I go Rob-bed of my answer.
I agree with Rob, re the precision you need for this application should be fine for 1/5 Vcc if the difference between black and white varies by, say, 5% I do not think anyone can see that on the grayscale.
Most of the tolerance will be static, adding the capacitor Rob suggest, should remove most, if not all, dynamic variance.
If, however, you want to compare two frames you must add some hysteresis to your comparator or you will have total confusion around the middle grey.

Have fun,

Erik