I’ve been discussing filters for a while now, and I guess I need to go back one step and explain what a filter is and what it can do.  In photography, a filter is a camera accessory consisting of an optical element that can be inserted in the optical or light path. The filter can be a square or rectangle shape mounted in a holder accessory, or, more commonly, a glass or plastic disk with a metal or plastic ring frame, which can be screwed in front of the  lens.

Filters allow added control for the photographer of the images being produced. Sometimes they are used to make only subtle changes to images; other times the image would simply not be possible without them. The selection of the proper filter is actually far more important than any choice of lens or camera. The filter has a large and real effect on your image. Lens brand and sharpness does not.

The negative aspects of using filters, though often negligible, include the possibility of loss of image definition if using dirty or scratched filters, and increased exposure required by the reduction in light transmitted. The former is best avoided by careful use and maintenance of filters, while the latter is a matter of technique; it usually will not be a problem if planned out properly, but in some situations does make filter use impractical.  Worst case is that the light degradation is inevitable and is something I have to live with when using some filters.

Photographers classify filters according to their use. If you are aiming for cleaner, sharper pictures with less haze, photographers would pick up UV Filters which absorbs ultraviolet rays.  In fact, most photographers leave a UV filter on their camera lens at all times to protect the lens from dust, moisture, scratches, and breakage.

There is what are called Sky Filters - which reduce blush tones in outdoor shots.  We do not have to worry because this keeps skin tones natural and free of reflection from nearby objects.  It can also serve as a permanent lens protector.

Protector Filters are filters which protect your lenses from expensive front element damage which could be caused by dirt, knocks or scratches.

And you can choose also polarizer filters.     The circular polarizer and moose filters  are essential for outdoor photography; deepens intensity of blue skies; reduces or eliminates glare.  Circular Polarizing filters are used on auto focus cameras.  When using non-auto focus camera, use linear polarizer filters and linear focus.

The Neutral Density Filters are devised to reduce the amount of light without affecting the color.  This also eliminates overly bright, washed out images. This is also great for video.

One special kind of filter is the HOYA HMC Ultra Thin Filter.  This is a series or set of high end filters designed to avoid vignetting problems which occur with wide angle lenses.

Filters are commonly used in black and white photography to manipulate contrast. For example a yellow filter will enhance the contrast between clouds and sky by darkening the latter. Orange and red filters will have a stronger effect. A deep green filter will darken the sky too but will lighten green foliage and will make it stand out against the sky. Also see diffusion filters, which are used to reduce contrast.

Yes, filters are needed even if you use a digital camera and edit with Photoshop. Polarizers and grad filters need to be used before the lens and are not well, if at all, simulated electronically. Digital cameras are excused from most of the color conversion filters, since you dial these in as white balance settings. I still use a mild warming filter (81A) on all my digital cameras since I prefer the look I get, even with the WB adjusted warm.