best to test for critical applications.
Acrylics can be cast with surface textures to produce a wide array of special effects. They are lighter in weight than glass, and are typically made in thinner dimensions, although glass has been available to fit Cokin holders for some time. They also can be colored with a respectable degree of consistency. The cost/weight/precision features of acrylics fit certain portions of the imaging market well. Glass generally reigns otherwise.
I should note here that most manufacturers, while perhaps known best for one or other of the above filter materials, offers certain products in some or all of the others, as well. It is just a matter of available technology and market demand.
As with acrylics, glass surfaces can be cast or otherwise altered to produce certain effects. A notable one is star effects. Fine lines are etched into the glass surface using acid; these lines refract point sources of light to create the brilliant star effects we are all familiar with. Additional effects can be made through inventive surface textures, producing fog and mist, and further alterations and creative distortions for imaging enhancement.
Some of the more effective early portrait diffusers were made by Kodak using cast glass; the surface had smooth raised lines on it that would soften fine details and yet leave focus sharp. This method still has its adherents; variations involve adding light-altering materials and textures to the inside of laminated glass, either to the internal glass surfaces or by incorporating special materials in the laminate itself.
A final differentiator our discussion is surface coating. Coatings can take the form of spectral attenuators, as some makers, like Heliopan, have long produced certain colors and color effects, such as grads, using coatings. The most important use for coatings, however, has been to reduce surface reflections. Anti-reflection coatings maximize light transmission while minimizing contrast-reducing glare. They are most readily applied to glass. They do add cost, and are susceptible to deterioration apart from other filter functions, but also add important functionality.
Coatings reduce the amount of change in the speed of light as it goes between glass and air. The greater the difference, measured as the difference in the refractive indices of the material it is traveling through, the greater the amount of light lost to reflection. Multi-layer coatings break that difference down into many small steps, further enhancing coating efficiency and reducing reflections. This is especially important at night where bright lights against dark backgrounds may greater-than-average reflection problems; and when using multiple stacked filters can cause unwanted internal reflections between them.
Coatings have typically been offered on filters made of solid glass, since coating techniques traditionally require high temperatures not suited for acrylics or laminations. More recent advances in technology, however, have eliminated much of that difference – efficient and durable coatings on laminated glass, especially, have risen to meet the challenge and are much more widely available than before.
With this comparison of features and functions in mind, you can see why there are the variety of filter materials and production methods available. You can knowledgeably select the one that’s best for your application. And most importantly, you can more confidently achieve your goal of success in your imaging efforts.
In over 30 years of making optical filters, Ira Tiffen created the Pro-Mist, Soft/FX, Ultra Contrast, GlimmerGlass, and others, netting him both a Technical Achievement Award from the Academy of Motion Picture Arts and Sciences and a Prime-Time Emmy Award. Elected a Fellow of the SMPTE in 2002, he is also an Associate member of the ASC, and the author of the filter section of the American Cinematographer Manual.
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