General principles of calibration

In the clinical testing when different tests are compared with the LEA charts, printing quality is one of the variables to be considered. If the edge quality of the optotypes differs from one test to the other, then the symbols are not correctly compared. Ideally all tests to be compared should be printed by the same printer on the same material. Edge quality varies on computer screens, especially in optotypes with curved contours or oblique lines. 

In the calibration of optotypes the test subjects should have high normal visual acuity, corrected or uncorrected, and the measurements are preferably done binocularly in order to avoid variation of performance when using the non-dominant eye at threshold. Young children should not be used as subjects in calibration of paediatric symbols against the Landolt C because laterality may not have been developed so that children have difficulties with both pointing and using expressions 'left/right'. Some otherwise well functioning school children with CP may not have the concept of directions so that "up/down" cannot be used and instead “left/right” expressions like "the lines (of E)/ the gap in the ring points where your mother sits or points where your therapist sits". The definition of the threshold is then difficult because the child easily gets bored.

When different tests are compared in examination of clinical patients with varying degrees of visual impairment or uncorrected refractive errors, those results cannot be used for calibration. The differences in visual acuity values when using different sets of optotypes depict how these tests behave in non-foveal vision or when refractive errors are not corrected.