What is a camera filter?

A camera or lens filter is a piece or multiple pieces of glass being attached to the front of the lens. In the case of lenses of Single Lens Reflex(SLR) cameras, the lens are threaded in front allowing filters of the same size to be screwed on. For some smaller consumer digital cameras, manufacturers may design special adaptors to allow filters to be attached securely to the camera. These filters tend to much smaller due to the small size of the lens on digital cameras.

The main purpose of using a filter is to change the light which enters the lens in a way desired by the photographer. The filter, being in the optical path, may block away certain colours or bend the light in a way to create special effects. A secondary purpose of using a filter for many photographers is to protect the front element of the lens, which is the first piece of glass or plastic in the lens. A filter offers protection against stains, fingerprints and dust. Good filters often have a special layer of coating which allows dust and prints to be easily wiped off. In addition, a filter adds a layer of protection against scratches or knocks. Since good lenses can be very expensive and in many cases more expensive than the camera itself, it makes economic sense to protect the lens as it is much cheaper to replace the filter rather than repair the lens.

However there are some disadvantages of using a filter. Firstly, the optical quality of a filter may not be as good as the lens since a filter is probably much cheaper. The filter may degrade the image quality of a photograph. Secondly, the filter not only blocks out unwanted light, it will also block off desired light to a certain degree. For example many polarizing filters take away 2 to 3 full stops of light. In many cases, the better the optical quality of the filter, the more amount of desired light it lets through. Thirdly, some filter systems prevents you from using the lens hood due to the size or shape of the attachment. An example is the Cokin Creative System whereby you attached a square filter holder in front of the lens. The attachment is usually much larger than the lens hood. Another example is when using a polarizing filter, you will need to rotate the filter and having a lens hood restricts access to the polarizing filter.