Firstly, why are you worried so much about the corners. Your photo looks fine to me and unless you are pixel peeping why worry. Secondly you can shoot at smaller apertures (like f:16) but most likely won't help too much. Getting a bigger dome will help but at a cost and pain for travel factor. Using a diopter on the lens can often help and is quite inexpensive compared to a new dome. One thing to check on is the corner sharpness of the lens on land; test it in the garden; it can't be much better UW than it is on land.
- Timz likes this