What Is a Lens Filter?

The lowdown on camera lens filters and effects

A camera lens filter is an additional piece of glass (or sometimes plastic) that’s mounted at the end of a camera lens (the end that faces the subject) to alter the light that travels through the lens to the image sensor. On DSLR cameras, lens filters are used to achieve a variety of effects. Here’s what you should know about the different types of photography filters.

What Is a Lens Filter?

A camera lens filter can be clear, gray, graduated, or a variety of different colors. They’re used to alter the light that travels through the lens to the image sensor, and they’re used for a variety of effects. 

Initially, photography filters were used in film photography to enhance black and white photography. By adding a different colored filter, the photographer could add depth, improve contrast, and reduce light glare that could ruin an image.

Modern lens filters still work with DSLR cameras, but many DSLR cameras have features built into the shooting modes, camera settings, and even in the actual hardware of the equipment that perform many of the same functions. So, it’s not unusual for digital photographers to use lens filters for strictly creative enhancements and alterations to ‘standard’ photographs.

Types of Camera Lens Filters

Lens filters take two forms: square or round. Square filters filters are designed to slip into a mount that screws onto the front of the camera lens, immediately in front of the outer lens glass. Then, you can choose which filter you would like to use, slip it into the filter mount, and begin photographing your subject. 

Square camera filters can be slightly less expensive than round filters, because one set of filters can be used with multiple sizes of lenses. Square filters are also considered by some photographers to be more versatile, since you can leave the filter mount attached to your camera and add or remove filters as needed. 

The more common type of lens filters that you’re likely to encounter are round filters. These filters screw onto the front of a camera lens (assuming there are screw threads on the lens—a few DSLR lenses do not have these threads).

The problem you may encounter with these filters is that you need a different size for each different size lens you own. For example, if you own a 35mm lens that has a 57mm front ring, you’ll need to have 57mm filters. If you add an 18mm lens to your kit that has a 67mm front ring, then you’ll need to purchase another set of filters that will fit the larger front ring.

One way that some photographers get around buying multiple sets of round filters is to buy filters that fit the largest size lens they own, and then use step-up rings to build up smaller size lenses so they’ll fit. This is accomplished by stacking filter rings—essentially screwing them together until there is an end that fits the camera lens and an end that fits the larger lens filter.

Regardless of whether you choose to use square or round lens filters, you can find a large variety of these filters, each that performs its own job. For example, one piece of advice you may have received early on is to always use a UV (ultra-violet) filter.

UV filters are supposed to help reduce hazy or gray-ish casts on images, and some photographers keep one attached to their lens at all times to help protect the lens. However, most DSLR cameras on the market have mechanisms in place to prevent this from happening. So, in truth, UV filters aren’t usually necessary.

One general rule of thumb in photography is: the more glass you put between the image sensor and the image subject, the more there is to go wrong. Adding another layer of glass can reduce the amount of light that reaches the sensor and cause images to be too dark. That additional filter can also introduce a host of other effects that aren’t present when you’re not using it, so it’s always way to take some test shots when adding a new filter to ensure you’re getting the look you are trying to achieve.

What Are Photography Filters Used For?

In addition to the UV filter previously mentioned, there are a few different types of filters for use with both digital and film cameras. Those other types of lens filters include:

  • Polarizing Filters
  • ND (neutral density) Filters
  • Variable or Circular ND Filters (also called Graduated or Gradient filters)
  • Colored filters
Man holding gradient neutral density ND filter on background of summer sunset.
 Koldunova_Anna / Getty Images

With all these different types of filters, it can be confusing about which type of filter should be used for what conditions. Here’s a quick breakdown of what each type of filter is used for:

  • Polarizing: Much like polarizing sunglasses, polarizing lens filters can help reduce reflections and improve the dept of color in images. The drawback of polarizing filters, however, is that they can wash out the sky, which means you won’t get the depth often associated with the perfect clouds. Polarizing filters are great for reducing water glare, and are often used in landscape and outdoor photography.
    • Circular Polarizing Filters: A subset of polarizing filters is circular polarizing filters. These filters perform the same function as a standard polarizing filter, but can be adjusted to get the perfect pop of color and reduction in glare. Just add the filter and then turn it until the image looks exactly the way you want it to look. The one drawback is that these filters can easy be bumped or moved as you move the camera around, which means you may need to readjust them frequently to keep the filter turned to the perfect position.
  • ND (Neutral Density) Filters: The purpose of ND filters is to reduce the amount of light that enters the lens flows through to the image sensor. So, these filters are often used in very bright conditions, including during sunrise and sunset when the photographer wants to catch those few moments the sky is on fire with color. Those colors can be very bright, which will cause a loss of detail in the final image; and ND filter can help reduce that loss of detail (which is often called blowout in an image). Photographers also use ND filters when they want to create long exposure images or reduce the shutter speed in a bright environment. ND filters come in a variety of ‘darknesses’ and are usually labeled ND# (where the # is the equivalent of a number). For example, and ND8 filter reduces about ⅛ of the light that travels through the lens to the image sensor. The key to remember is that in ND numbering, the lower the number, the less light it will let in. So, an ND1 filter will appear almost black and will allow very little light through to the image sensor. 
    • Circular/Variable Neutral Density: Circular or variable ND filters are a subset of ND filters where the filter is actually graduated or has a gradient of darkness that increases and decreases. Rather than changing ND filters each time you need to go darker or lighter with the filter, you can simply turn the circular ND filter until the desired effect is reached. 
  • Colored Filters: Colored filters are a bit of a throwback to film photography. Film photographers used different colored (red, blue, green, yellow, orange) filters when shooting black and white photographs to help increase the depth in an image, since removing color from an image can reduce the contrast and shading that indicates depth. Different colors will increase or decrease shades of gray, which can be used to achieve the photography you want to capture. Colored filters are also often used in creative photography to add a color cast in an artistic way.
Color photo filters isolated on white background.
aquatarkus / Getty Images 

Some photography filters can be stacked to achieve different effects. For example, you can stack polarizing and ND filters, or you can stack different levels of ND filters to go even darker if you’re shooting in very bright conditions and need that additional reduction in light.

How Do I Know Which Camera Lens Filter to Use?

If you’re still not clear about when to use each different type of lens filter (and maybe even if you are), the best thing you can do is to take some example photographs using each type of filter so you can see how they alter your images. Here are a few additional tips to keep in mind:

  • Cheap filters may not work as well as higher quality filters. Some cheap filters can cause vignetting and color shifts, or can introduce noise into an image. Be sure to do your research before you purchase a set of camera lens filters, and choose good quality filters. A good rule of thumb is that a filter should cost around 10 percent of the price of the lens on which you’re using it.
  • Some of the effects you can achieve with camera lens filters can also be achieved in post-production using progams like Photoshop or Gimp. Try using a filter and using your editing program in post-production to see which effect you like best.
  • Lens filters can protect the front glass on your lens if you’re shooting in extreme conditions. If you’re planning to be climbing around on rocks, in sandy or wet areas, or on the ground a lot, a lens filter and help protect the front of your camera lens from dirt, dust, scratches, and even those unexpected bangs. That’s probably not a great reason to keep a filter attached to your camera all the time, but in some circumstances, you may be thankful if you do.