Our goggles have a focussing ring at the front and a dioptre adjustment at the rear (eyepiece end) and as long as they are set up properly - ie do the focussing with the front ring as best as you can get it and then fine tune it with the dioptre adjustment - there should be no eyestrain at all. I know that my right eye is about -0.5 and my left eye -1.5 from my prescription for reading glasses and a quick glance at the dioptre adjusters after focussing tells me I have got them set up right.
If you have access to a Hoffman box it is worth while spending time experimenting with different settings and it is also a very good way of monitoring the performance of your goggles.