For four observers, thresholds improved an average of 5% (range 11% to +14%) equally for trained and untrained patterns and remained stable during subsequent training with the same targets in a related form discrimination task not involving blur. Because it transferred across target sets, the very slight improvement was indeed in the perceptual capacity to compensate for optical image degradation and not in form discrimination, but its defocus equivalent was quite minor, well less
than 1
4 diopter. Previous claims for blur adaptation must therefore rest on more complex factors that are
not fully excluded in clinical settings.