Ok, so A has a 1 pixel width circle made the "normal" way, it is untidy...
B is a circle made with a 2 pixel width circle with a half overlap of a white circle to halve it's thickness.
C is a circle made by screen capturing the outline of the paint brush when appropriately sized...
And D is a paint brush making a solid black circle, reducing the size by 2 digits and making a white circle, to leave a black outline...
The bottom half circles are copies of the above circles with the bottom half copied, rotated, inverted and put in an xor layer, this cancels much of the image, but I believe if it were Identical it would be blank, showing only white...
I used this to show the differences between the 2 halves, which are worse on C... But never perfect surprisingly.