The Filter Glossary

Here we break down some of the more common technical and marketing terms floating around so that photographers can be empowered to make an educated choice when purchasing their next lens filter.

Are there any terms that we didn't include? Let us know.

Lens filters are usually fitted with rings made of either brass or aluminum. Compared to brass, aluminum, which is a softer, lighter metal, is somewhat more prone to deforming (and thus getting stuck on your lens). However, aluminum that has undergone a hard anodizing process is made as strong as steel and less likely to deform compared to untreated aluminum.

Anodizing is an electrochemical process that converts the metal surface into a pleasing, durable and corrosion-resistant finish.

See hard anodized.

The glass of some filters is painted black on the edges. This is claimed to reduce internal reflections that reduce contrast. Our labs tests, however, have demonstrated no meaningful benefit to black-rimmed glass. Because almost all filters have black mounts pressed right against the edge of the glass, the frame itself effectively serves the same purpose as black paint.

Lens filters are usually fitted with rings made of either brass or aluminum. Brass, as a harder, stronger metal, reduces the likelihood of getting stuck (a.k.a. “binding”) because it’s somewhat less likely to deform. Some also prefer the slightly heavier, more premium feel of brass over aluminum.

A lens filter that reduces off-axis light, such as reflections from water, windows and cloud on a bright, sunny day. The "circular" part of this filter is a variation produced that works with autofocus systems found in cameras. The effect can be increased and decreased by turning the frontmost ring of the filter and observing through the lens or LCD.

A quality clear or UV filter will impart no coloration to the final image. Although the coatings on a clear filter may sometimes impart a subtle color to the glass itself, this should not affect the final image.

The contrast of an image can be diminished by unwanted reflections that occur within lenses and camera bodies as well as lens filters. A quality lens filter will avoid adding any additional reflections, thereby keeping the contrast of the final image as high as possible. Without a UV filter, UV haze can be a cause of decreased contrast under special circumstances—primarily at very high altitudes.

When a lens or lens filter fails to focus all colors on a single point, chromatic aberration occurs. It is considered a negative quality.
Identifying chromatic aberrations is easy. It looks like color fringing at the edges of high-contrast areas, such as a hard edge of black and white area: a star in the night sky or window next to a dark metal frame.

This is likely a phrase carried over from the transition period between film and digital. Ultimately, there is no inherent characteristic of a lens filter that would make one better suited for digital versus film. While old film stock might be more prone to UV haze, digital sensors and modern film stock are immune to UV haze at all but the highest altitudes. It’s also important to note that a quality UV filter should not have any adverse effect on your image, whether you’re shooting film or digital.

Because long telephoto lenses can have very wide front diameters, screw-on filters can become impractical due to cost. As an alternative, drop-in filters allow photographers some leeway in terms of sizing. These can include telephoto lenses with rear-mounted filter holders (to allow for smaller filter sizes), as well as front-mounted rectangular filter systems that allow for very large filters typically made of plastic or resin rather than glass.

This is a term commonly used with color filters. The filter factor measures the amount of light the filter allows to pass through. With a value ranging from 1 to 16, it indicates the necessary exposure adjustment you need to make for a properly exposed photo. UV and clear filters have a filter factor of 1, indicating that no additional exposure adjustment is needed.

The metal frame used to hold the optical filter glass in place, and parallel with the imaging sensor plane.

Caused by internal reflections and refractions in the filter, lens, and camera body, flare is the veiling of the image due to stray light, often occurring when shooting into strong light sources. These effects are generally considered to be undesirable.

Some filters have front threads, and some do not. The threads allow one to "stack" filters - i.e. mounting two or more filters atop each other. Filters without front threads are most often "slim" filters for ultra-wide lenses.

See slim filters.

Like flare, ghosting is caused by internal reflections and refractions in the filter, lens, and camera body. In the case of ghosting, the displacement of the reflected light is severe enough that a duplicate object (typically a strong light source, especially within a dark scene) appears in the frame.

Glare is the effect of light scattering while passing through a photo lens filter. Negative effects are a loss of contrast. Glare can also be caused by internal reflections within a filter, which is why filter rings are black to reduce reflection of light from extreme angles to the surface of the filter. Glare is also further reduced by the application of anti-reflective "multicoatings".

Factories that produce glass are called glass foundries. They typically make glass for use in all kinds of products, including windows, windshields, kitchenware, eyeglasses, and optics. Different recipes and manufacturing processes are used to create different “flavors” of glass that have specific qualities desirable for specific applications. There are many foundries around the world producing quality glass for a wide variety of applications, including lens filters.

Corning is a glass manufacturer that produces a ion-exchanged-toughened glass that they call Gorilla Glass. Famous for its use in smartphones, Gorilla Glass is known for its resistance to scratching and shattering. Other foundries produce glass that have the same qualities.

Hard anodizing is an improvement on traditional anodizing, in that is yields a harder-wearinng, more corrosion- and wear-resistant coating. It often lends the treated parts a durability approaching tool steel.

Aluminum is generally softer and lighter than brass. Quality aluminum-frame lens filters use hard-anodized aluminum, which increases the strength of the frame to that of steel and makes it less prone to corrosion and thread chafing. This helps to prevent the filter from jamming due to wear or deformation.

Some filters feature "hardened optical glass," which is claimed to be chemically enhanced to be four times as strong. In our tests, filters advertising this feature did exhibit the best shatter resistance of the filters we tested. However, their claims of being four times stronger were only true in comparison to the weakest filters of the bunch. Compared to the more competitive filters, our tests showed them to be only about 33% stronger.

See UV Haze.

When it comes to lens filters, the term “HD” or “high density” has no practical meaning. Filters marketed as such are not inherently different in any way from other lens filters.

This is the bottom line when it comes to optics. Almost every feature of a lens filter’s optics—including anti-reflective coatings, chemical composition, flatness, surface precision, and glass manufacture—serves to aid the filter’s light transmission, which is a measure of the percentage of light that passes through the filter unchanged. By our standards of testing on ISO-certified machines, the best clear and UV filters in the market transmit at an average of about 99 percent in the visible spectrum. (Note: Different measuring standards and methods of reporting can result in varying claims of transmission percentage. For example, some manufacturers claim as high as 99.8 percent because they’re reporting the absolute highest point of the spectrum as opposed to taking an average (the way we do). That’s why it’s essential that any comparisons between filters be made using the exact same machines and the same standards of reporting.)
See video

This is a term used by some manufacturers to indicate the presence of multiple coatings for increased resistance and/or optical performance.

See resistance coatings.
See multi-coating.

This term indicates the presence of multiple coatings on the glass for increased resistance and/or optical performance. See resistance coatings

Multi-coating is the most frequently touted feature in the industry. It sparked a spec war between brands in 2007, with claims of up to 32 coatings (including optical and resistance coatings). In reality, however, counting coatings isn’t a useful way to assess a filter’s quality. When shopping for a filter, it’s advisable to consider optical performance and resistance properties directly rather than looking at the number of coatings.

This is a term that some brands use to describe some of their resistance and optical coatings. Describing a filter or lens coating as “nano” indicates that the coating solution contains particles small enough to be measures in nanometers.

See resistance coatings.

A photographic or video filter used to reduce the amount of light entering a lens, without adding a hue (color). There are multiple uses for this, including longer exposures and using a wider aperture in daylight for a shallower depth of field.

Glass specially made for use in optical products, including eyeglasses, camera lenses, telescopes, and lens filters can be referred to as “optical glass.” This type of glass is usually designed with stricter requirements for transparency and flatness to reduce refraction and dispersion.

A polarizing filter is used to manage reflections, darken skies and suppress glare from specualr objects like cars, windows and polished surfaces.
The effect can be increased and decreased by turning the frontmost ring of the filter and observing through the lens or LCD.

Filter glass is cut from long rods of optical glass. Polishing these cut discs of thin glass is essential to provide a surface free from surface imperfections, unwanted refractive properties and parallel front and back surfaces. Unpolished glass is not suitable for photographic optics, as it would severely decrease the quality of the image prior to entering the lens.

The glass in lens filters is mounted in one of two ways. In some filters, it’s screwed in using an internal threaded metal ring surrounding the glass, making it possible to unscrew the glass from the frame. Press-fit mounts, on the other hand, use heat to expand the frame, insert the glass, and let the frame cool and contract again so that the glass is locked in by pressure. Because the latter method does not allow the glass to be unscrewed after production, it helps to prevent the accidental dismounting of the glass due to normal wear and tear of the lens filter. (Chiaro filters have press-fit mount assemblies.)

Because UV haze is usually not a problem with modern film stock and digital sensors, photographers these days tend to use UV filters more for the physical protection of their lens than their optical effects. To reflect this practice, photographers and brands are starting to refer to UV filters as protective filters. This term is also applied to colorless filters that have no UV-blocking coatings (often called “clear” filters, though “clear” can be used to describe UV filters as well).

This is a general term that describes the light bouncing off the various surfaces of the glass. This can cause flare, decreased contrast, and loss of light. Reflections can be caused by the various elements in the lens and within the camera itself, as well as by lens filters. The goal of a filter is to avoid adding any additional reflections.

Refraction refers to the change of direction of light as it passes through a transparent material such as glass. While camera lenses rely on carefully controlled refraction to form an image, the ideal lens filter would be completely flat and cause no additional refractions. Refractions caused by a lens filter can result in flare, decreased contrast, and loss of light.

This is a general term used to describe all the various coatings applied to filter to help keep the surface of the glass clear. This includes hard coatings that help resist scratching, as well as repellent coatings that allow for easy removal of oil, water, dust, and fingerprints.

A German glass foundry made famous in the photography community for supplying optical glass to Leica and Zeiss, Schott was an early pioneer in developing standardized raw glass qualities and specifications. While many brands boast of sourcing Schott glass, however, the reality today is that many foundries—Hoya, Ohara, Sumita, Luoyang, Hikari, and Asahi being just some of the players—now manufacture high-quality optical glass to the same specifications. Because glass type is only one of many factors involved in light transmission, photographers looking for the filter with the highest image fidelity should focus on light transmission rather than glass manufacturer.

Among the many possible resistance coatings, scratch-resistant lens filters include a hard coating that provides resistance to scratches.

This is the most common type of filter, distinguished by its threaded metal frame that allows it to be screwed directly onto the front of a lens. These filters come in standardized sizes to match the front diameter of the lens. Most screw-on filters are also threaded on the front, making it possible to stack multiple filters.

Sharp cut filters are designed to eliminate wavelengths outside specific spectrum. For instance, and "sharp cut 370" filter would block as much as possible for wavelengths below 370nm, but allow everything above it.

UV coatings on filters operate by cutting off light transmission at lower wavelengths in the non-visible UV spectrum. UV filters that advertise "sharp-cut UV" claim to make that cutoff sharper than that of a conventional UV filter, with the goal of transmitting more visible light while blocking more UV light.

When using wide-angle lenses, there’s a chance that the frame of a lens filter might encroach upon the image, introducing unwanted vignetting. By using thinner glass and a thinner frame, slim filters are designed to be usable on lenses with shorter focal lengths. Some slim filters forego the front threads to slim down the frame even further, sacrificing the ability to stack additional filters or universal hoods on top.

Because different filters affect the image in different ways—e.g., polarizers, neutral density, UV haze—photographers sometimes stack one filter atop another to use both at the same time. Most filters have screw-in threads on the front end to make this possible. In terms of image quality, the downside to stacking is that it compounds the effect of each filter’s imperfect light transmission, potentially degrading your image. Stacking filters can also cause vignetting. To avoid these issues, photographers are advised to remove any filters that they don’t need in a given situation rather than stacking filters unnecessarily.

The primary reason that a filter maker would source thinner glass would be to allow for a thinner mount to avoid vignetting when either using wide-angle lenses or stacking filters. All slim-mount filters necessarily have thinner glass, due to the simple fact that thicker glass can’t accommodate slim mounts. Thinner glass might also have an advantage when it comes to image fidelity, simply because the light has less material to travel through—but photographers who care about image fidelity would be better served by focusing on the light transmission rather than the glass thickness.

In film photography, UV light can introduce color casts and decrease contrast in film stock, so a UV filter can sometimes make a perceptible difference in helping to capture a clear image. When it comes to modern digital sensors and modern film stock, however, UV only affects images when shooting at very high altitudes. Nevertheless, using a UV filter on a digital camera can help protect your lens from debris or scratches while also ensuring that you’ll get clear, neutral images at any altitude.

A negative effect when the edges of a photograph are darkened by obstructions. Taller filter frames acn cause this on wide angle lenses.

The portion of the eletromagnetic spectrum that is visible to the human eye, namely 390 (violet) to 700nm (red).

An original term used by Schott to describe some of their glass, including B270. Some filter manufacturers have taken to using this term in their marketing, but it’s ultimately meaningless.

While water tends to pool and cling to uncoated glass, water-repellent coatings on filters cause the water to bead and roll off (think Rain-X®). A quality water-repellent coating will also ensure that no residual streak is left behind.

While an improperly or loosely constructed lens filter might allow dust and moisture to reach the front element of the lens, this should not be a problem for any decent lens filter. Most lens filters are “weather-sealed” whether or not this term is used in their marketing.

Wide angle lenses for full-frame (35mm) imaging sensors are 28mm to 35mm lenses. Extra wide lenses are focal lengths wider than 28mm. In regards to filters.