Difference between revisions of Astigmatism
Jump to navigation
Jump to search
(Expand a bit on the analogy with chromatic aberration, which I find a lot easier to fit into my head.) |
(→Analogy with Chromatic Aberration: add analogy for the two different ways to prescribe cylinder.) |
||
| Line 23: | Line 23: | ||
* It's not possible to get everything into focus using only spherical lenses. | * It's not possible to get everything into focus using only spherical lenses. | ||
As a compromise, focusing the green light on the retina causes a little bit of myopic blue blur and hyperopic red blur. This corresponds to the "circle of least confusion" in astigmatism. | As a compromise, focusing the green light on the retina causes a little bit of myopic blue blur and hyperopic red blur. This corresponds to the "circle of least confusion" in astigmatism. | ||
On a prescription, there are two different conventions for specifying the cylinder. This corresponds to either quoting the spherical correction to focus red on the retina, and then the additional "minus" required to focus blue; or a spherical correction for blue, and then how much that can be reduced by for red. The average of the two, or the "spherical equivalence", is then the correction required to put green on the retina. | |||
If the object is moved away, beyond your blur horizon, so that your eye can no longer keep the green light focused, | If the object is moved away, beyond your blur horizon, so that your eye can no longer keep the green light focused, | ||
all colours will suffer myopic blur, but blue will have the worst blur. This corresponds to the directional blur | all colours will suffer myopic blur, but blue will have the worst blur. This corresponds to the directional blur | ||
in astigmatism. Adding some spherical correction would allow you to push the green back into focus. | in astigmatism. Adding some spherical correction would allow you to push the green back into focus. | ||
Chromatic aberration could be treated by adding some material which applies the opposite chromatic error - bending the blue light out a bit more than the red light, to cancel the error introduced by the eye. This corresponds to cylinder correction. (But as with all analogies, it's starting to stretch a bit thin...) | Chromatic aberration could be treated by adding some material which applies the opposite chromatic error - bending the blue light out a bit more than the red light, to cancel the error introduced by the eye. This corresponds to cylinder correction. (But as with all analogies, it's starting to stretch a bit thin...) | ||