Filters are pieces of glass that attach to the front of your lens and serve a variety of purposes. Filters can be for protection or for achieving a certain effect or look in your photograph without post-processing.
There are two main systems for filters. Most manufacturers make screw-on filters that attach directly to the front of your lens. When purchasing this style of filter, you need to know the diameter of your lens (indicated with the Ø symbol). While you can attach more than one filter to your lens, you may run into the problem of the edges of your filter showing up in the corners of your image, which is less than desirable. Also, if you have multiple lenses with different diameters, you have to buy separate filters for each one.
The second system for filters is the Cokin series of filters and holders, where you attach a filter holder to the front of your lens and the filters themselves are larger than your lens and generally rectangular. With the Cokin system, you simply buy a different adjustable piece to attach the holder to lenses of different diameters, but you can use the same filters on all your lenses. The Cokin filters generally cost more up-front but are worth it if you need to use them with different lenses or want to use many filters simultaneously.
Personally, I currently use screw-in filters, and well-regarded brands include Hoya and B+W. If you are already further invested in photography, equipment, and lenses, you may want to consider the Cokin series of filters and holders. I am seriously considering making the switch myself when I can next justify the expense. If you are just starting out and only have a few lenses, then you can stick to screw-on filters, but if you know that you want the flexibility of the Cokin system, then it makes sense to invest in their filters from the start.
UV FiltersUV or ultraviolet filters are among the most common photography filters. Many new camera or new lenses come packaged with UV filters. A UV filter is a basic measure of protection for your camera and your lens. A quality UV filter has no negative impacts on your images or image quality, but the UV filter protects the surface of your lens from direct contact with dust, dirt, sand, fingerprints, and all other matter of debris. A UV filter can also protect your lens from breakage due to human error, especially if you drop or bump the front of your lens up against something. Recommended brands for UV filters include B+W and Hoya. Read more about the importance of UV filters in this post on Lens Accessories.
|B+W UV Filter|
Circular PolarizersA circular polarizer is a two-piece filter. The near piece screws into the front of your lens, and the far piece is free to rotate. Just like polarizing sunglasses or car windows, a circular polarizer changes which wavelengths of light are visible through the filter. This effect can serve to emphasize the blue in blue skies and cut down or eliminate reflections in water.
|Hoya Circular Polarizer|
Neutral Density FiltersA neutral density filter blocks some amount of light from your lens and sensor. These filters are called "neutral" because they should not have any impact on the color or color cast of your final image. (Inexpensive neutral density filters often leave a blue or purple color cast in your photograph.) Being able to cut down on the amount of light reaching your sensor is useful in many situations when you want a longer shutter speed.
|Fotga Variable Neutral Density Fader|
(This is one I don't recommend unless you just want a cheap filter to see what the effects look like.)
There are many different varieties of neutral density filters. The one pictured above is a variable neutral density filter. Like a circular polarizer, it has two connected elements that can be rotated. The version of the left shows the minimum amount of light being blocked, while the version of the right shows the maximum amount of light being blocked. This allows you to control how much light is being blocked simply by turning the filter. Tiffin makes a well-regarded variable neutral density filter, but there are many different styles and brands of variable neutral density filters or neutral density faders available.
|20-second exposure obtained using a Variable Neutral Density Fader in mid-afternoon|
|Zykkor Split ND Filter |
(Again, this is a very cheap version to try out the effects. You can already see the purple color cast to this filter.)
|Sunrise shot with the Zykkor Split ND Filter, which allowed significantly more color and light in the reflection to balance the brightness of the sky.|
|Another example shot with the Zykkor Split ND Filter|
For this photograph of the sunflowers, I used the split ND filter again to block out some of the light from the sky. Otherwise, the sky would have appeared much lighter, and the storm clouds would have lost some of their dark and brooding look.