Could be you or the sight Three things can cause this, first is astigmatism. Rapscallion did a great job explaining why your astigmatism show up more in low light vs bright light. Second is oil on the lens causing distortion. Clean the lenses well with Q-tips and good glasses cleaner. And the third is the quality of the coatings on the lenses. High end sights have better coatings to produce the sharpest dot image. In every case, turning the brightness down to the minimum you can see will help.