They have some guards in place, but it isn’t like a business and you’re still dealing with a person. They seem to ban people pretty fast tho
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Could be field curvature. The article describes how you can test it (eg shoot a field with grass and then see where the sharpness is).
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Thanks, I’ll take a look and try the lens before I give up on it 
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Good suggestion which is readily enhanced by Edge Detection combined with the Threshold function in the GIMP.
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I haven’t tested my lens yet, but I found that I have more photos of this scene and one of them doesn’t have the sharpness issue, even though it’s the same focal length and aperture. It’s quite likely that I have focused much further.
I remember that I was trying to focus hyperfocal a lot that day, so I would avoid blurry foreground, but the field curvature probably doesn’t really allow for that…