What are filters? When should they be used?
First of all, let's define the word 'Filter' In strict photographic terms, a filter is something that allows certain types of light to pass through it and blocks (or reduces) other types of light.
But the term has come to mean any kind of attachment that modifies the light in some way, so Polarisers (which are correctly known as polarising screens and not filters), close-up attachments and other accessories, such as multi-image prisms, are also known as filters.
Which filters should you buy? That's a very popular question and my usual answer to it is "Filters are used to solve problems - which problem do you need to solve?" In other words, don't use a filter unless you need to. My reasons for advising against the use of filters without a good reason are that
Unless a filter has been made perfectly and is optically flat, there will be a loss of quality - what's the point of having an expensive lens and then degrading the quality of your photographs by fitting a filter to it?
Filters add 2 more surfaces to your lens - 2 more surfaces that can be marked with smears, fingerprints, scratches and dust, reducing the quality even more.
Filters need to be coated to reduce flare, and this reduces (slightly) the amount of light that passes through the lens.
Filters are (normally) fitted at the very front of a lens and because of this they 'attract' flare. Lenshoods are essential when filters are used, but there are many circumstances where a lenshood used without a filter will prevent flare but cannot prevent it when a filter is fitted.
Not all filters are fitted to the front of a lens. Filters are also used on light sources (studio lights or even on small flashguns) either to change the colour of the light, for special effects, or to polarise the light (in conjunction with a polariser fitted to the camera lens). And then there are 'slot in' filters, normally used with extreme wide angle lenses, where the filter slots in to a special slot at the rear of the lens.
Different types of filter material
The filters fitted to lights are known as lighting gels and are basically sheets of dyed plastic. They are not optically flat and so cannot be used on lenses.
Filters fitted to lenses are of 3 main types:
1. Glass filters are the most expensive and the most long-lasting
2. Optical Resin are a special form of plastic - they are cheaper than glass but they are more easily damaged by scratches. They are normally supplied as squares (available in 3 sizes) that fit into special filter holders. The best known makes are Lee and Cokin.
3. Gels are extremely delicate and are easily scratched. Their main use is for 'slot in' filter holders. Some 'gels' are now made from polyester, which is more durable and which, unlike gelatine, can be cleaned.
What effects do filters have?
Filters modify the characteristics of light in various ways, here are some examples:
Filters for black & white film
If you are using black and white film you can lighten or darken the tone of coloured subjects simply by using coloured filters. A coloured filter will lighten the tone of the same colour and darken the tone of a complementary colour. For example, a red filter would make red lipstick look very light and will make a blue sky look almost black. A green filter would make grass look very light and would make rosy cheeks look darker. If you are making a copy of an old black & white photo that has yellow stains, you would use a yellow filter to reduce or remove the stains. If you were to use a filter designed for black & white on colour film the result would be a picture that looked as if you were viewing it through a filter of that colour.
Filters for colour film
Colour compensating filters These are filters designed to correct the colour temperature of the light and are normally used with transparency film, where correct colour temperature is critical. With colour negative film, used to make prints, correct colour temperature is far less important because compensations are normally made automatically during the printing process.
Colour conversion and colour correction filters are used to 'convert' the colour of the light to suit the film - e.g. to allow film designed for tungsten studio lighting to be used in daylight.
Colour effect filters are used to produce special effects on colour film. Perhaps the best-known of these are the graduated filters, clear at the bottom and gradually changing to a colour, often used to colour skies. They do more than just colour the sky, because the coloured portion of the filters block some of the light and so prevent over-exposure of the sky. Filters of this type are normally supplied in rotating mounts, so that the filters can be positioned exactly as required.
Filters for both colour & black & white
Diffusion filters are used to provide a 'soft focus' effect, and are available in various designs and different strengths
Net filters have a broadly similar effect. The consist of fine mesh (nylon stocking) sandwiched between glass. The white net filters spread the highlights into the dark areas and the black net filters have the opposite effect.
Star filters consist of plain glass etched with lines, in a grid pattern. They create a star effect from any point source of light (very popular with TV game shows etc). To get sharply defined 'stars' you need to use the lens at very small apertures. Star filters are available in various designs, e.g. 2 star, 4 star, 8 star and are also useful for providing gentle overall diffusion, e.g. for portraits.
Neutral density filters are used to restrict the amount of light passing through them into the lens, they are used to allow wider apertures or slower shutter speeds to be used. They are available in strengths from 0.3 (1 stop) to 1.2 (4 stop)
Polarisers are used to reduce or remove reflections. Many people wrongly believe that they work only with shiny surfaces, but in fact they work with the sky too, this helps to remove internal reflections and so increases the blue saturation of the sky. They do not work at all with unpainted metal, and for maximum effect the lens axis should be between 30-30° to the reflecting surface. Polarisers are more effective with telephoto lenses than standard lenses, and when used with wideangle lenses the polarising effect can be very uneven. Polarising filters need to be rotated in their mounts until, looking through your SLR viewfinder, the reflections are at their minimum. If you are using a non-SLR camera just look through the filter with your eye, rotate it for maximum effect and then fit it to your camera in exactly the same relative position. The result may not be perfect but it will be fairly close.
Polarisers are available in two distinct types, circular and linear. This refers to their characteristics, not their shape! Circular polarisers are the same as linear, except that they also have a quarter-wave plate added, which increases the cost. The results produced by the two types are similar but not identical, and there is no point in paying the extra for a circular polariser unless you need one. You need to buy a circular polariser if your camera uses a semi-silvered mirror in either the metering or autofocus system.
Close Up lenses
Also known as supplementary lenses are simple meniscus lenses that screw in front of the normal camera lens to allow you to focus on closer subjects. They are available in 4 strengths, with No.1 being the weakest and No.4 the strongest. If necessary you can even combine them for greater effect, if they are combined the stronger lens should be fitted to the camera lens first. The physics behind these attachments is simple but the application of it is brilliant - they shorten the effective focal length of the lens to which they are fitted. This means that the lens is extended further from the body than it 'should' be and so it will focus closer!
There is always a quality loss when using these attachments, but they are ideal for occasional close up use.
Split field lenses A bit of an oddity really, these are really half of a close-up lens. If, for example, you want to photograph a tiny flower and have the mountain in the background in focus too, you can use one of these attachments, with the lens at the bottom and the empty bit at the top. The close-up lens will focus on the flower and the camera lens will focus on the distant mountain.
Filter factor
All filters (but not diffusers or close up lenses, which are not really filters at all) will allow some light to pass through and will block light of other wavelengths (colours). Because of this there is a 'filter factor', which is a measurement of the amount of light reduction. This varies between almost negligible (very slight colour correction filter) to very strong (deep red filter) and the actual amount is normally engraved on the filter mount or, with plastic (sorry, I mean 'optical resin' filters the filter factor is engraved or printed on the edge of the filter itself. '2X' or 'X2' means a factor of 2, which is 1 stop. If your camera has automatic metering then, in theory at least, the filter factor will be taken account of by the camera meter. I say in theory because metering systems are not equally sensitive to all colours, resulting in incorrect measurement. The safest answer is to take a meter reading without the filter, say it reads 1/125th at f11. Put the camera on manual mode and set it to 1/125th at f11. Now fit the filter and adjust either the shutter speed or aperture setting to take account of the filter factor - e.g. if the filter factor is x4 (2 stops) set the shutter speed to 1/30th or set the aperture to f5.6.
Stacking your filters
Most screw-in filters can be 'stacked', using more than 1 filter to combine effects. And the special filter holders used for square filter systems have slots that will take more than 1 filter. But bear in mind that each filter added will substantially reduce the quality of your photo.
Digital cameras
The effects of filters on digital cameras are almost identical to those on film cameras with transparency film fitted.
Filter care
Filters are difficult to clean, and some cannot be cleaned at all, so keep them spotlessly clean, don't touch the surface and keep them in filter wallets when not in use. With screw-in filters it is a good idea to lubricate the filter thread with graphite (pencil lead to you and me) to help prevent jamming.
Which brand?
Like most things in life, you get what you pay for. Top brands such as B&W are expensive but will produce better results. For occasional use, or if budgets are tight, you can save a lot of money by buying Cokin or Jessops own brand filters.
Which size?
It makes good sense to buy just 1 size of filters, to fit your largest lens, and to buy relatively cheap step-up rings to increase the filter mount size of your smaller lenses.
And finally
I hope this helps to explain the basics, but there are many specialist filters and attachments that I haven't mentioned here.
Garry Edwards is a commercial photographer who has written a series of detailed tutorials and articles, including Portrait & fashion, Still Life, Glamour, Wedding photography etc., plus an article for models and photographers who photograph them. Details can be found by clicking on this link
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