Figure B above shows corneal astigmatism against the rule where the horizontal meridian has the steeper curvature.

When evaluating the fit of a rigid contact lens, dynamic flourescein patterns are observed immediately after the patient has blinked. The following are patterns of a spherical base curve on a spherical cornea, and toric corneas showing astigmatism with the rule, against the rule and oblique. Flourescein patterns are best observed using a cobalt light.

Figure A below shows an ideal fluorescein pattern where the dye is evenly distributed.

In figure B the oblique astigmatism shows less dye along the obliquely oriented flatter meridian. Pooling occurs in the other steeper portions of the cornea covered by the contact lens.

Figures C and D represent with-the-rule astigmatism, where the flatter corneal meridian is horizontal. After a blink, the lens can be pulled up high, fall low. The lens will tend to move to either side following the steeper meridian.

Figure E shows the less common against-the-rule astigmatism. Here, as the lens moves to follow the steeper meridian, it is pushed almost all the way off to one side.