What is a Lens Filter?
In photography, a filter is nothing but a camera accessory consisting of an optical filter that can be inserted into the optical path. The filter can be of a square or oblong shape and mounted in a holder accessory, or more commonly, a glass or plastic disk in a metal or plastic ring frame, which can be screwed into the front of or clipped onto the camera lens.
Camera lens filters can serve different purposes in digital photography. They can be indispensable for capturing scenery in extremely difficult lighting conditions, they can enhance colors and reduce reflections or can simply protect lenses.
Since digital photography is all about the quality and intensity of light, it’s often necessary to modify the light before it enters the lens and so does the lens filters. Some photographers think that some built-in features of Photoshop and Lightroom can simulate the filter behavior, making the use of filter redundant in digital age. As I will demonstrate below, some filters in fact can never be simulated in software and some actually help in getting even better results during post-processing. In this article, I will talk about the different types of lens filters available, what they do, when and how to use them.
My lens came with a lens cap, so why do I buy a filter?
If you ask most consumer-camera owners why they keep a filter on their lens, a majority will reply, ‘Protection’. Yes they do, in fact, filters protect the lens surface against dust, moisture, occasional thumb prints and even sometimes from little external damages. But the primary function of a lens filter is really to improve the quality of a photo you take, depending on which type of filter you are using and how you are using.
Types of Lens Filters
Lens filters come in different shapes and forms, as shown below. The most popular lens filters are circular, screw-on filters. Those mount directly onto the filter thread in front of a lens. They come in different sizes, depending on the lens filter thread. The standard and the most common size of screw-on filters for professional lenses is 77mm.
- Circular screw-on filters – Most common type that mounts directly on the lens filter thread. Examples of circular screw-on filters include UV/Clear/Haze filters, circular polarizers, neutral density and color filters. Circular filters also come in different thicknesses – some are thick that can potentially add vignetting, while others are ultra-thin to diminish vignetting, but make it impossible to put a lens cap.
- Square filters – A popular choice for landscape and other photography. A filter holder directly attaches to the lens filter thread and can hold one or more filters. The most popular sizes are 3×3 and 4×4. Can be stacked together in certain situations, which can negatively impact image quality and add reflections.
- Rectangular filters – Another popular choice, primarily among landscape photographers. Mounted just like square filters via a filter holder system. Because it is impractical for graduated neutral density filters to be circular (due to different sizes of high-contrast areas and composition), rectangular filters are the primary choice for landscape photography. Unlike square filters, they have more room to move up and down. The most popular size is 4×6, although larger and smaller filter sizes are also available.
- Drop-in filters – These filters are used inside long telephoto lenses, due to the large size of the front lens element. Only clear and polarizing filters are used for drop-in filters.
Overview of different lens filters
|Lens Filter||Photography Type||Purpose|
|UV/Clear/Haze Filter||Any||Protects the front element of a lens from dust, dirt, moisture and potential scratches. High quality UV filters can be permanently mounted on lenses with a minimum impact on image quality.|
|Polarizing Filter||Any||Filters out polarized light, dramatically reducing reflections, enhancing colors and increasing contrast. Can be used for any type of photography. Polarizing filters are typically circular, allowing for easy control of the effect of polarization.|
|Color/ Warming/ Cooling Filter||Any||Corrects colors, resulting in a change in camera white balance. Some color filters can subtract colors, blocking one type of color and allowing other colors through. These types of filters were popular for film. They are rarely used in digital photography, since their effects can be easily applied in post-processing.|
|Special Effects Filter||Any||There are a few different types of special effects filters. Star filters make bright objects look star-like; softening/diffusion filters create a “dreamy” look used for portraits, multivision filters create multiple copies of a subject; infrared filters block infrared and pass visible light; bokeh filters have a certain shape cut in the middle of the filter that makes bokeh highlights have the same shape, etc.|
|Neutral Density (ND) Filter||Landscape, Flash Photography||Reduces the amount of light entering the lens, thus decreasing camera shutter speed. Useful for situations where motion blur needs to be created (rivers, waterfalls, moving people) or large apertures must be used with flash to avoid overexposure.|
|Close-Up Filter||Macro Photography||Also known as “diopter”, a close-up filter allows a lens to focus closer on subjects. These filters are only used for macro photography.|
|Hard-Edge Graduated Neutral Density (GND) Filter||Landscape Photography||Hard-edge GND filters are primarily used in high contrast situations, where the sky is much brighter than the foreground and the horizon is flat. These filters are always rectangular (giving the ability to move them in all directions) and are typically used with filter holders.|
|Soft-Edge Graduated Neutral Density (GND) Filter||Landscape Photography||Soft-edge GND filters are also used in high contrast situations, but where the horizon is not necessarily flat. The soft edge allows for smoother transitions, making the use of a filter less evident. Soft-edge GND filters are also rectangular and are normally used with filter holders.|
|Reverse Graduated Neutral Density (GND) Filter||Landscape Photography||The reverse GND is a specialized filter used by landscape photographers when shooting against the sun while it is setting close to the horizon. While a regular GND filter gradually transitions from dark to clear towards the center, a reverse GND filter transitions from dark to less dark from the center to the edge.|
Different lens types in detail :
UV/Clear/Haze Filters – The purpose of a UV / Clear / Haze filters today is to simply protect the front element of a lens. In the past, these filters were used to block UV from hitting the film. All digital camera sensors have a UV/IR filter in front of the sensor, so there is no more need to use UV filters on DSLRs. Many photographers use these types of filters for protection, because it is easier and cheaper to replace a filter than to try to repair a scratched or broken lens element. I personally prefer to keep a clear filter on my lenses at all times, because they are easier to clean.
However, UV filters have the potential to decrease image quality by increasing lens flare, adding a slight color tint or reducing contrast. Multicoated UV filters can dramatically reduce the chance of flare, and keeping your filter very clean minimizes any reduction in image quality (although even invisible micro abrasions will affect sharpness/contrast). High quality UV filters will not introduce any visible color cast.
Polarizing Filters or Polarizers are perhaps the most important of any filter for landscape photography. They work by reducing the amount of reflected light that passes to your camera’s sensor. Similar to polarizing sunglasses, polarizers will make skies appear deeper blue, will reduce glare and reflections off of water and other surfaces, and will reduce the contrast between land and sky.
Note how the sky becomes a much darker blue, and how the foliage/rocks acquire slightly more color saturation. The intensity of the polarizing effect can be varied by slowly rotating your polarizing filter, although no more than 180° of rotation is needed, since beyond this the possible intensities repeat. Use your camera’s viewfinder (or rear LCD screen) to view the effect as you rotate the polarizing filter.
The polarizing effect may also increase or decrease substantially depending on the direction your camera is pointed and the position of the sun in the sky. The effect is strongest when your camera is aimed in a direction which is perpendicular to the direction of the sun’s incoming light. This means that if the sun is directly overhead, the polarizing effect will be greatest near the horizon in all directions.
Additionally, using a polarizer on a wide angle lens can produce an uneven or unrealistic looking sky which visibly darkens. In the example to the left, the sky could be considered unusually uneven and too dark at the top.
Color/ Warming/ Cooling Filter – Cooling or warming filters change the white balance of light reaching the camera’s sensor. This can be used to either correct an unrealistic color cast, or to instead add one, such as adding warmth to a cloudy day to make it appear more like during sunset.
These filters have become much less important with digital cameras since most automatically adjust for white balance, and this can be adjusted afterwards when taking photos with the RAW file format. On the other hand, some situations may still necessitate color filters, such as situations with unusual lighting (above example) or underwater photography. This is because there may be such an overwhelming amount of monochromatic light that no amount of white balance can restore full color—or at least not without introducing huge amounts of image noise in some color channels.
Neutral Density (ND) Filter – The purpose of neutral density filters is to reduce the amount of light that gets to the camera and thus decrease the shutter speed and increase exposure time. These types of filters are particularly useful in daytime, because of the abundance of light that cannot be significantly reduced by stopping down the lens aperture and decreasing ISO. For example, if you are photographing a waterfall and your starting point is ISO 100, f/2.8, 1/2000 that results in good exposure, stopping down the lens to f/22 will only slow down the shutter speed to 1/30 of a second. This would be too fast to create a “foggy” look for the falling water. By using an 8 stop neutral density filter, you could slow down the shutter speed all the way to 2 seconds while keeping lens aperture at f/11 instead of f/22 (using apertures beyond f/11-f/16 in normal lenses decreases image quality due to diffraction).
Neutral density filters can be both circular and rectangular. There are no benefits to having a rectangular neutral density filter, so it is best to buy a circular ND filter for size and portability benefits.
It is sometimes necessary to stack neutral density filters to decrease the shutter speed even more. Try not to stack ND filters with wide-angle lenses to avoid vignetting.
Graduated Neutral Density (GND) Filter – Graduated neutral density (GND) filters restrict the amount of light across an image in a smooth geometric pattern. These are sometimes also called “split filters.” Scenes which are ideally suited for GND filters are those with simple lighting geometries, such as the linear blend from dark to light encountered commonly in landscape photography (below).
Prior to digital cameras, GND filters were absolutely essential for capturing dramatically-lit landscapes. With digital cameras one can instead often take two separate exposures and blend these using a linear gradient in photoshop. On the other hand, this technique is not possible for fast moving subject matter or changing light (unless it is a single exposure developed twice from theRAW file format, but this increases image noise). Many also prefer using a GND to see how the final image will look immediately through the viewfinder or rear LCD.
GND filters come in many varieties. The first important setting is how quickly the filter blends from light to dark, which is usually termed “soft edge” or “hard edge” for gradual and more abrupt blends, respectively. These are chosen based on how quickly the light changes across the scene, where a sharp division between dark land and bright sky would necessitate a harder edge GND filter, for example. Alternatively, the blend can instead be radial to either add or remove light fall-off at the lens’s edges (vignetting).
Placing the blend should be performed very carefully and usually requires a tripod. The soft edge is generally more flexible and forgiving of misplacement. On the other hand, a soft edge may produce excessive darkening or brightening near where the blend occurs if the scene’s light transitions faster than the filter. One should also be aware that vertical objects extending across the blend may appear unrealistically dark
A problem with the soft and hard edge terminology is that it is not standardized from one brand to another. One company’s “soft edge” can sometimes be nearly as abrupt a blend as another company’s so called “hard edge”. It is therefore best to take these on a case by case basis and actually look at the filter itself to judge the blend type. Most manufacturers will show an example of the blend on their own websites.
The second important setting is the differential between how much light is let in at one side of the blend versus the other (the top versus bottom in the examples directly above). This differential is expressed using the same terminology as used for ND filters in the previous section. A “0.6 ND grad” therefore refers to a graduated neutral density filter which lets in 2 f-stops less light (1/4th) at one side of the blend versus the other. Similarly, a 0.9 ND grad lets in 3 f-stops less light (1/8th) at one side. Most landscape photos need no more than a 1-3 f-stop blend.
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