This Neutral Density filter reduces intensity of all wavelengths or colors of light equally, giving no changes in hue or color rendition. The purpose of standard photographic neutral density filters is to allow the photographer greater flexibility to change the aperture, exposure time and/or blur of subject in different situations and atmospheric conditions. Instead of reducing the aperture to limit light, the photographer can add a ND filter to limit light, and can then set the shutter speed according to the particular motion desired and the aperture set as needed.Neutral Density filters have a four main uses: 1) To enable slow shutter speeds to record movement in subjects such as waterfalls, clouds, and cars. 2) To decrease depth of field by allowing wider apertures to be used. This helps separate subjects from their background. 3) To decrease the effective ISO of high speed film and allow it to be used outdoors in bright situations. 4)To allow cine and video cameras to film subjects such as snow, sand or other bright scenes that would normally cause over-exposure. Some examples of when to use an ND filter include: 1) Blurring water motion (e.g. waterfalls, rivers, oceans). 2) Reducing depth of field in very bright light (i.e. daylight). 3) When using a flash on a camera with a focal-plane shutter, exposure time is limited to the maximum speed (often 1/250th of a second, at best) at which the entire film or sensor is exposed to light at one instant. Without an ND filter this can result in the need to use f8 or higher. 4) Using a wider aperture to stay below the diffraction limit. 5) Reduce the visibility of moving objects. 6) Add motion blur to subjects.The number on the filter you need will depend on the thread size of your lens. For example, the kit lens which comes with the Canon Rebel XT is 58mm, so you would need a 58mm filter.