A GUIDE TO FILTERS
Lens filters date from the days of film, when personal computers and digital editing software hadn't even been conceived of. Back then, things had to done right in camera. If you wanted a warmer photo, you added a yellow filter. If you wanted more punch to your black and white photographs, a red filter was placed over the front. They pretty much came in any colour imaginable. Today photographers tend not to bother with such things, primarily because the vast majority are using DSLRs and any warming, cooling or colouration can be done at the click of a button in post processing. In short, they're a waste of money.
However, even in the digital age we still can make use of some of these basic technologies because either their effects cannot be recreated in post, or they simply save us a lot of time by getting it right in camera. The filters I'm referring to are circular polarizers (CPL), neutral density filters (ND), ultraviolet filters (UV) and star filters. They come in two formats, either screw in to a particular thread size, or square filters systems like Cokin P, Lee, Hitech and Kood, that drop into a mount that attaches to the lens.
I will start with the latter first as they require the least explanation. In essence, UV filters are fitted to the front of a lens to protect the front element from scratches and accidental knocks. They used to be employed to filter out UV light below certain wavelengths (commonly 370nm and 390nm) but Digital SLR cameras are highly insensitive to ultraviolet light and so fitting one serves little purpose other than scratch protection in the grand scheme of things.
The downside to fitting them to a lens is that they can lower the image quality, particularly if a cheap brand is used. They can introduce flare as well. Personally I don't bother with them on my lenses. Why spend hundreds, even thousands of pounds on a lens because of its sharpness and flare resistance in the form of nano crystal coatings, only to have that diminished by some crappy glass stuck to the front? It makes no sense to me!
I do, however, have a UV filter fitted to all my lenses. "Hypocrite!" I hear you scream. Nope, I'm not, as none of the filters actually have the UV glass in it. "Why?" you might well ask. Well, it serves to keep the lens cap further away from the front element. If any grit was to get caught on the reverse of it, I wouldn't damage the coating on the lens. It's a neat trick and one I highly recommend. Either that or purchase the generic petal lens hoods, which screw into the filter thread and keep the cap even further away from the front element.
It would be remiss of me should I fail to mention that a scratch on your front element in all likelihood will do bugger all to impact image quality. The element you should be most concerned with is the rear one. Any dust, scratches or fungus upon that will be picked up by the sensor due to the proximity. On the front, it won't make any discernible difference. Keeping your front element scratch-free is more about resale value than anything else. Although I too like my babies to have pristine glass (yes I love my lenses), if I see a decent optic on sale with a mark on the front element, it doesn't put me off - provided I can get it for a decent price. It's entirely up to you, though whether you use UV filters or not and is more about your own peace of mind than anything else.
Next up are Circular Polarizers. Don't confuse these with the older linear polarizers. Those are for old film cameras and won't work properly on today's DSLRs - that's the reason why it was so cheap and no one else on eBay had bid! CPLs are quite ingenious creations and allow you to capture things that you otherwise wouldn't be able to do - at least not without hours of messing about in editing software or some compositing. CPL filters block out light in a particular direction of polarization. The effect of this is that reflections are removed from things like windows, car paintwork and water. Now you know how they captured all the rocks and pebbles under the water in the those mountain lake photos! It won't work on metallic objects, so be aware. It doesn't end there though. A CPL will also saturate colours more, making your skies bluer and everything else richer by removing reflections. They're very handy things to own and have the ability to transform the look of a photo.
Last up are the neutral density filters. They're essentially light reducers and come in various densities, usually ranging from ND2 to ND10. The purpose of ND filters is to enable you to use longer shutter speeds due to the reduced light levels. This is what gives you the milky water photos. It also is useful for architectural photography by eliminating people from the scene. As long as nobody remains static in the image for an extended period, they will be ghosted into invisibility as they walk through the frame. The only exception to this is if someone is carrying a light source bright enough to be recorded onto the sensor. An example of this is street photographs where the cars are invisible but the head and tail lights appear as white and red strokes across the photograph.
The other purpose of the ND filter is to enable you to use a wider aperture than would be possible without one. Imagine this scenario. You want to do a portrait of someone but just isolate the eyes and want to opt to use the lens wide open at f/1.4 on your 85mm. However, the scene was so bright that even at 1/8000 sec, ISO 100 your images were over exposed. Without the filter your only other option to get correct exposure would be to use a smaller aperture, like f/2.8, f/4, etc.
You can also get variable ND filters that you rotate to control the amount of light that is cut out. Good quality ones of these can be very expensive but, they are very useful - particularly for videography where the shutter speed cannot be altered and changing aperture will affect the depth of field and change the look of the scene.
ND filters come in another variety too. Graduated ND filters (sometimes called ND Grads or GNDs) are ND filters that fade from dark to transparent, either softly or abruptly. They come in different strengths like the standard ND filters to control the amount of ambient light. The purpose of ND Grads is primarily for landscape or seascape photographs. By placing the darkened part of the filter against the bright sky or evening sunset, a photographer can balance the exposure of the two in camera. The only away around this without one would be to bracket the photograph (taking two or more photos at different exposures - one for the foreground, one for the sky) and combine them later in post production. This can be time consuming and fiddly. It's much better to get it right in camera, than to spend hours messing about in software.
Finally, star filters are simply an effect filter that creates radiating shafts from a light source. You can achieve this effect by using very small apertures but, sometimes a small aperture is not desired. The star filter will produce these shafts by way of the etches on the glass filter. They generally range from 4 stars up to 16 and are more an aesthetic choice than serving any other real purpose. You could recreate these effects in post production but, again, it's all time spent staring at the screen, which could have been created at the time of capture. If your Photoshopping skills aren't up to much, then use one of these filters instead.
That's pretty much it as far as filters go. They vary in price greatly from cheap and cheerful brands up to the likes of Lee filters which are incredibly expensive but, generally are the choice of professionals due to quality control standards and no colour cast. Those filters mentioned above are really the only ones you'll ever need if you shoot a DSLR. They can prove invaluable and be huge time savers too. They afford greater creative control and so should not be absent from your camera bag.