The approximate relation of unaided visual acuity and required correction (in dioptres) from 6000 refractions in young adults (males and females <30 years). The size of the plus cylinder for a given uncorrected astigmatic error was approximately 1.4 times greater than the spherical equivalent for an oblique axis, increasing to 1.6 times for a vertical axis (with the rule astigmatism) and 2.0 times for horizontal axis (against the rule astigmatism). The latter reducing to 1.8× when the VA was worse than 20/150. This demonstrates that against the rule astigmatism has a worse unaided VA and requires a larger cylinder correction than an equivalent with the rule astigmatism (that is, for any given spherical equivalent)29
| Visual acuity | 20/30 | 20/40 | 20/50 | 20/70 | 20/100 | 20/150 | 20/200 | 20/250 | 20/300 |
| Astigmatism axis horizontal | 1.00 | 1.50 | 2.00 | 2.50 | 3.00 | 4.00 | 4.50 | 5.50 | 6.25 |
| Astigmatism axis vertical | 0.75 | 1.25 | 1.50 | 2.00 | 2.50 | 3.25 | 4.00 | 4.25 | 5.50 |
| Astigmatism axis oblique | 0.75 | 1.00 | 1.50 | 1.75 | 2.25 | 2.75 | 3.50 | 4.25 | 5.00 |
| Spherical myopia or absolute hypermetropia | 0.50 | 0.75 | 1.00 | 1.25 | 1.50 | 2.00 | 2.50 | 3.00 | 3.50 |