ggplot alpha levels appear different on fill and border of points (ringing artefact)

Question

I am plotting many points using ggplot with a constant transparency value for all points.

What I find is that the circular points have a more transparent fill than

Answer 1

Changing stroke to 0 seems to have hte desired result:

ggplot(data, aes(x,y)) + 
  geom_point(alpha=0.2, colour="dodgerblue", fill=mycol, stroke=0,  size=5) +
  theme(panel.background = element_rect(fill = 'black', colour = 'black'))