What is a photographic filter?
A photographic filter is an optical accessory placed in front of a camera lens to modify the light reaching the sensor. Depending on the type of filter, it can reduce light intensity, control reflections, balance exposure, alter color or create a specific visual effect.
Although many effects can now be created during image editing, certain filters still have an important role in digital photography. Polarizing filters and neutral density filters, for example, produce effects that cannot always be reproduced convincingly afterwards.
Choosing and using photography filters
Screw-in filter or filter holder?
Filters are available as screw-in filters or as rectangular or square filters used in a filter holder system. Screw-in filters attach directly to the front of the lens and must match its filter-thread diameter.
A filter holder system is more flexible when several lenses are used. The same filter can often be used with different lenses by fitting the holder with the appropriate adapter ring. This can be more economical than purchasing the same filter in several diameters.
Do you really need a protective filter?
Some photographers use clear or UV filters primarily for lens protection. A protective filter can help shield the front element from fingerprints, dust, spray and minor impacts.
Modern digital cameras generally do not require a UV filter to correct ultraviolet light because the camera sensor already incorporates UV and infrared filtering. Adding another piece of glass may also introduce reflections or slightly reduce image quality. Whether to use a protective filter is therefore largely a matter of personal preference and shooting conditions.
Light reduction
Many filters reduce the amount of light reaching the sensor. The amount depends strongly on the type of filter. A clear protective filter has virtually no practical effect on exposure, while polarizing and neutral density filters can reduce the transmitted light considerably.
When using automatic exposure modes, the camera's light meter normally compensates for this reduction automatically. With strong ND filters, however, manual exposure calculations may sometimes be useful.
Reflections and image quality
Adding a filter introduces extra optical surfaces that can cause reflections, flare or reduced contrast, particularly when a strong light source is in or near the frame. High-quality multi-coated filters are designed to minimise these effects.
Low-quality filters may also reduce sharpness or introduce unwanted colour casts. This is particularly noticeable with high-resolution cameras and strong neutral density filters.
Vignetting
Filters can occasionally cause vignetting, especially when several filters are stacked or when thick filter mounts are used on wide-angle lenses. The filter rim may then become visible near the corners of the image.
Slight vignetting can usually be corrected in software such as Lightroom or Photoshop. Vignetting can also be added intentionally as a creative effect to draw attention towards the centre of an image.
The photograph below shows an example of deliberately applied vignetting.
Types of photography filters
Photography filters are available for different purposes. Some control the amount or direction of light, while others modify colour or create a specific optical effect. The following are some of the most commonly used filter types.
UV and skylight filters
UV filters were traditionally used with film to reduce the influence of ultraviolet radiation, which could create haze or a bluish cast, particularly at high altitudes.
With modern digital cameras their optical effect is generally very small because the sensor assembly already filters most ultraviolet light. For this reason, UV filters are now often used primarily as protective filters.
Skylight filters have a subtle pink or warm tint and were originally designed to reduce excessive blue tones in daylight photography. Their colour-correcting function is far less important with digital cameras because white balance can be adjusted in-camera or during RAW processing.
Polarizing filters
A polarizing filter reduces certain reflections and glare and can increase colour saturation and contrast. Circular polarizing filters (CPL) are normally used with modern digital cameras because they are compatible with autofocus and through-the-lens metering systems.
Polarizers are particularly effective at reducing reflections from non-metallic surfaces such as water, glass and wet vegetation. They can also make blue skies appear darker and increase the contrast between the sky and clouds.
The filter can be rotated to control the strength of the effect. Its influence on the sky is strongest when photographing roughly at a right angle to the direction of the sun.
Neutral density (ND) filters
A neutral density filter, or ND filter, reduces the amount of light entering the lens while ideally leaving the colours unchanged.
This allows photographers to use slower shutter speeds or wider apertures in bright conditions. ND filters are commonly used to create motion blur in waterfalls, rivers, clouds and other moving subjects, or to use a shallow depth of field in strong daylight.
Graduated ND filters
A graduated neutral density filter transitions gradually from dark to clear. It is commonly used to reduce the brightness of a sky while leaving the darker foreground largely unaffected.
Graduated ND filters are especially useful in landscape photography when the brightness difference between the sky and foreground is greater than the camera can conveniently record in a single exposure.
Colour graduated filters
Colour graduated filters work in a similar way to graduated ND filters but introduce colour rather than simply reducing light. They are available in colours such as blue, orange and tobacco brown and can be used to create or strengthen atmospheric effects.
Infrared filters
An infrared filter blocks most visible light while allowing selected near-infrared wavelengths to reach the camera sensor. The resulting photographs can have a distinctive surreal appearance, with foliage often appearing unusually bright.
Digital cameras contain an internal infrared-cut filter, so their sensitivity to infrared light varies considerably. Strong IR filters also make the viewfinder or live image very dark on some cameras, which can make focusing and composition more difficult.
Depending on the camera and filter, it may therefore be easier to focus and compose before attaching the filter, or to use live view and manual focusing.
Centre spot filters
A centre spot filter keeps the central part of the image relatively clear or sharp while softening the surrounding area. It creates a dreamy effect that was particularly popular in portrait photography.
Similar effects can also be created experimentally with a clear filter and a small amount of petroleum jelly around its outer area. Petroleum jelly should never be applied directly to the front element of a lens.
Special-purpose and creative filters
Some filters are designed for specific photographic situations or primarily for creative effects. Night, star and close-up filters are examples of filters that can change either the recorded light or the optical behaviour of the lens.
Night and light-pollution filters
Light pollution can be a major problem in night and astrophotography. Artificial lighting can brighten the sky and reduce the contrast between stars and the background.
Light-pollution filters are designed to suppress selected wavelengths associated with certain types of artificial lighting. Their effectiveness depends on the light sources present, and modern LED lighting can be more difficult to filter because it emits light across a broader spectrum.
These filters can improve contrast under suitable conditions, but they may also alter colour balance. Colour correction may therefore be necessary during RAW processing.
Star filters
A star filter contains a fine etched or patterned surface that turns bright points of light into star-shaped rays. Depending on the filter, four, six, eight or more rays may appear around a light source.
A similar but not identical starburst effect can occur naturally when photographing point light sources with a small aperture. In that case, the shape and number of rays are influenced by the aperture blades of the lens.
Star effects can be used in both daytime and night photography, especially when small, bright highlights are present in the scene.
Close-up filters
A close-up filter, also known as a close-up lens, works like a magnifying lens placed in front of the camera lens. It reduces the minimum focusing distance and allows small subjects to be photographed at greater magnification.
Close-up filters are specified in diopters. Common strengths include +1, +2, +3 and +4 diopters. A higher diopter value provides stronger magnification and a shorter working distance.
Close-up filters are a relatively inexpensive way to experiment with close-up photography, although a dedicated macro lens generally provides better optical quality and greater flexibility.