What Is a CPL Filter? 5 Essential Photography Tips

A CPL filter (circular polariser) is a glass attachment that screws onto your camera lens to reduce glare, darken blue skies, and boost colour saturation. For landscape, travel, and outdoor photographers, it is one of the most valuable accessories you can own. The effects cannot be replicated in post-processing — once glare is captured, it is there to stay.

Here is what we cover: what a CPL filter is and how it works, the difference between circular and linear polarisers, 5 essential photography tips for getting the most out of your CPL, when to use it (and when not to), CPL vs ND filters, and a practical buying guide.

What Is a CPL Filter?

A CPL filter — short for circular polariser — is a camera lens filter that screws onto the front of your lens and rotates. It contains two glass layers bonded together. The front layer is a linear polariser that blocks light waves oscillating in certain directions. The rear layer is a quarter-wave plate that converts the light to circular polarisation so your camera's autofocus and metering systems work correctly.

Unlike a UV filter (which just protects the lens), a CPL filter actively changes the light entering your camera. It selectively removes polarised light — the light that causes glare on water, reflections on glass, and the washed-out look in bright skies.

The effect is controlled by rotating the front ring of the filter. As you turn it, you see the polarisation effect strengthen or weaken in real time through your viewfinder or LCD screen.

How Does a CPL Filter Work?

Light from the sun is unpolarised — it vibrates in all directions. When it reflects off a non-metallic surface like water, glass, or wet leaves, it becomes polarised: the vibrations align in a particular direction. This polarised light is what creates the bright glare you see reflecting off a lake or a shop window.

A CPL filter acts like a picket fence for light waves. Picture a rope shaking up-and-down trying to pass through a vertical picket fence. Only the wiggle that matches the gap gets through; the rest is blocked. Rotate the fence 90°, and nothing passes at all. This is exactly how a polariser works on light — it passes only light waves oscillating in a specific orientation and blocks the rest.

The quarter-wave plate at the back of the filter does something clever. It converts the linearly polarised light into circularly polarised light, which is what modern DSLR and mirrorless cameras need for accurate autofocus and metering. This is why you need a circular polariser for modern cameras, not an old linear one.

A CPL filter typically reduces incoming light by about 1 to 2 stops. This means you may need to use a slower shutter speed, wider aperture, or higher ISO to compensate.

For the full physics behind this, see our guide on how polarisation works.

CPL vs Linear Polariser: What's the Difference?

The question "circular vs linear polariser" comes up often. The short answer: for any modern camera with autofocus, buy a circular polariser.

A linear polariser does the same polarising job optically, but it interferes with the beam-splitters and phase-detection sensors inside DSLRs. The result: inaccurate autofocus and exposure. Circular polarisers solve this with the quarter-wave plate, which is why they are the standard for modern photography.

5 Essential Tips for Using a CPL Filter

Here are the five tips that make the difference between a good polariser shot and a frustrating one.

1. Use the 90-Degree Rule for Maximum Effect

A CPL filter works best when your subject is at a 90-degree angle from the sun. Point your thumb at the sun, extend your index finger — where your finger points is where the polarisation effect will be strongest.

If you shoot directly toward the sun or directly away from it, the polariser will have little to no effect. The sky will not darken, and reflections will not reduce. Position yourself so the sun is to your side for maximum impact.

2. Rotate and Preview Before Shooting

The CPL filter's front ring rotates continuously, and the effect changes as you turn it. Look through your viewfinder or use live view and rotate the filter until you see the desired effect.

The maximum polarisation setting is not always the best. Sometimes partial polarisation looks more natural. For skies, a subtle darkening often looks better than full polarisation, which can make the sky look unnaturally dark. Experiment with different rotations for each scene.

3. Watch for Uneven Skies with Wide-Angle Lenses

Wide-angle lenses — especially those wider than 24mm — cover a large field of view. The angle of light varies across the frame, so the polarisation effect can be uneven. One part of the sky may look deep blue while another section looks normal.

This is one of the most common mistakes with CPL filters. If you see an uneven sky in your frame, either zoom in, recompose so the sky takes up less of the frame, or rotate the filter to a less intense setting.

4. Compensate for the Light Loss

A CPL filter blocks 1–2 stops of light, depending on the brand and the polarisation angle. Your camera's meter will detect this and adjust, but you should be aware that:

• Shutter speeds will be slower — use a tripod if needed
• In low light (dusk, indoors, shade), the filter may push you into unusably slow shutter speeds
• ISO may need to increase to compensate

For handheld shooting in bright daylight, the light loss is rarely a problem. For low-light or indoor shooting, take the filter off.

5. Remove It When You Do Not Want the Effect

A CPL filter removes reflections. Sometimes you want them — when shooting a mirror-like reflection on a lake, the glossy surface of wet streets at night, or a sunset reflecting off water. In these cases, take the filter off.

Similarly, remove the CPL for:

• Sunrise and sunset shots (the sun in frame causes flare, and the filter has little effect)
• Indoor or night photography (not enough light)
• Portrait photography (the filter does not help and costs you light)

For a complete buyer's guide on choosing the right filter, see our article on the best polarising filters for photography.

When to Use a CPL Filter

CPL filters shine in these specific situations:

Landscape photography. Darken blue skies, increase contrast with white clouds, and reduce atmospheric haze on distant mountains. This is the most common use and where the filter makes the biggest difference.

Water scenes. Cut glare from lakes, rivers, and oceans so you can see beneath the surface. This works best when shooting at an angle to the water, not straight down.

Foliage and vegetation. A polariser removes the harsh reflected light from leaves and grass, making greens appear richer and more saturated. This is especially effective after rain, when leaves are wet and highly reflective.

Through glass. Shooting through a shop window, car window, or museum display case? A CPL filter can eliminate or reduce the reflections, letting you capture what is behind the glass.

Architecture and cityscapes. Reduce glare from building windows and metallic surfaces, improving contrast and colour in your urban shots.

When NOT to Use a CPL Filter

Just as important as knowing when to use one is knowing when to take it off.

• Shooting directly into the sun — the filter has little effect and adds risk of lens flare
• Low light conditions — the 1–2 stop light loss can ruin handheld shots
• When you want reflections — mirror reflections on water or wet streets are often the point of the shot
• With very wide-angle lenses — uneven sky polarisation can ruin a landscape
• With variable ND filters — stacking a CPL on a variable ND can create strange colour artefacts and uneven darkening

Many photographers keep a CPL filter on all the time and rotate it to the neutral position when they do not want the effect. This works, but be aware of the light loss and the potential for uneven skies.

What to Look for When Buying a CPL Filter

Not all CPL filters are created equal. Here is what matters:

Thread size. This is the diameter of your lens's filter thread in millimetres. It is marked on the lens barrel or lens cap with a "ø" symbol (e.g., ø67mm). Common sizes are 52mm, 58mm, 67mm, 72mm, 77mm, and 82mm. If you have multiple lenses with different thread sizes, buy a filter for your largest lens and use step-up rings for smaller lenses.

Glass quality. The optical glass determines image sharpness and colour accuracy. Multi-coated filters reduce lens flare and ghosting. Premium brands use German Schott or Japanese AGC optical glass. Budget filters may introduce colour casts or reduce sharpness.

Thin vs standard. Thin (slim) CPL filters are designed for wide-angle lenses to prevent vignetting (dark corners). Standard thickness filters are fine for normal and telephoto lenses.

Brand reputation. Well-known brands include Hoya, B+W, Tiffen, Urth, NiSi, Marumi, and K&F Concept. For a deeper look at specific models, see our roundup of the best polarising filters.

Common misconception: a CPL filter is just a UV filter with extra steps

Many photographers assume a CPL filter is just a fancy UV filter — something to protect the lens that also does a bit of colour enhancement. This is wrong.

A UV filter is a plain piece of glass that blocks ultraviolet light (which the human eye cannot see anyway) and protects the lens. It does not change the image at all. A CPL filter is an active optical tool that physically blocks specific light waves. It changes how light reaches your sensor, and the effect cannot be replicated in software.

The other common misconception is that a CPL filter makes everything "better" all the time. It does not. It removes reflections, which is sometimes the opposite of what you want. Knowing when to use it and when to remove it is what separates a skilled photographer from someone who just screws on a filter and leaves it.

External resources

• Wikipedia: Polarizing Filter (Photography) — technical background on polarising filters including the difference between linear and circular types
• B&H Explora: Why and How to Use a Circular Polarizer Filter — practical tips from one of the largest photography retailers
• Shotkit: What Is a CPL Filter and Why Should You Use One? — comprehensive guide covering CPL uses, ND comparison, and when to avoid