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Hmmm... i'm still struggling with this a little to be honest - If I take a shot even at F8 and don't nail the focus, I am still getting irregular results.
for example, when I say 'nail focus':
At 35-40mm and F8 - If I focus just a little deeper than my central subject at distance everything looks awesome (this is to account for the inward curvature, leaving the middle acceptably in focus, while ensuring the edges are still within the curved plane) . However, if I focus too deep, I retain sharp borders, but lose centre detail and similalry if I focus 'correctly' on the centre subject the borders drop well out of focus.
The margin of error just seems very small, even at F8.
I will post some samples when I get home - perhaps its being aggravated by my 'test scene' not being flat, but focussing at 35mm, F5.6-8 and 25 plus metres away, it should be a complete non-issue (hyper focal would be 7-10 metres)