A more scientific method than the Snellen chart for determining visual acuity is imperative for research studies and is desirable in many clinical situations. The psychometric method using the constant contour interaction S-chart of Flom has been regarded as an accurate and reliable visual acuity measure and has been used widely for research purposes. This paper compares visual acuity measured by the S-chart with that measured by the Snellen and Bailey--Lovie charts. The results indicate that the Bailey--Lovie chart is a valid, reliable and rapid method of measuring threshold visual acuity for research purposes.