Posted 16 February 2014 - 02:33 PM
I don't have specific knowledge of this setup, but usually the extension requirements are for two reasons: to avoid hitting the lens, sure, but also to put the dome far enough forward that the lens can focus on the virtual image formed.
The Sigma 15mm has a pretty tight minimum focus distance (5.9 inches), so I'd think it would work in the small dome...but you may still need the correct extension to get the len nodal point closer to the center of curvature of the dome.
Other things you might try are stepping down a bit, which can certainly help edge and corner sharpness. You mention having difficulty with manual or auto-focus so I assume it's not an issue with not finding a good high-contrast point for the autofocus to work with.
Current rig: Sony SLT-alpha55 in Ikelite housing, Sigma 105mm f2.8 DC Macro w/ Ike 5505.58 flat port or Sigma 8-16mm f/4.5-5.6 DC HSM behind UWCamStuff custom 5" mini-dome. Dual INON z240 Type IVs triggered with DS51 for TTL mimicry, or DS51 alone with home-made ringflash assy for macro.
Topside, unhoused: Sony SLT-alpha99, Sigma 150-500mm + 1.4TC, Sigma 15mm diagonal fish, Sony 24-70mm f2.8 CZ, Tamron 180mm f2.8 Macro...all the gear and nary a clue...