Early treatment diabetic retinopathy study （ETDRS） charts was designed for vision research， which incorporated features of the original Bailey-Lovie charts. Compared with Snellen chart. The design of ETDRS chart is much more scientific and reasonable， accuracy and repeatability is better， but it takes more time； the logMAR testing results of ETDRS chart can be used directly in statistics. low vision can be quantified and evaluated reliably by the ETDRS chart， by changing the viewing distance to 4， 2， 1 meter， conversion of logMAR values in different testing distance is applicable. Due to discrepancy between theoretical and virtual testing values， it′s possible to exaggerate visual gain and loss， we should be cautious of converted values of different testing distance.