Shot through dirty double paned tinted glass at ISO 800, it's fairly reasonable. Had no trouble locking focus on this one, but in many cases I have trouble locking focus on birds in front of busy backgrounds like lots of tree limbs. In order to stop motion of busy creatures like birds and squirrels you have to use a fast shutter speed and that, of necessity, raises the ISO higher than you'd like with a slow lens (f/5.6 zoomed) like this. In brighter light, the IQ is excellent.