Lens filters explained

Every filter box sells you an effect, and a lot of those effects now come free with a slider in your editor. Cross those off and the shopping list is down to two or three pieces of glass.

1. What filters are actually for

Physically, a filter can do three jobs: protect the front element, pass light vibrating in one direction only, or cut the amount of light.

Warming, contrast, diffusion and star effects are a slider away, and baked into the frame they can't be undone. Pay for filters that change the light before it enters the lens: polarisation and long exposure.

2. UV and protectors: buying insurance

Digital sensors already sit behind a filter stack that handles ultraviolet, so a UV filter does next to nothing optically. Think of it as a cheap, replaceable piece of glass in front of an expensive one, which is insurance rather than an upgrade.

• Worth having on at the coast with salt spray, on sand, in rain and snow, in crowds, around small children, and scrambling through trees.
• Take it off for backlit scenes and night shots with bright lights in frame. Every extra glass-to-air surface means more ghosting and flare.
• Don't buy a cheap one. A poor protector hurts the image far more than no filter at all.

3. The polariser: the one you cannot fake

A circular polariser only passes light vibrating in one direction, so you rotate it to dial the effect in, and it peaks at one angle. It does three things no editor can:

• It kills reflections. Water, glass, wet roads and waxy leaves lose their sheen, and you see the rocks under the surface and the real colour of the foliage. Reflections off metal stay put.
• It deepens blue sky, most strongly at 90 degrees to the sun and hardly at all when you face towards or away from it.
• It lifts saturation by stripping off the white surface glare that was washing the colour out. Woodland after rain shows it best.

The costs: it eats roughly 1.5 to 2 stops, so three to four times the ISO or a shutter speed three to four times slower. On an ultra-wide the sky covers so wide an angle that you get an uneven band of darker blue. When you don't need it, turn it to the weakest position.

4. ND and variable ND: buying time

A neutral density filter cuts light evenly and, ideally, without shifting colour. You buy one to use a slow shutter or a wide aperture in bright light. The maths is one line: each stop of density doubles the exposure time.

The method is in the long exposure guide: meter without the filter, then slow the shutter by however many stops the filter takes.

A variable ND is two polarisers stacked, so turning one changes the density smoothly. Video shooters love them. Near maximum density, though, a dark X appears across the frame (sooner on wider lenses), and the colour cast is usually worse than a fixed ND. Stay in the middle of the marked range and leave the last two or three stops alone.

5. Graduated filters and the alternatives

A graduated ND is dark at the top and clear at the bottom, for holding back a bright sky. They come soft (wide transition), hard (narrow) and reverse (darkest through the middle, for a sun sitting on the horizon).

The catch: the bright/dark boundary has to be roughly a straight line. Sea horizons and open plains work. Ridgelines, treetops and city skylines don't, because anything poking above the line gets darkened with the sky and turns murky.

Usually it's easier to bracket and blend, or expose for the highlights and pull the shadows up later. Digital files hold far more recoverable detail in the shadows than in the highlights; see the histogram guide and RAW or JPEG. A grad still earns its place when you want the frame finished in camera, or when waves, branches or people are moving and a blend would leave ghosts.

6. Casts, vignetting and flare

• Buy one set for your largest common filter thread and use step-up rings on smaller lenses.
• On wide lenses get slim filters, and stack two at most. Threaded filters add depth fast, and at 16–24mm equivalent the stack starts showing in the corners.
• Cheap strong NDs often lean magenta or green, worse as density goes up. Shoot RAW, photograph a grey card with the filter on, and apply that white balance across the set; see white balance and colour temperature.
• With a bright light in frame, take off every filter you don't need. Multi-coated glass is clearly better than single-coated, and a fingerprint on the glass multiplies flare.
• A dirty filter is far worse than none. A microfibre cloth earns more bag space than another piece of glass.

7. FAQ