A quote from popphoto.com states the following:
Quote: As with most fisheyes, the front element of this 10–17mm (a 15-26mm equivalent, due to the 1.5X 35mm lens factor on Pentax DSLRs) bows out in front of the barrel, leaving its arched glass surface unusually exposed. Pentax claims that its SP coating, applied using a new evaporation process, improves surface resistance to water and finger oils, making it easier to clean. Still, beware of scratches!
Myself find its cap very useful and have used it on several trails in almost all conditions from heavy snow to desert heat and find it the easiest cleanable front element of all my humble lens collection.
Just my 2 cents and happy distorting, JR