Prescription lenses for shooting glassea

[vbull]

So, for shooting purposes what difference does it make where the datum is, whether at 9 or 12 o'clock ?

[Sir_SwampThing]

So, for shooting purposes what difference does it make where the datum is, whether at 9 or 12 o'clock ?

I image that the rotation of the lens affects the amount of correction? If the lens is mounted with the datum point in the wrong position, it would possibly increase the astigmatism instead of correcting it?
Or am I way off?

Cheers
Chris

[derekm]

I image that the rotation of the lens affects the amount of correction? If the lens is mounted with the datum point in the wrong position, it would possibly increase the astigmatism instead of correcting it?
Or am I way off?

Cheers
Chris

Yes and no. Rotating the lens will put the correction in the wrong axis relative to your eye and screw up the image.

[vbull]

Thanks, the penny's dropped.

[derekm]

So, for shooting purposes what difference does it make where the datum is, whether at 9 or 12 o'clock ?

It doesn't per se. Nothing to do with shooting purposes, it is aligning the cylindrical form built into the lens to your eye. The datum, reference point or whatever you want to call it lets you turn the lens to the correct position. If the lens maker puts the mark at 9 o'clock, you put it in the frame at 9. 12 o'clock, install at 12.

If you already have glasses with astig. correction, look through one lens and rotate the glasses. The image will become distorted and the penny will drop.