Some patients require critical vision or are critical observers and will not tolerate any blur. Other patients do not seem to notice even significant blur. Often this can be detected during the examination. The patient who indicates that the chart gets very blurred with the addition of +0.25 D. is obviously more sensitive than the patient who indicates no change in the appearance of the chart with a increase of +0.50 to +0.75 D. over the best correction.
The case history can also give one an indication of the patients need for critical vision. They are often aware and quick to indicate that they want very good vision.
In the case of the critical observer, you may not be able to allow a 0.50 D. to 0.75 D. of uncorrected cylinder which might be tolerated by a less critical observer. A rigid or a toric hydrogel lens may be required. For some patients a rigid lens may give better vision than a hydrogel lens even with no residual error.
