Filters are accessories that are normally used and associated with camera lenses, and are most commonly fitted to the front of then. There are two primary uses of lens filters. One is to alter or balance the colour of the light source, thus affecting the colour of the tones that appear in a shot that is taken. The other is to alter or balance the tonal values within a scene being shot, thus affecting the overall exposure. Some filters that are used are capable of doing both at the same time.

Most of the colour light balancing filters used with film cameras to ensure the correct colours result under a range of different lighting conditions are not needed or used with digital cameras since this aspect comes under the control of white balance settings. Please see our page on  White Balance  for more details. 

However there are some filters that can and are used with digital cameras. There are those that alter tonal values and exposure, and some that, whilst they still affect the quality and colour of light, do so in a way that digital camera white balance cannot.

Filter design types

There are two main types of filter made. The circular filter made in optical glass with a metal rim, which screw into the front of a lens via the filter thread normally found there, and the square filter made in CR39 optical resin which fit into special holders which are also fitted on the front via the filter thread. The round glass filters are more durable than the square resin type and not affected over time by UV light, but the downside is they can only be used with one size of lens filter mount unless 'stepping rings' are employed. The square resin types are more prone to scratching but there are certain types that work better as square and don't have the same flexibility of use when they are round filters. As with most products the better quality products cost more. Yet in this particular case buying the cheapest available is not a wise move since they effectively control the quality of the light entering the lens, and thus have a major impact on overall image quality besides just the effect that they are supposed to have.

Hoya are the worlds largest maker of optical glass. This Japanese firm supplies most of the camera and lens makers with their raw glass. So it seems a wise choice to use the round glass filters made by them when you can. They are widely available. The Kenko brand of optical filters and lens accessories is a subsidiary of Hoya, as is the Tokina lens brand. B+W is another maker of quality round glass filters. They are a subsidiary of the German optical glass firm Schneider and use Schott glass in their filters. Again high quality is assured. Cokin is the name long associated with square resin filters as they first developed them, and Lee is another make of quality square filters.

Using filters with Digicams

Some digicams have filter threads on the front of their lenses, or adapter tubes that allow round glass filters and other accessories that are attached via the filter thread to be used. This was certainly the case with early digicams that were produced before lower priced DSLR's arrived. But the vast majority don't have any means of attaching a filter. But this does not mean that filters cannot be used should you so desire. Cokin make a digital camera adapter plate that is attached to the camera via the tripod bush and allows the square type filters to be used. It is basically a digicam version of the filter holder used with other cameras. We doubt that you would want to bother with UV filters like this, but polarizing, ND, and grey grad types are a different proposition, and their use might prove beneficial. Please refer to  Digital Camera Lenses 6  for more information regarding the mounting of filters on DSLR lenses and digicams.

Filter Factors

All filters are given a 'filter factor'. This is the amount of light they stop from entering a lens. Most filters stop a certain amount of light, there are only a few that don't. The most common way of describing this loss is in Ev stops. A filter may be said to have a light loss of 1 stop, or 2 stops, and so on. This is the amount that the cameras exposure needs to be increased to offset the use of the filter. Although this is often a drawback in their use, it is also used as an advantage in some cases, as you will see when reading about the different types.

General purpose filters

The UV filter.

The Ultraviolet filter works on wavelengths that are mainly outside the visible light spectrum but which can still have an adverse affect on image quality. They cut through atmospheric haze and reduce the overall blue cast it produces and this results in increased tonal saturation. Because they do not reduce the amount of visible light entering the lens and are colourless they do not have an adverse affect on exposure values, and are thus often recommended for use as permanent fittings on the front of lenses, where they can also act as protection for the front lens element, preventing scratches etc. This is a worthwhile consideration as accidental damage to front lens elements are serious should they occur. A bad mark or scratch can render the lens useless in a worst case scenario. Replacing a UV filter, and they are not that expensive in relation to the cost of a lens (about 10%), is far better than having to replace a lens, should the worst happen. They can be considered an almost universal fitment, especially as they are of equal value to colour or monochrome images. UV's are best purchased as round glass filters and left permanently fitted on the front of a lens.

Many outlets that sell cameras, digital and film, attempt to sell Skylight 1A/1B filters instead of UV's. They will tell you that they are the same. They are not. Skylight's are colour compensating filters. They are very weak, just slightly pink, and their use is just to inject a little bit of 'warmth' into a shot taken on slide film to offset the blue cast that can occur. They have a slight effect on exposure, increasing it by about half a stop. And on a digital camera with the white balance on auto it will correct for them if possible, as it would for any colour cast. So for the digital camera user they offer no advantage over the use of a UV filter, quite the reverse in fact.

The Polarizer filter

A  Polarizing filter is concerned with selective filtration. Light travels from the sun in waves that vibrate in all directions. When these waves hit certain types of materials they bounce off in just one direction, the waves are re-orientated. This causes reflections, which are most often found in water and glass, and in the air, thanks to the droplets of water that exist as air moisture. A polarizer works like a grid of narrow slats that only allows the light waves orientated at the same angle as the slats to pass through, and blocks all others. As a result it can block light wave reflection. 

The polarizer rotates in it's mounting and by altering the angle of rotation the light waves allowed to pass through can be chosen. The result is that used at the right rotation setting, not only can reflections be minimized and often completely removed, but tonal saturation is improved as a result. Blue skies appear darker, plant leaves look a deeper richer colour and so on. The strength of the effect obtained is found and adjusted by rotating the filter back and forth in the mount, and the strongest effects of all are found when the filter, and thus the lens and camera, are between 45º-90º to the subject reflections.

Two types of polarizing filter exist. There is the plane polarizer, normally referred to as a linear polarizer, and the circular polarizer. The linear polarizer was the first type to be developed and can be used with manual focus  cameras. Because autofocus in cameras also uses plane-polarization, in a device called a beam splitter, to obtain the light that the autofocus system uses, which the use of a normal plane/linear type polarizer interferes with, another type was designed when autofocus became commonplace in cameras. This is the circular polarizer. These are now the most common type of polarizer to find for sale since they can work with any type of camera, and most cameras are of the autofocus type these days. These are about 50% dearer than linear versions.

Again because polarizer's are used in a round mount, to enable them to rotate, the round glass types are most commonly used. They are quite expensive. So it is wise to buy the largest size you need, to use with the lens with the biggest front element and filter size you have, and employ stepping down rings when using the polarizer with smaller lenses. It is the most economical way. As a general rule they are used mainly with lenses with shorter focal lengths, but not extreme wide angles because of the effect on skies they can have. With any lens past a very moderate wide angle, where a large expanse of sky can be captured, the colour change can be huge and very noticeable, mid-blue at one side and almost black at the other. A polarizing filter is not one to leave permanently attached to a lens. Polarizing filters are quite 'dense', and as a result they reduce the light entering a lens by about 3 stops, and are thus are best used in bright light conditions. As such they can also be used in place of ND filters. See below for details of what these are.

Neutral Density (ND) Filters. 

Neutral Density filters are designed purely to stop light entering the lens. This sounds a really strange thing at first, because to most people's minds that is the last thing you want to do, as normally the problem is one of not getting enough light for a decent exposure value, a high enough shutter speed or aperture setting. But there are occasions when for particular situations and photographic effects, there is too much light. Say when a combination of a slow shutter speed and wide aperture are required but the light levels are too high to achieve this. This is a particular problem when film is used, a fast ISO film has been loaded say, and the ISO speed is fixed. With digital capture it is less of a problem since ISO speed can be changed. But once you have reached the slowest you can use then you are back in the same position.

So then a Neutral Density filter is used which reduces the amount of light entering the lens, allowing the use of a different shutter speed and aperture combination. These filters come in a variety of 'strengths', the most common being ND2 -1 stop, ND4 - 2stops, and ND8 - 3stops.

These filters are not colourless and look grey coloured, the depth of the tone depending on their strength, but they do not actually affect colour tones but move them down the tonal scale, making them darker. But the camera metering allows for that and compensates, which is the whole point of using them, to obtain longer exposures that require longer shutter speeds or wider apertures. 

These filters can be obtained as round glass or square resin. The most useful strengths are probably the ND4 at 2 stops and ND8 at 4 stops. The ND2/ND1 strengths - at 1 stop and ½ stop difference - don't seem worth using as the difference is too small to have a worthwhile effect.

Neutral Density Graduated filters and the 'Grey Grad'.

As well as plain ND filters there are graduated types that vary the amount of light they stop over the image frame. The strength of the colour tone varying over the size of the filter. These are made in a range of colours but the most useful for use with both colour and monochrome is undoubtedly the 'Grey Grad'. As it's name suggests this is just grey in colour, like the plain ND filters, and it is used mostly for balancing exposure between sky and land. When the sky exposure value is very high, and the land exposure below is not. So either detail in one part or another is lost, since one exposure cannot cover both extremes. Using a grey grad in this situation reduces the light from the sky, producing a more balanced exposure, and detail is retained in both.

Again, as it is grey in colour like the plain ND filter, using it has no effect on colour tones, just tonal values, and they are made, as are all ND grads, in a range of strengths. In this case the ND2 type is worth using. It's effect is strong enough to make a visible difference to tonal values without making a tonal change that is visible in the image overall. Using too strong a grad can result in this, as if a tonal line has been drawn.

Graduated filters are really the preserve of the square type filter. You can get round glass grads but using them is very restrictive. Why? Because you need to be able to move the filter up and down in relation to the image frame to find the best balance between the different exposure levels in the scene, to arrange the biggest tonal change to occur usually, at the horizon. Then any exposure changes don't stand out as un-natural. If you can't do this then you have to move the camera and lens up and down, and the image perspective you then get might not be where you want it.

You don't even have to use a square filter holder to use the odd square resin grey grad, we don't, just hold it against the front filter element and move it up and down. It's crude, but works, which is the main point.

We have occasionally seen written some strange advice as to how to use graduated and ND filters. This is that you should meter the scene first, set the exposure, and then fit the filter. This is utter nonsense. If you follow this advice the exposure will be wrong. The whole point of using these filters is to affect the exposure, the Ev value. Fit them and then let the camera meter the scene as you would normally. This will produce an altered Ev value, which can be used to provide the aperture/shutter speed combination desired.

Black & White filters 

Besides these four filters which are good for both Colour and Black & White imagery, there are a range of coloured filters that are normally used with the taking of monochrome shots. Many digital cameras can only take colour shots, but if you have one that can also produce B&W straight off then they may be of some interest. 

All aspects of digital camera Black & White photography is covered in our page on the subject,  Black & White

It might be strange to some that B&W photography can involve the use of coloured filters, but the fact is it does, if you choose to do it. Taking a monochrome shot through a coloured filter alters the tonal values in a way that chemical darkroom work cannot replicate. It changes the relationship the tonal values each colour produces has to others in the scene depending on the colour of the filter used. The main filter colours employed are, Yellow, Green, Blue, Red, and Orange.

• Yellow ; absorbs UV & blue:  Most Yellow B&W filters are weak in strength and don't absorb all the blue. They are essential to give any contrast between clouds and sky. They tend to darken a blue sky, accentuate clouds, and they also lighten foliage. 1stop light loss

• Green ; absorbs UV & blue & red:  This lightens foliage and slightly darkens sky to a stronger level than yellow. It also makes red objects and skin tones darker. So it's ideal for predominantly green landscapes and adds a healthy tanned look to outdoor portraits. 3 stop light loss

• Yellow-Green ; absorbs UV & blue & red. Theoretically perfect rendition of landscapes, correcting foliage and sky. Useful when people are also visible in the image. 2-3 stop light loss

• Blue ; absorbs yellow, green & red:  This lightens blue subjects and increases haze in landscapes. But it is good indoors for portraits taken under tungsten lights. 2-3 stop light loss

• Red ; absorbs UV & blue & green:  It darkens blue skies and water, increases contrast, deepens shadows, cuts haze, and lightens red objects. It's the filter with the strongest effect. It exaggerates cloudscapes and dark blue skies can become virtually black. is excellent for cutting haze in distant shots and good for rendering detail in brickwork. Anything green however comes out very dark.  3 stop light loss

• Orange ; This has mainly the same effects as red, but to a lesser degree. It tends to absorb more blue than green, so is recommended for UK skies. Cuts through haze and increases contrast. Subdues freckles on faces, but can render foliage too dark. 2-3 stop light loss

As you can see each colour has particular effects, some of which mean that careful use must be made of each type.

Generally the most used filters are Yellow, Green, Orange and Red. Blue is rarely used, and only for specific shots. We have a combination Yellow/Green filter, an Orange, and a Red. 

'Effects' filters

The filters we have listed and described above are those that we feel are of most use and benefit to digital camera users. Besides these there are a huge number of 'effects' filters made. Some, like the graduated types made in a range of colours have some uses, sometimes, whilst others are quite frankly just pure rubbish, nothing more than costly gimmicks.

The best use of filters is when, apart from the UV, they are used for specific purposes.

'Digital filters' - alteration in image editing software

There are many photographers using digital cameras and digital imaging that will advise you not to bother to use any filters on your camera as the effects that they produce can be replicated using image software editors. This is only partly true. And also depends on the software you have, and how experienced you are at using it.

It is certainly true that most of the effects produced by the coloured filters used in Black & White can be re-produced on the computer, with the right software package, and of course colour balancing can also be undertaken if the camera hasn't got it quite right itself. 

But please note that this does not apply to the UV, Polarizer, ND, and Grey Grad types. These can and should be used at the image taking stage. The mode used, colour or monochrome, is immaterial to the effect they have, which cannot be replicated afterwards. You can generate a graduated filter in image editing software, and it will do exactly what the real filter does. But this only works with the tonal values that exist in the image file being altered. If sky detail has not been captured for example, this won't restore it. And there is no way of replicating the effects of a polarizing filter digitally at all, nor that of the UV or ND.