While testing various wide angle lenses on the a7R, I discovered a trick that is useful with any camera with an EVF and focus peaking. It can give you a quick idea of the loss of sharpness at the periphery of an image without making even one exposure. Out of focus, focus field curvature, corner smearing, it’s all the same to this technique. As a special deal, I offer it to all my viewers at half my usual price.
Turn on focus peaking. On the a7R, set the sensitivity to low so that the whole finder isn’t full of blinkies. Set the color to red; you want something that will really stand out. Focus on the scene as you normally would and set the aperture to the one you’re concerned about. Find some central element that’s causing the peaking to show red. Move the camera back and forth and watch the peaking fade out as you get close to the edge. You can get a good sense of how much you’re losing, and where you’re losing it.
Things that are barely triggering the red shimmer will show slight losses as you move away from the center. Things that really get it going will be harder to extinguish as you move them towards teh edges, but their loss will indicate more softness.