Canon G16+Ikelite + 6" dome autofocus problem

Sorry if this has been answered already.  This is  a Canon G16, Ikelite housing + 6" (wet) dome.   Autofocus will not work when when fully zoomed in.

Is this the expected behavior?  I can certainly "fix" the problem by moving closer, but I'm at a loss to explain why.

Is the camera in macro mode. That's how I always used my Dyron wide angle port with my G12 years ago.

The minimum focus distance of many compacts changes with focal length so you need to back off to get within AF working range.  Or put it into macro mode to get it to focus closer. 

When you say AF will not work - do you mean it refuses to even try to focus or that it hunts and can't find focus?

12 hours ago, ChrisRoss said:

The minimum focus distance of many compacts changes with focal length so you need to back off to get within AF working range.  Or put it into macro mode to get it to focus closer. 

When you say AF will not work - do you mean it refuses to even try to focus or that it hunts and can't find focus?

Thanks,  Sorry I was unclear, it will not even try and focus

That does sound odd, you say you have it behind a dome - is this in macro mode or normal mode?  It might be that the minimum focus distance is quite long at maximum zoom so it  can't reach focus on the virtual image?  The specs say min focus is 40cm at max zoom which on a 6"dome I believe will not reach focus as the virtual image is 20cm from the dome surface for an object at infinity. 

If you leave the camera in macro mode it will focus from 0.4 to 50cm.  This is well within the range of virtual image location.