OK, as other have mentioned previously about this lens, I've also started noticing some pretty soft corners underwater on my Canon 10-22mm behind a Athena Dome, 40mm S&S ext., S&S 350D housing. Topside, the corners are not that noticeable even at wide aperture settings. Someone suggeseted the use of a diopter but also recommended to avoid the cheap b&w diopters and try to find a double element diopter. Given the 10-22mm is 77mm filter size, that I think only leaves me the option of the Canon 500D close up lens. Would this be the right choice? Also (bear with me..), as the lens would focus a virtual image imposed by the dome, how does the diopter effect a WA lens in terms of infinity focusing? It's not the same as slapping a diopter on a macro lens and then being limited to a certain focus length right?