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re :

Try this Onde screen capture for Mac, it can capture circular region. Firstly, you need to press Command at the same time you should click "Ellipse", then you can grab a circular region very accurately.i think it can help you .Good luck😀
 
I would have thought it would be simpler to grab a regular screenshot and then do an elliptical crop in Preview, but if there's something out there that does it more quickly for you then it might be worth a shot.
 
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