Oblique and WTR/ATR astigmatisms

[RetroRat]

I hope this is the correct forum to be posting this question, but my understanding of these aberrations (are they even aberrations?) isn't very good at all.

Can anybody recommend an article that goes into some of the principles behind them or point me in the right direction? Thanks so much in advance

[uncut]

Are you wanting something beyond text references, or just explanation of corneal astigmatisms?

[RetroRat]

:P To put it in perspective, I didn't know that they were corneal to begin with!

An explanation would be very much appreciated, uncut. Thanks

[WFruit]

You want this article here: http://www.optiboard.com/forums/show...ectacle-Lenses by Darryl. Make sure you read both parts.

[uncut]

@RetroRat:

Think of subdividing regular astigmatism:

WTR(with the rule) is when the greater refractive power is in the vertical meridian.
ATR(against the rule) is when the greater refractive power is in the horizontal meridian.

Both types of astigmatism are suggested to also be within 20 degrees of 180 and 90.

Oblique is astigmatism in which the principal meridians do not lie within 20 degrees of 180 or 90.

In contact lens fitting, especially gas permeable the type of astigmatism would influence lens parameter choices.

[uncut]

@RetroRat:

In the context of corneal topography the above post also means:

WTR: cornea's flattest meridian is horizontal.
ATR: cornea's flattest meridian is vertical.

[RetroRat]

@RetroRat:

Think of subdividing regular astigmatism:

WTR(with the rule) is when the greater refractive power is in the vertical meridian.
ATR(against the rule) is when the greater refractive power is in the horizontal meridian.

Both types of astigmatism are suggested to also be within 20 degrees of 180 and 90.

Oblique is astigmatism in which the principal meridians do not lie within 20 degrees of 180 or 90.

In contact lens fitting, especially gas permeable the type of astigmatism would influence lens parameter choices.

When you say 'greatest refractive power' for WTR and ATR astigmatism, is that the power displaced most from plano, that is to say the spheric power in hyperopes and the cyl power in myopes (as they would be further away from plano)?

Thanks for the info and links, it's also given me something to google a bit further as well (probably think up some more questions shortly too, so thanks in advance for being patient with this newbie hehe)

[uncut]

When you say 'greatest refractive power' for WTR and ATR astigmatism, is that the power displaced most from plano, that is to say the spheric power in hyperopes and the cyl power in myopes (as they would be further away from plano)? Correct , and remember that the majority of corneas are "with-the-rule".
....
Thanks for the info and links, it's also given me something to google a bit further as well (probably think up some more questions shortly too, so thanks in advance for being patient with this newbie hehe)

uncut