Automatic objective optometers provide a possible alternative to conventional clinical methods of refracting the human eye. Quantitative evaluation of one of these instruments (Dioptron), including a comparison between optometer and subjective refractions, was undertaken to determine how automatic optometers may be utilized most effectively in clinical situations. The results show a high degree of instrument validity and reliability. The conclusion is that the Dioptron has substantial promise as a means of estimating ametropia-as a replacement for retinoscopy and for screening or monitoring ametropia. However, the use of this instrument as a substitute for subjective refraction procedures does not seem warranted at this time.