What they are
Filters are coloured (usually) pieces of glass, resin or gel that are placed in front of (or, in some cases, behind or inside) a lens to make adjustments to the image. To most people, a filter is fitted in front of the lens, but some lenses for large format cameras have the filters fitted behind them. The front element of such lenses can have a large area to cover and hence be expensive, as larger filters cost more, whilst the rear element is smaller. Mirror lenses (where they permit filters) normally have a slot in the lens where a filter is inserted, and the optical design assumes that a filter will in fact be used, so they are supplied with a clear filter in place to preserve the optical properties of the lens.
It seems almost too obvious to be worth mentioning, but when I first used an SLR camera, I was momentarily taken aback to find that the world had taken on a yellow tint due to the filter on the lens – not a problem when you are not viewing through the taking lens. Using a strongly coloured filter on such a camera will give you a strongly coloured view of the world as well as a dimmer image.
When people talk about “slim filters” they are usually referring to the thickness of the filter mount but the thickness of the material used to make the filter matters as well. All filters will degrade the image to some extent, and the thicker the filter the greater the effect. Here’s why.
The red lines represent incoming rays of light. The one that strikes the filter vertically passes straight through without being refracted, but the oblique rays are refracted as they enter the filter. On exit, they are refracted again, by the same amount but in the opposite direction, and so emerge parallel to their original direction. The path through the filter and out is shown by the blue lines. The green lines show where the rays would have gone if they had not been refracted, and you can see that the filter has changed the rays’ apparent point of origin. The thicker the filter, the more the rays will shift from their original direction.
For this reason, the thinner the filter, the better. A thicker filter material will also increase the overall thickness of the filter and mount combination, and this can cause vignetting with extreme wide angle lenses.
Filters can be uncoated, single coated or multicoated. As with lenses, coating reduces reflections and improves the performance, so go for multicoated ones. As a recap, uncoated filters will lose 5% of the light on entry, and 5% (of the 95% that gets through) on exit; multicoating brings the percentage down to 1% on entry and 1% (of the 99% that gets through) on exit.
There is a caveat on the above; the actual amount of light reflected as flare depends on the difference between the refractive indices of the materials on each side of the surface where flare occurs. Hence, some materials have lower amounts of flare, whether coated or uncoated.
Glass filters are usually considered to be the best, and are often made by makers of optical glass. The problems that can occur lie in ensuring consistency of colour from batch to batch if the glass is coloured; or the physical arrangement if a sheet of thin coloured material is sandwiched between two sheets of glass. Glass filters really do need to be multicoated to cut reflections, due to the refractive index of the glass. Coloured glass is made by adding pigments to the glass, and these are not available in as wide a range of colours as dyes.
Resin filters have less need of coating, are easier to make to a consistent colour, and can be made in a wider variety of colours than glass filters. They might in theory be more prone to scratching than glass filters, but with a modicum of care they should remain problem free. The major makers of resin filters can supply a vast range of colours to suit all needs.
Gel filters are very thin sheets of gelatine; easily carried as they are small and light, and, like resin filters, available in a vast range of colours. They were the first choice of photographers using reversal films where exact colour balance was important, simply because of the range of colour correction filters made in gels. This also led to their use in correcting the colour balance of slides after exposure, by sandwiching them into a mount with an “off colour” slide to correct the colour temperature.
Being thin, they have a negligible effect on the performance of lenses. They also have the advantage of being the cheapest filters, but they are easily damaged.
Because of their lower refractive index, resin and gel filters are less prone to flare than glass ones, and therefore have less need of coating.
As well as different materials, filters come in two different types according to their method of attachment to the lens. Lenses normally have a thread at the front into which attachments can be screwed, and the simplest way to attach a filter is to screw it into this thread. The alternative is to use “system” filters which are square or rectangular in shape and fit into a special holder that is itself screwed to the front of the lens. Lenses normally have the size of the thread engraved on the front of the lens
Screw in filters
These are the smallest and in some ways the easiest to use. The filter is circular in shape, and fitted to a mount that directly screws into the lens. It adds the least amount of extra size and weight to the lens, and many people have a screw in UV or skylight filter permanently fitted to their lenses for protection. I’ll come to this use of filters shortly, and give the pros and cons of this practice.
The disadvantages of screw in filters are the time taken to attach a filter (or remove it and change it for another) and the possible need to buy the same filter in a number of different sizes if you have lenses that have different sized filter threads. My normal lenses have 49mm, 55mm or 77mm thread sizes. To get round the problem of needing to buy the same filter in a number of different sizes, stepping rings can be used.
Before extreme wide-angle lenses became common, the alternative to having several sets of filters in different sizes was to buy filters in the largest size you needed, and use stepping rings to convert. (The other alternative was to select your lenses based on the filter size – the major manufacturers usually had a standard filter size and made the commonly used focal lengths available in that, although often in a slower version.) Stepping rings are simply rings where one side screws into the lens as a filter does, and the other side has a screw thread to screw a filter in to. You can therefore attach a filter in one size to a lens in another. You can go both ways – smaller filter to larger lens as well as larger filter to smaller lens – but using a too small filter is almost certain to cause vignetting. Going the other way may cause vignetting, since the stepping ring increases the overall thickness. But it isn’t certain, and for the more normal focal lengths it usually works well.
The photo above shows a stepping ring (in this case one that allows 49mm filters to be attached to a lens with a 46mm screw); the comparative increase in width of the combination can be seen.
Filter mounting ring
Screw in filters are usually made with aluminium mounting rings; these can be prone to sticking and create difficulties in unscrewing (although to be fair this is not a major problem for most people). Filter wrenches are sold to enable removal in hard cases. Brass mounts are less likely to stick, and the more expensive filters may come in brass mounts.
If you use this type of filter, and if all your lenses take the same size (or at most two sizes) of filters, one way of carrying them around that saves space is to use specially made end caps and carry the filters in a pocket as a stack.
System filters use the stepping ring approach. The filters themselves are usually square or rectangular, and oversized. You screw a filter holder onto the lens, and slot the filter into that. You need a holder for each thread size, but (provided that the filters are large enough) the same filters can be used with almost every lens. Usually, you can fit more than one filter into a holder, if necessary. The advantages of a filter system are that you only need to buy a single instance of each filter, and they are fast to slot in and out of the mount.
Graduated filters have been made in circular mounts, with the division usually about half way up the filter. I’ll cover graduated filters later in this chapter – they are filters which are not uniform in colour or shade. In use, they rely on the area that needs the graduated part occupying about half the frame. The soft transition gives the leeway required to make them work, but using a square filter in a holder allows you to place the transition exactly where you want it.
Some filter systems have different sized filters. The smaller ones are cheaper, but won’t work with all lenses, so you need to watch out for this. Additionally, the holders occupy more space than a conventional mount, so special ones are made for extreme wide angle lenses.
It is best to avoid stacking filters if you can; every filter adds to the light loss and flare potential indicated above. Plus adding a second filter adds two more surfaces that can have dust on (increasing flare again). If using an extreme wide angle lens, the physical width of the filter stack may cause vignetting in the image corners.
This is only really a problem with extreme wide angle lenses; a thick mount can cause vignetting for the same reason as stacking filters can – the mount projects far enough forward to intrude into the picture area. Makers of screw in filters usually have a range of thin mount filters for use with extreme wide angle lenses, and the makers of system filters make special thin filter mounts which only take one filter.
Filters by definition “filter” the light – they remove certain parts from the incoming beam. Therefore, less light gets through, and an increase in exposure is called for. The amount is given by the “filter factor” which is normally engraved on the filter mount (when the filters are mounted) or given in a specification sheet. In theory, cameras with through the lens metering should allow for the filter factor automatically. In practice, the metering cells do vary in sensitivity according to the colour, and usually give incorrect exposure when used with orange and red filters.
When we come to look at individual filters, we’ll find that they are given descriptive names. Those like “skylight” are reasonably unambiguous, but when it comes to describing a filter as “yellow”, “medium yellow” or “deep yellow” it’s clear that the names are anything but precise.
“Wratten numbers” are often quoted for filters, and have the advantage of greater precision in meaning. They are named after a pioneer in the manufacture of coloured filters, Frederick Wratten, whose company was bought by Eastman Kodak in 1912. “Wratten numbers” are therefore often associated with Kodak, although other makers also use them. The names Wratten used are fairly arbitrary, but they do give a consistency which terms like “light yellow” cannot. I’ll note the Wratten designations under each filter type.
General purpose filters
Different filters are used for black and white photography to the ones used for colour. Also, the filters used vary depending on whether they are being used with negative film, slide film, or digital. In this chapter we’ll look at filters generally and the ones that are used with all media, and ignore the specialised types.
UV and skylight (no exposure increase)
Although different, I’ve grouped these together because in practice both are usually used not for their primary purposes, but as a means of lens protection.
UV (ultra violet) filters remove ultra-violet light, which film records as blue. Scattering of light by the atmosphere results in a blue haze in the distance; this haze can be reduced slightly by a UV filter, although UV filters normally only have this effect at higher altitudes than those found in the UK.
Unlike the colour contrast filters used in black and white, UV filters are named from the light that they remove, rather than the light that they pass. This makes them the solitary exception to the rule that filters are named according to the light that they pass; a red filter, for example, passes red and blocks everything else. As we’ll see when we come to infra red and ultra violet photography, true UV filters exist – and are also called UV filters.
Skylight filters (Wratten 1A) do the same job, but with an additional slight “warm up” of the light (skylight is bluish, and hence colder than direct sunlight).
Having disposed of the actual use of these as filters, let’s look at the pros and cons of using them as permanently mounted lens protectors. There is no clear-cut answer that everyone accepts, so all I can do is give the arguments on both sides.
In favour of using them:
- If you brush up against something, the filter rather than the expensive lens will be scratched
- If the environment is harsh or dirty, there’s less risk from cleaning a filter than cleaning a lens. Some environments are sufficiently harsh that deposits on the filter are effectively not removable without damage – a clear cut case where use of a filter is necessary.
Against their use
- They add two more surfaces that need to be kept clean, and will add to light scatter and flare
- A broken filter can go on to scratch the lens anyway
- If you use a lens cap when the camera is not in use the lens is protected
- A lens hood by projecting forwards from the lens offers protection against brushing up against something
When the subject is debated, one side will cite people they have known who have suffered damage to a lens because there was no filter protecting the front surface, and the other will counter with people who have suffered a damaged lens precisely because a filter was broken and inflicted the damage itself. The glass used in filters is often more easily shattered than the glass used in lenses, so this argument does carry more weight than might appear at first sight. You pay your money and take your choice. In any event, you shouldn’t needlessly stack filters, so if you do have one permanently mounted, you should remove it before fitting a different filter. This does add to the time taken
Polariser (2 stops exposure increase)
Light can be polarised when it’s reflected from almost anything except unlacquered metal. If you have Polaroid sunglasses, you can see the effect of the filtering on reflections from a water surface. If you move your head round to get the correct angle, the reflections can be cut dramatically. The same applies to reflections from a shop window or a display case.
Using a polarising filter can reduce the amount of reflections, but the filter must line up with the plane of polarisation of the light. Since this is variable, polarising filters use a special mount that lets you rotate them within it. A white dot (usually) lets you see where the filter has been turned to, which might seem redundant until you consider that not all cameras let you view through the taking lens, and you might have to hold the filter to your eye to make the adjustment before fitting it to the camera. One point to be aware of is that some polarising filters come with mounts that are difficult if not impossible to rotate when a lens hood is attached, because they require you to access the front of the filter.
The light from a blue sky at 90° to the sun is polarised, and a polarising filter will darken the sky in this direction dramatically. The polarisation is strongest at 90°, but is still polarised over an angle of 15-30° about this position. This means that if you use a polarising filter to darken the sky, the difference in the effect will be noticeable if you use a wide angle lens.
The other effect of a polarising filter is to increase the saturation of colours. When light is reflected from a surface, there will be a component that is specualrly reflected (in other words, the colour of the light will be unchanged). When the surface is coloured, as no colour has been removed from this component it will act to reduce the colour saturation. Removing this small amount of polarised light will increase the colour saturation. This can also be seen in the photographs above, where the leaves are greener in the version where the polarising filter was used.
The amount of light cut by a polariser will depend on how much the incoming light was polarised; the usual figure given (if you need to allow for it) is two stops. Going on from there, if you use two polarising filters, and use them to stop light in different planes, you can significantly reduce the light. If the filters were 100% efficient, you could stop all light passing by having the planes opposed. There is a caveat here; the transmission of polarisers does vary with the wavelength of the light, and therefore using crossed polarisers can multiply the inevitable small colour cast.
The more expensive and less efficient circular polarising filters are required by most modern cameras that have auto focus or through the lens metering. “Circular polarising filters” refers to how they polarise the light, and not to their shape. Circular polarising filters can be square.
There are two ways in which a polarising filter can be made. Some makers use resin entirely, but most sandwich a sheet of polarising material between sheets of glass. This method does mean that there is a very small chance of the sealing failing. The addition of the extra layers doesn’t greatly affect the light scatter within the filter, as the amount of scatter depends on the difference of the refractive indices of the media, and the difference between glass and polariser is far smaller than that of glass and air.
Neutral density (various exposure increases)
Sometimes you may want to use an exposure time that is too long even when using the smallest aperture on your lens and slowest film speed or ISO that you have available. Neutral density filters are designed for such an eventuality. They reduce the light without having any other effect (in theory; some do in fact have a slight colour cast, and if you use more than one at a time to reduce the light even further, you may see a colour cast). See also above, on stacking filters.
They are designated by the amount of light that they hold back, either in terms of stops (hence the numbers 2, 4, 8 etc) or the density. Density is measured in logarithmic units, and the values corresponding to 2, 4 and 8 are 0.3, 0.6 and 0.9. When measuring in stops, you multiply the factors to get the factor for the combination: using a 2 and a 4 is equivalent in terms of light reduction to using a single (2 x 4 =) 8 times filter. Densities, being logarithmic, add, so using a 0.3 and 0.6 together is equivalent to a 0.9.
As mentioned above, two polarising filters can be used together as a variable strength neutral density filter, although clearly no filter factor is available in this case, and metering must be done through the combination. This is possible with both hand held light meters and built in TTL ones.
Long exposure times could produce effects that our eyes do not normally see. We can empty a street by a long exposure, or make the sea flat by averaging out the waves. Another possibility is the effect on clouds, which move at a far greater speed than most of us realise.
These long shutter speeds are difficult, if not impossible, to achieve without the use of a very dark neutral density filter – ND filters which allow a 10 stop increase in exposure (which is about a 1,000 times increase in exposure time).
Graduated neutral density filters (no exposure increase)
Standard neutral density filters are uniform in tone, and cut back the light equally in all parts of the frame. Graduated neutral density filters are uneven, cutting more light at one side than the other. The idea is that if a sky is too bright to allow you to give enough exposure to the foreground without burning out the sky (meaning giving so much exposure that you lose detail and “burn out” the highlights) you can use a graduated ND filter with the darker half covering the sky, and the lighter over the foreground.
They come in two varieties: hard and soft, which refers to the transition between the dark and clear parts. A hard filter has a very clear line of demarcation between the two regions, and is the best choice if you can position the “join” along the horizon. A soft filter has a gentle transition between dark and clear halves, and is the more useful of the two for general situations. We saw a graduated filter (although not a neutral density one) when we looked at filter systems. In that instance, the filter was a graduated tobacco filter, which was as popular in the 1970s as the HDR effects are today.
The problem with graduated ND filters is that they can have a noticeable effect on foreground objects inconsiderate enough to protrude above the horizon, such as trees, hills and mountains, and lighthouses – not to mention telegraph poles and pylons. In an ideal situation, you wouldn’t really want to use them, and it may be worthwhile considering if there are any alternatives.
Graduated filters are intimately connected with the dynamic range of your recording medium, in that some media need them more than others. Users of negative films (colour or black and white) rarely if ever use them; digital camera and colour slide film users normally employ them. With a digital camera, it is worth exploring the possibility of being able to use RAW processing to increase the dynamic range of the image beyond that which would be recorded in a jpg file. When we looked at characteristic curves, dynamic ranges and the specifics of digital cameras we saw that one of the effects of the processing which creates a jpg was a reduction in the dynamic range of the image, which is precisely the problem that graduated filters are used to overcome. In some instances, and for some subjects, it might be possible to take more than one image at different exposures and combine the resulting images later in software to capture detail in both highlights and shadows. If you do this, don’t forget that leaves, branches and clouds can move in a landscape, and be prepared to substitute rather than blend the results if so.
I gave the exposure increase as “none” on the grounds that you have set the exposure for the foreground, and are using the filter to cut the light from the sky. Clearly, you don’t want to increase the exposure to compensate for the light you’re trying to hold back. Through the lens meters may however give a different reading if used with the filter in place.
As a final point, there is no reason why the foreground rather than the background or sky shouldn’t be held back. Some subjects have a bright foreground that may well need holding back a little. In the photo below, the upper half of the frame is reasonably exposed, but many of the brightly lit flowers in the foreground are burned out.
Centre spot filters
A variant on the graduated neutral density filter is the centre spot filter. When we looked at lenses, we saw that wide angle lenses could have problems caused by light fall off as the rays entered the lens progressively further from the lens axis. Some lenses made for large format cameras (where “off axis” can be a much greater distance than it is with smaller formats) have centre spot ND filters made for them, specifically to cut the light on axis and even up the light at centre and corners.
Using filters off camera
We’ve only looked at filters attached to the camera lens; but you can also use filters over light sources, to balance the light or change its colour.
One unusual effect depends on remembering that coloured lights are additive; mix red, green and blue and you get white. If you put a coloured filter over the lens, and use a filter of complementary colour on the light source, you get a subject that appears to be illuminated with white light where the filtered light source provides the illumination, and has the colour of the on camera filter elsewhere. This technique is usually used with flash outdoors to provide unusual effects; but the same can now be done with image editing software.
Other lens attachments
To many people, anything that screws into the filter thread of a lens is a filter; but this isn’t the case. A filter indicates by its name what it does – it filters light and removes certain components. There are a number of attachments that can be screwed into the filter thread other than filters.
Close up lenses
These are perhaps the most common, and we looked at them when we looked at close up photography in the chapter on using lenses.
Diffusers and soft focus attachments
Also in the chapter on using lenses, we glanced at portrait lenses. These are lenses which are specially designed to give a variable soft focus effect; but soft focus attachments are available which although not as flexible, are certainly less expensive. They work by diffusing the light, and a similar (and variable) effect can be obtained by smearing Vaseline on a UV (or skylight) filter – in which case, viewing through the lens at the taking aperture is advisable. A nylon stocking can also be used for the same effect.
Not quite the same, but similar in principle, are fog filters, designed to simulate fog in outdoor scenes. This is borderline as a “mainstream” filter, and takes us into the realm of special effects.
Special effects devices
There was a time (in the 1970s, mainly) when lens attachments were very much in vogue for “creative” effects. And the results were in many cases not worth the effort and expense. Fashions change; and many of the effects that were once the preserve of the lens attachment are now handled in Photoshop. One such that springs particularly to mind is the “multi-image” lens that gives multiple images (obviously) of the subject in the frame.
Two that may still be of interest are the starburst and diffraction attachments. The starburst (sometimes called a cross screen) gives four “spikes” to a point light source, giving a star effect. Two used together will give eight spikes. A diffraction attachment works rather like a prism, in that it splits the light into its component colours, giving a rainbow effect to shafts of light in an image.
My recommendations are for older books.
Filter Practice by Hans Clauss and Heinz Meusel would be my first choice for filters used on the camera lens; The Photoguide to Filters by Clyde Reynolds covers filters used in other places as well; and Filters and Lens Attachments (Kodak, no author given) covers the “special effects” attachments as well as filters.