2 totally different axis measurements for same eye?

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nospam@everest.net - 16 Mar 2006 22:09 GMT

When they use the machine to determine the axis of the right eye, it
says 165 degrees axis. But when they take me back to do the
subjective exam, the best results are at 40 degrees.

Is there some explanation for this? If so, which would be the correct
axis for the prescription? They wrote me a prescription for 162 axis
2 weeks ago and I couldn't see through it without rotating the
glasses.

So they made a new one with a 174 degree axis, but I can't see clearly
until I rotate it about 40 degrees clockwise (as viewed by my eye when
I'm wearing the glasses).

If I look at an astigmatism wheel chart shaped like a clock, with the
right eye and no glasses, the clearest line goes from 7:30 to 1:30,
which is a 45 degree line from lower left to upper right.

The blurriest line goes from 4:30 to 10:30. which is a 45 degree
angle from lower right to upper left.

Where should the axis be?

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Dom - 16 Mar 2006 22:42 GMT

> When they use the machine to determine the axis of the right eye, it
> says 165 degrees axis. But when they take me back to do the
[quoted text clipped - 17 lines]
>
> Where should the axis be?

One possible (partial) explanation is if one result was presented in
plus-cylinder form and the other in minus-cylinder form. In this case,
the axes should be 90 degrees apart.

Another possibility is if your cylinder is very low (0.25) then the axis
result can vary due to measurement error. Theoretically it shouldn't but
in practice sometimes it does.

The other possibility is that someone made a mistake! If the subjective
result was 40 degrees then why did they make your glasses at 162?

Dom

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Mike Tyner - 16 Mar 2006 23:53 GMT

> When they use the machine to determine the axis of the right eye, it
> says 165 degrees axis. But when they take me back to do the
> subjective exam, the best results are at 40 degrees.

Dom covered the reasons it could be wrong. If your cylinder value is low
(like 0.25 or 0.50) the autorefractor is occasionally way off.

> If I look at an astigmatism wheel chart shaped like a clock, with the
> right eye and no glasses, the clearest line goes from 7:30 to 1:30,
> which is a 45 degree line from lower left to upper right.

Taken at face value, you'd use the "times 30" rule: 1:30(=1.5) x 30=45
degrees.

This assumes the cylinder is written in minus, the sphere value is zero or
slightly +/-, and the target is not close.

-MT

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