(This article is quite long, and has varying complexity for amateurs to advanced users. The difficulty level will be indicated as "Basic" or "Advanced" in brackets at each topic if possible)

Filters are some of the cheaper photographic equipment that you can buy. Yet their usefulness is often understated. On the other hand, there are photographers who abuse them. Filters can help turn a ordinary photo into a great one, not a lousy photo into a good one. They can help spice up a image, but if you use the same filter effects for all your images you'd end up with a very boring portfolio. Yawn !
 

Types of filter systems

[Basic]

There are various types of filter systems. The most common type is the screw-on filters. These filters come in acrylic or glass pieces in a metal ring, which screws on to the front of your lenses. Advantage of the ring filters is that they are usually very high in quality and more durable. However, because they are specific in filter thread sizes, you cannot use a 55mm filter on your 67mm filter-size lens, since the filter is too small to fit on it. The solution is to get the filters in the filter-thread size of biggest diameter lens you have, then get step-down rings to use them on the smaller lenses. However, it may still be expensive if your biggest lens is 77mm in diameter, because big screw filters will cost a fortune.

The second in popularity is the square filter system. It consists of a metal ring which screws onto the front of your lens, and a filter holder which attaches to the ring in front of the ring. This filter holder has a few slots which you can slide the square filters into. The filters are usually made of acrylic. The most popular brands of such square filter systems are Cokin (cheap) and Lee Filters (expensive). The main advantage of such systems is that you only need to get one filter holder, then buy the filter rings of different sizes to fit your different lenses. The filters can all remain the same, and it will save you lots of money than buying large diameter screw-on filters. The other advantage is that you can position graduated filters exactly the way you like it (see graduated filters).

Other types of filter systems are less commonly used. They include the gelatin filters, which are flimsy pieces of coloured gelatin to be carefully mounted in front of the lens through a special holder. Because they're so thin, they supposedly provide higher quality. But they get easily scratched and must be frequently replaced. They're the domain of the professionals. Another type is the bayonet mount filters. They're like the ring filters, except that instead of screwing onto the lens, some specific lenses have bayonet mounting which the filters click-lock onto. Examples include Carl Zeiss lenses for Hasselblads and Schneider lenses for Rollei.
 
 

Types of filters

[Basic]

There are literally hundreds of types of filters, ranging from the most basic UV filter to the exotic special glass filters. We will talk about the most commonly used filters, and pass up on the gimmicky multiple image filters and exotic glass filters.
 

Skylight and UV(0)

[Basic/Intermediate]

When you first buy your lens, the salesman probably sold you a Skylight or UV(0) filter. These two filters are the most common filters around. They do not have any noticeable effects on the image. The main function of the two filters is to protect the lens against debris, dust, water droplets and other foreign matters. Skylight is a (very) little pinkish, and UV(0) is colourless and supposed to reduce a little haze. Personally, it does not matter to me whether it is a UV or skylight on the lens.

Does putting one as a protection degrade the lens ? This is a very hotly debated topic. Some photographers claim that any filter on the lens will degrade the optical results, since the lens designers did not compute a filter in the formulations when designing the lens. So the best way to ensure quality is not to have any filters at all. Now, this is a matter of personal opinion. My view is that the UV/Skylight filter WILL protect your lens. I have seen lens being saved because the filters were there. The filters were smashed instead of the lens. Even if the situation is not that drastic, the filters could still protect your lens against sand, dust or water contaminants. If you use your lens without a filter, and claim that this offers best optical quality, it will only be true for a few months. You will need to clean your lens very often because of the amount of dirt on it. And then your lens will be permanently damaged. By all means, use a high-quality multi-coated filter from Hoya or B+W, and you will be keeping your lens safe without much compromise in optics.
 

Polarizers (circular vs. linear)

[Basic/Intermediate]

The Polarizing filter is one of the most useful filters if you know how to use it. The light that we encounter everyday is unpolarized light. This means that the light rays scatter in all directions. The polarizing filter will filter this light and only allow light rays of a certain orientation to pass through. This is known as polarized light.

A polarizing filter will eliminate reflections from a non-metallic surface. Let's say you're taking a picture of a lake with Japanese Koi swimming in it. However, you can't really see the fishes because the water is reflecting the sky. So you fit a polarizing filter on your lens and rotate it until the reflections are eliminated, so that you can see the bottom of the lake ! The same principle can be used if you’re shooting through glass. As long as the reflection is not by a metallic surface, it can be removed through a polarizer.

(Note: a mirror reflection cannot be eliminated, because the coatings for the mirror is actually mercury, which is a metal)

Another use for polarizers would be to darken the skies. Polarizers can be used to deepen the blue in the sky. It can be used to bring out dramatic cloud formations, and to give the picture more punch. The elimination of reflection in the picture means colours are more saturated and has more impact. However, the blue skies trick will only work if the sky is already blue. It will not turn an overcast grey sky blue ! You need to have blue skies, and the sun should be around 45 degrees to the area of the sky you intend to deepen the blue. Light polarizes the most at 45 degrees. This is true whether you're shooting the sky or eliminating reflections. You can eliminate the most reflections if you're standing 45 degrees with respect to the reflecting surface. For the blue sky photography, stand so that your shoulder is directly pointing at the sun. This means that light can least be polarized at noon ! A lot of people heard about the wonders of polarizers without understanding how to use it, and are disappointed when the results do not match up to what they heard. You lose around 2 stops of light when you use a polarizer. So either use a faster film, or bring along a tripod.

There are two types of polarizers: the linear and circular polarizers.

Both linear and circular polarizers perform the same function, i.e. to polarize the light passing through. Linear polarizers are used for the manual focus cameras; circular polarizers are used for autofocus cameras. Autofocus cameras use a beam-splitting module to split the light coming into the camera into two paths - one to the viewfinder and the other to the meter. The light entering the meter is already partially polarized by the beam-splitting module. Using a linear polarizer will act as a second polarizer and affect the metering of the camera, leading to erroneous meter readings. Circular polarizers use a 1/4 wave-retarder foil to convert the incoming light so that it appears unpolarized to the meter, leading to accurate metering. If the salesman try to push you a linear polarizer for your AF camera, saying that the AF will not be affected, don't be fooled. The AF will probably still work, but the metering may be way off !!

Sunset and graduated filters
[Intermediate]
 

When the sky is much brighter than the ground, the contrast in the scene may be too high for your film to register. You'd end up with a picture with correctly exposed ground, but overexposed sky. Or a correctly exposed sky but underexposed ground. This is especially true if you use slide films.

So what should you do ? The answer is to lower the contrast of the scene so that both areas can fit into the latitude of the film. The way to do that (other than darkroom printing) is to use graduated filters. Graduated filters are filters with which are half-coloured, and half-clear. The coloured portion serves to darken the bright areas of the scene, to bring it in line with the darker areas. 

Graduated comes in a wide variety of colours, from orange to blue, to mauve and even tobacco colour. Try to avoid those tacky colours. Graduated filters are very useful for architecture and landscape photographers. Stick to the traditional colours like orange and blue. Professionals are known to avoid all colours and use a neutral density version. It lowers the brightness of the scene without adding any colour cast. 

Avoid the Cokin version of the ND graduated. It sucks big-time. It adds a violet/mauve cast to the pictures, depending on what film you use. The Singh-Ray ND filter is reputed to be very good (see web-site address below). The square filter system has the advantage here, because it allows you to position the graduated edge more precisely than the screw-on filters. Be careful where you position the split in the graduated filter. Try to merge it with some "natural" occurring lines like the horizon. Use a wide aperture to ensure that the graduation is not too sudden.

Trivial knowledge:
Graduated Neutral filters are the creation of Harrison & Harrison, as are the Strip filters used in the motion picture industry, first used in the motion picture - Dr. Zhivago
 
 

Colour temperature correction filters (and colour temperature chart)

[Intermediate]

81-series

80-series

Depending on the situation, the light may be warm or cold. Light temperature is measured in units of degrees Kelvin ("K"). Light which measures 5500 K is considered neutral. Any temperature below 5500 K is known warm light, and temperature higher than that is cool. Neutral light is 5500 K and is the light at 12 noon, with the sun directly overhead in a cloudless sky. Warm light is yellowish in colour, and can be seen when the sun is low on the horizon, such as in the evening or dawn. Cold light is bluish in colour, and is present after the sun sets or before sunrise. Click here for the colour temperature chart.
 

Sometimes, you'd like to portray a certain mood in your pictures, such as warmth in a family portrait or coolness in a winter scene. Yet the light may not be what you wished for. In those cases, colour correction filters can help to achieve the effect that you want. Warm-up filters give a warming effect (slightly hay-orange) to the image. Warm-up filter is denoted as the 81 series. It comes in different strength, from A to C. 81C is the strongest warm-up filter, and 81A gives the most subtle warmth. Likewise, the 80 series is the cool filters, being bluish in tone. It comes in 3 strengths, with 80C being the strongest effect and 80A the faintest tint of coolness. Color correction filter series were first introduced to photographers by Harrison & Harrison, Optical Engineers in 1938.
 

Diffusion filters

[Basic/Intermediate]

Diffusion is when you add softness to the image. Softness is different from blur. When the image is blur, it is either caused by camera shake or unsharp focus. When the image is diffused, it is sharp and correctly focused, but there is a softening effect due to filters. You can see that underneath the softness the image is sharp.

You may not want every picture to look razor-sharp. Sometimes, by diffusing a image, you enhance the image by hiding blemishes or giving it a certain romantic mood. Diffusion filters are often used in portraiture and landscape photography. Use them with care though. Overdoing the effect will lead to tacky pictures.

There are various special filters for diffusion. Some of them have concentric rings engraved. These give the image a out-of-focus effect, and the diffusion can be controlled by controlling the aperture size. The bigger the aperture, the more diffused the image. This design is a very old design, one of the earliest for diffusion filters. An example of such filter is the B+W Soft Focus filter.

The second type is the uneven surface design. It gives a overall softness to the image. The effect is quite pleasing if used well. It works great for landscapes and portraits which need more diffusion to hide blemishes. Examples of such filters are the Nikon Soft and the Hoya Diffuser.
 

The third design is the filter with lens-shaped droplets on the surface. This design gives a sharp image with subtle diffusion. The underlying image retains it's original sharpness, with a slight halo from the bright areas seeping to the darker areas. Great when you need subtle diffusion with a sharp image. This design is unique in the sense that the diffusion is not affected by the aperture opening. Such designs include the B+W SoftImage and the legendary (and bloody expensive) Carl Zeiss Softar series.
 
 

Special effects filters

[Basic]

Special effect filters are those prism filters which creates multiple images of one object, or creates streaks originating from the subject to give a sense of speed. In my opinion, these filters are incredibly tacky and should be avoided at all costs. I have never seen more than 10% of such shots being appealing to the eye. Most of the time, people jusy over-use them, making the images cliche and boring, and they hardly looks credible. Avoid… avoid…. avoid…
 

Star burst filters

[Basic]

This nearly fell into the special effects category, but it is rescued from the trash bin because it does have some merits. Star-effects filters create star-burst effects from points of light in the image. It is a clear piece of glass engraved with criss-crossing lines. The number of points in the star-burst is determined by the number of intersecting lines on the filter. Used with care, it can create a special mood to the image. Avoid those with too many points in the star-effect. A 4-point star-burst is fine, a 6-point star-burst is stretching the limits a little, and the 8-point star-burst is ridiculous looking. And avoid using it with images that has too many points of light. Using it on a Christmas tree or Christmas street light-up is too tacky. The entire image will be full of the damn stars. Unless you fancy your images looking like WonderWoman's underwear, forget about such pictures.

Prism filters 

Even worse than star-burst filters are the prism filters. Shaped as multi-faceted prismic surface, they create multiple images of the same object. Even before I was a beginner in photography, those images looked so tacky they looked like they came out of the 60s. I know some photographers who are absolutely enchanted by such filters and would love to hit me on the head with a tripod, but heck... I would still say this... prism filters suck big time.

Colour filters for B/W photography

[Basic/Intermediate]

How about letting B+W speak about the filters for black and white photography? (the following is an excerpt from the B+W web-site)

Colour filters affect black and white films by darkening the complementary colours of a filter and their closely related colours on the colour wheel while lightening its own colour and those similar to it. The result is a clear differentiation of grey tones. In addition, the degree to which a filter suppresses colours depends on its density. For example, a dark filter blocks a larger percentage than a lighter filter.

Lighter coloured filters are generally used for realistic, natural tonal reproductions. Intentional tonal changes are recommended when important portions of a subject consist of two different colours with the same brightness intensity.

Improved tonal separation, as in still-life photography (subject and background), does not always require a strong filter. One should consider how the various colours are influenced. If a tonal separation can be achieved with a lighter coloured filter, then it should be preferred over a darker one due to it's lower filter factor. Black and white filters give the best effect when the colour tendency of the light is considered in relation to the degree of filtering. Where a light yellow filter may be best in morning, a darker filter would be needed to shoot the same scene in the afternoon in order to compensate for the stronger blue tones in the light. Also, the modern panchromatic black and white films reproduce green, yellow-green, yellow, and bright orange in somewhat darker gray tones than they appear to the human eye. Slight counter filtering is recommended when the subject has important details in one of these colours. Portrait and group pictures which show face detail should be only lightly filtered because a clear change in the reproduction of eye colour is almost unavoidable. Blue filters tend to enhance even slight skin blemishes.
 
 
 

  Close-up filters

Close-up filters act like expensive magnifying glass. They magnify the size of the image so that it appears bigger on the negative. They come in many degrees/strengths, indicated by the diopters. The higher the magnification, the higher the number. Typically, it comes in a series, from +1 to +5. High quality close-up filters consists of 2 or more elements. They offer superior optics over the single element type. You can stack one over another for a more powerful close-up, with the higher-diopter filter closer to the lens. Do not, however, use more than two at any one time, as the image quality will definitely suffer. Close-up filters are a cheap way of getting into close-up photography. They are worth looking at if you do not need such high-power magnification or do close-ups often. They are not convenient to use, however, since the focusing distance is fixed and you can't really control the framing precisely. A highly-regarded close-up filter series is from Nikon. They have around 4 such filters, named 5T, 6T etc. Also check out my other article on the Nikon Micro 105mm.
 
 

Which brand of filters ?

Some people would rather die than to be spotted with a Hoya filter on their thousand dollars lenses.

The few brands of filters on the market is quite distinct. I would say that no one brand is completely condemned, or offers everything. I would pick the filters from different manufacturers, depending on the filter that I want. There is no brand loyalty involved here. Some brands offer certain outstanding products. Unless you're a snob and go after brands instead of the image quality, you should probably own filters from different manufacturers.

Hoya

Hoya is known for its affordable prices and above-average quality. They offer value-priced filters in most sizes. The cheaper range is uncoated, but of surprisingly good quality. If you're not particular about filter quality or you're on a tight budget, Hoya filter is a very good buy. And do check out the Super HMC (Highly Multi-Coated) filters. They are considerably more expensive than the regular range, but offers superb multi-coating that increases light-transmission up to 99.5%. I have the Super HMC UV(0) filters on almost all my lenses. They are not exactly cheap, but one of the best around for a reasonable price. 

Nikon

Americans Nikon users seem to love this brand. I do not have a lot of experience with it, but I have heard that it is very good. And expensive it is too. The multi-coating is said to be very good. I have tried the famous Nikon Soft I filter, and I must say that it indeed matches up to its reputation. Try the Nikon Soft if you want a soft diffusion throughout your image. As for the rest of the Nikon series, I'd reserve my comments.

Canon

For some strange reasons or another, nobody ever liked Canon filters. In fact, I've never seen anyone using one! The exception is probably their excellent multi-element close-up filter. 

Marumi

You remember I said something about no brand being entirely condemned ? Well, I eat my words. This brand is atrocious ! The catalogue is really crappy, with 1970s looking photos and cheap paper printing. The packing is like junk, and with dusty looking stickers slapped over the yellowing plastic boxes. The labels have stupid and uninformative wordings. I haven't used the filters themselves yet, but I have the feeling that I won't want to.

B+W

German manufacturer of filters, under the wings of legendary lens-maker Schneider. This company turns out high-quality German filters at a reasonable price (not cheap, just reasonable). Do not take a look at their catalogue, because the photos are incredibly old-fashioned (1970s) and makes the filters look bad. Very high-quality construction, with anodised brass rings and Schott glass. Their claim to fame is the ultra-parallel glass used in their filters, to reduce distortion. Worth taking a look if you have excess cash. But is it worth the premium over the top-end Hoya filters? Personally, I think not.

Carl Zeiss

Another legend from Germany. Carl Zeiss makes the incredible optics for Hasselblads. Most of their filters can be seen in the Hasselblad's product catalogue. If you want to buy all your filters from Carl Zeiss, you'd better start digging a hole in your backyard and hope that you hit a oil-well, because that's how expensive they are. I'd highly recommend the Carl Zeiss Softar I & II. They are incredible diffusion filters (some say the world's best diffusion filters), but are incredibly expensive. The other filters from Carl Zeiss are probably excellent, but they will not be good value-for-money.
 
 

Okay, that's it… I'm dead beat. This is the end of my article !!!!

Happy shooting !!!

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