What are filters used for in photography?
With film, filters are an essential part of photography, used either to overcome technical issues or for creative effect. However, when it comes to digital images, editing software has replaced any creative reasons why you might use a filter and built-in camera features have overcome many of the technical issues. In fact, just four types of filter remain useful, and even those have quite specific uses.
What types of filters are there?
Ultraviolet and protection: If you buy a camera in a shop, the first accessory the salesperson will try and sell you is an ultraviolet (UV) or ‘protection’ filter. With film, UV filters prevented UV rays adding a blue tint to images when there was too much UV light present (typically at high altitudes on sunny days), so they had a technical use. Digital sensors have their own ways of coping with this interference, though, so you no longer need a UV filter to filter out UV light.
A lot of photographers still use a UV filter to protect their lens against dust and scratches, or use a protection filter, which is essentially a clear piece of optical glass designed specifically for this purpose. Emma doesn’t use a UV filter for protection.
However, if the thought of scratching your lens and having to send it off for repair or replacement fills you with dread, you might choose to have an extra layer of protection. If you do, buy the best filter you can. Otherwise you are potentially putting a (relatively) cheap piece of glass on the front of an expensive lens, and your lens is only going to be as good as that cheap piece of glass.
Polariser: A polarising filter polarises the light passing through the lens, and because it is affecting the light itself the filter’s effects cannot be replicated convincingly using image-editing software. Polarisers have two main effects, the first of which is to make blue skies darker and more saturated. The second thing that a polariser can do is reduce or eliminate reflections on non-metallic surfaces, such as water, glass and wet leaves. Polarisers are particularly useful to landscape photographers, as well as architectural photographers.
There are two types of polarising filter: linear and circular. Both types generally look the same and have the same effect on your images, but they polarise light in slightly different ways. In a practical sense, the key difference is that the way a linear polariser works can interfere with a camera’s autofocus systems, causing it to behave erratically. So, unless you only ever focus manually a circular polarising filter is a better option.
In both cases, rotating the filter on the front of the lens controls the polarising effect. To maximise the effectiveness of a polariser, use it at 90 degrees to the sun: to find the ideal angle stand with one arm out to your side pointing at the sun and aim your camera in the direction of your nose.
A third, hidden benefit of a polarising filter is that it can double up as a 1- or 2-stop neutral density filter (see below).
Neutral density: Fitting a neutral density (ND) filter is a bit like putting sunglasses on your camera, in that it reduces the amount of light passing through the lens. This can have several practical uses:
You can use a longer shutter speed (to blur the water in a waterfall or river, for example).
You can use a wider aperture (to get a shallow depth of field on a sunny day, perhaps).
Extreme ND filters let you use very long shutter speeds at any time (exposure times can extend to several minutes, even during the day).
ND filters come in several strengths, measured by the number of stops of light that they block. Typical strengths are 1, 2, 3, 6 and 10 stops, and the filters can be combined to increase their effect. However, it’s worth remembering that whenever you use filters (even a single filter on its own) you are adding a layer of possible interference in front of your lens. For this reason you should ensure you buy the best quality filters you can. This is doubly true of ND filters, as you do not want the filter to introduce a colour-cast to your image; many cheap ND filters are not truly ‘neutral’.
Graduated ND: A graduated neutral density filter (or ‘ND grad’) has a neutral density coating over part of the filter while the rest is clear, with a graduated transition between the two areas. This type of filter is typically used to even-out the dynamic range differences between a bright sky and darker foreground in landscape photography, with the ND part positioned over the sky to reduce the contrast.
Like plain ND filters, ND grads come in a range of strengths, which refer to the density of the ND coating in stops. There are also two common transition types: hard and soft. A ‘hard’ grad has a small transition between the dark and clear parts and is used when there is a very clean horizon, while a ‘soft’ grad has a more gradual change from dark to clear that makes for a less obvious transition. This is useful when the horizon is broken by elements such as trees and buildings.
Filter types
Filters are either round and screw on to the front of your lens directly, or square/rectangular and drop in to a special filter holder that sits in front of your lens. If you only have one lens then it is probably easier to get a screw-on filter, but if you have multiple lenses with different diameters this can get expensive, as you potentially need to buy the same filter in multiple different sizes. In this case, a drop-in filter system is a better solution, as you can use the same filter and filter holder on different lenses, simply by purchasing an adaptor ring for each lens. Some manufacturers use a magnetic adaptor system rather than a drop-in holder, but the idea is the same. In both cases, check whether the system you are considering will let you stack multiple filters – this is particularly helpful if you want to combine ND filters or use a polariser and an ND grad at the same time.
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