@atbalogh sorry to reply to an old thread - but I thought I’d mention that @JohemianKnapsody did some more systematic investigation into field flatness in his (admittedly 100x) microscope. He found that part of the problem was that the top of the microscope stage wasn’t perfectly flat - or at least that the top of the sample riser wasn’t perfectly flat. When he added some sheets of paper on one side as a shim, he was able to get nice flat images. As @WilliamW says, it’s possible to do a relatively simple experiment to distinguish between field curvature and field tilt - and it’s tilt than can be corrected by this method. It sounds like you would have noticed if this was your problem, but I thought it would do no harm to double-check!