lanierb

Noone has an answer to this?  Question is, basically, should he try Canon 500d (2-element, +2, $150) or B&W (+4 -- assumes 6" dome, $50)?  I'm also interested in the answer...

Lanier

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?

Yes it does.

If I had to guess I would say that you probably mistakenly left the eye cup on the camera viewfinder and it is not allowing the camera to seat properly.  Try taking it off and see if it fits better.  The fit on these housings is very precise so something small like that can throw it all off.

"can the sharpness be affected by the focusing distance"?

The short answer is yes.  Normal lenses are designed to minimize aberrations at infinity focus, whereas macro lenses are designed to minimize aberrations throughout the range (and sometimes only at close focus).

"So if it's just about the image quality (at f/11 or f/16), it seems like a waste to spend money on the 60 mm."

There are many situations in which the 60 is vastly sharper than the 18-55, including anything at a wide aperture or close focus.  At f/16 on a 1.6 crop camera all lenses are essentially equally sharp (because they are diffraction limited), which is why those shots show no differences.  

Note also that there are some other issues when you use lenses underwater.  Flat ports add substantial chromatic aberration on wide angles so you won't want to use the 18-55 behind a flat port.  

You sound like you want to try the 18-55 first.  Why not just try it?  The dome is only $135.  My guess is you will quickly see its limitations and want something better.