What are optical filters in photography, and how do they work?

Digital cameras and photo-editing software have pretty much put an end to the need to cart around an extensive and expensive range of glass or plastic filters.

For instance, being able to change the white balance from shot to shot, either in-camera or later when you process your images, means that you no longer have to use colour correction filters to warm up or cool down an image before you take it.

The effects created by traditional red, green or orange filters in black-and-white photography can easily be emulated in software.

And why bother with special-effect filters to add soft focus, starbursts and coloured tints when you can experiment endlessly in the digital darkroom?

That’s not to say that all filters are redundant in digital photography. For instance, a close-up filter, which essentially acts as a magnifying glass, offers a cheaper alternative to a dedicated macro lens.

And the addition of a UV or clear protective filter will protect the front element of a lens; they come in particularly handy when you’re shooting near water and in dusty locations.

But the truth is that you only really need two types of creative optical filter: a polariser and a neutral-density filter. The reason for this is that their effects are time-consuming, and sometimes near-impossible, to recreate authentically in post-production.

A polariser acts like a pair of polarising sunglasses, cutting through glare and reflections to give richer colours and stronger contrast.

They are frequently used by landscape photographers to add drama to blue skies filled with fluffy white clouds, and to reduce the sheen on the surface of water and foliage. They’re expensive filters, but worth it.

Neutral-density filters come in two flavours: standard and graduated. Standard ND filters are a uniform grey tone, and are used to reduce the amount of light entering the lens.

They enable you to use large apertures in bright light to achieve shallow depth of field effects, and can extend shutter speeds to render a moving object as a blur.

Graduated ND filters are dark at one end and clear at the other, and help to reduce the contrast between a bright sky and a dark foreground. But as you’ll read above, they’re not always necessary in the digital age.

Whichever creative filter you choose, you’ll need to decide whether you want it as a stand-alone circular filter or part of a square filter system. Circular ones simply screw onto the filter thread on the front of a lens, while square filters require a filter holder and adapter ring to attach them.

Each system has its advantages. Circular filters are convenient as they can be left attached to your lenses. Square filters take up more room in your camera bag, but you only need one filter and a set of adaptor rings to cover all your kit.

Square filters are generally a better option when you plan on combining filters, or if you own a bunch of lenses with different filter thread sizes.

Using a polariser filter

A polariser is a two-part filter. Once it’s attached to your lens, you can freely rotate the front section to increase or decrease the effect. And what an effect it is!

It’s highly effective on sunny days, adding a rich, velvety quality to blue skies, but it can also add punch to pictures shot on damp, overcast days.

There are two types of polariser – circular and linear. These descriptions don’t refer to the shape of the polariser, but rather the way in which they polarise the light. Stick with the circular type, as the linear ones don’t mix well with a digital camera’s autofocus system.

One of the trade-offs with a polarising filter is that reduces the amount of light entering the lens by up to two stops, depending on the degree to which it is rotated.

This can result in slower shutter speeds and the potential for blurred shots, either through camera shake or subject movement, so you may have to compensate by increasing the ISO.

It’s going dark
Polarisers give their strongest effect when they’re used at 90 degrees to the sun. There’s a neat trick that you can use in order to discover the sweet spot for the effect…

Thumbs up
Point your index finger at the sun, stick your thumb out and rotate your hand. The direction in which your thumb is pointing is where you need to train your lens for the maximum effect.

Go slow
It can be hard to judge the point where the effect is at its strongest, so rotate the filter slowly as you look through the viewfinder. Avoid turning it in the direction that unscrews the filter too!

Wide woes
Polarisers can produce an uneven effect with ultra-wide-angle lenses, with patches of sky appearing darker than others. You’ll have to crop the shot or balance the effect in software.

Using graduated filters
Graduated neutral-density filters are grey at the top, blending to clear at the bottom. By placing the dark part over the sky, you can bring its exposure value closer to that of the landscape below.

Without it, you’d have to choose to expose for the land (resulting in a sky that’s too bright) or expose for the sky (and end up with land that’s too dark).

While essential to good landscape photography in the days of film, there’s less of a need for an ND grad with a digital camera.

You can simply take one photo with the sky exposed correctly and another with the land exposed correctly, then blend the two together in Photoshop or similar software.
You can also apply a grad effect digitally, although this technique is best used when the exposure difference is small.

Give me strength
ND grads are available in a range of strengths, which indicate how much light the dark section blocks – and come with either a hard transition or a soft transition between the dark and clear parts. Hard ones are best reserved for situations where there’s a clear horizon line, otherwise you’ll darken areas that protrude into the transition.

On the slide
The beauty of a square filter system is that you can position the effect where you want to; the transition is always in a fixed position with a circular filter.

It can be tricky to judge the position of the transition through the viewfinder, so switch to Live View, select a small aperture and activate the camera’s depth of field preview function as you slide the filter.

In a spin
Square filter holders rotate on the adaptor rings, so you can position the effect at an angle – such as along the edge of a cliff or building.

If you’re using a polarising filter in conjunction with an ND grad, then position the grad first before you adjust the polarising effect. If you don’t, the action of rotating the filter holder will change the position of the polariser.

Using neutral density filters

Standard neutral-density filters reduce the total amount of light entering the lens, allowing you to use much slower shutter speeds than you ordinarily could in a given situation.

As with ND grads, they come in a range of strengths that determine how many stops or exposure values they reduce the light by.

For instance, a three-stop ND (sometimes labelled 0.9ND or ND9) will allow you to use a shutter speed of 1/10 sec where you would normally use 1/80 sec. Standard NDs are available as both circular filters and square ones.

The latter are more convenient with very strong NDs, as it’s easy to slide the filter out of the way and tweak the focus and composition using the viewfinder.

Using ND filters to blur water and clouds
ND filters that are rated at six or 10 stops are popular with landscape photographers, as their impressive stopping power allows the motion of waves and clouds to be rendered as a silky-soft blur.