Neutral Density filters have several uses and offer the possibility to achieve otherwise unachievable results. ND filters appear grey and reduce the amount of light reaching the film. They have no effect on color balance. They have four main uses: 1) To enable slow shutter speeds to be used, especially with high speed films, to record movement in subjects such as waterfalls, clouds, or cars. 2) To decrease depth of field by allowing wider apertures to be used, which helps separate subjects from their background. 3) To decrease the effective ISO of high speed film (above ISO 400) and allow it to be used outdoors in bright situations. 4) To allow cine and video cameras (which have fixed shutter speeds) to film subjects such as snow, sand or other bright scenes which could cause overexposure.
Tamron's 500mm f8 lens has a fixed aperture of f8. This filter will allow you an effective f11 aperture. This can help in bright light situations.