For shorter focal lengths 20-24 or 28mm as well as for wide-angle zooms (24-50mm, 28-85mm, 35-135mm) System P is strongly recommended. It is available in diameters ranging from 48 to 82mm and eliminates vignetting (dark edges).
Color Compensating Filters - Control color by attenuating principally the red, green, or blue part of the spectrum. They can be used to make slight changes in the color balance of images recorded on color films, or compensate for deficiencies in the spectral quality of a light source. These filters will work perfectly with color temperature meters where even slight variances of color will cause a problem.