Cokin Z-PRO 084/840 Diffuser 2 filters have very little effect on the sharpness of the image. They merely diffuse highlights thus reducing contrast. As such, diffusers can be used to soften a landscape, or soften skin texture in a portrait to hide minor flaws.
Z-PRO Filters are made of CR39 Resin which is a reputable lightweight and unbreakable optical material also used for ophthalmic glasses. The Z-PRO Series is recommended for focal lengths of 20mm onwards (in the 35mm format).
Note: Cokin Diffusers are available in three grades, #1 (#083), #2 (#084), and #3 (#085) with #3 having the greatest effect