I am slightly short sited and have a slight astigmatism, I have been to several different opticians and the perscription is always slightly different. I don’t know whether it is normal but I always seem to struggle to focus on the eye chart and when they ask me which letters are clearest I always struggle to know. I have never been able to read the last 2-3 lines on the chart and whatever perscription I come away with I still struggle to read road signs. Why could this be?