Prescription lenses for shooting glassea

[derekm]

Just had lenses made for my Knobloch shooting glasses, only £15 for two different strengths, 2 way postage included from:-

https://www.reglaze-glasses-direct.c...SAAEgLaz_D_BwE

Best to send the lens and lens holder as I just sent a sample lens and the new ones came back slightly small and needed a little packing added for the holder to grip. Otherwise, 100% and better than the £30 odd (?) each I've heard quoted elsewhere.

At first I thought they had screwed up with the prescription, but they had put the datum mark for astig. correction at 9 o'clock when I expected it to be at 12 o'clock!

[RobF]

do they have the focal length set to the fore-sight?

[vbull]

Can you explain this datum mark for the astigmatism bit please ? I've looked at my prescription and can see nothing about astigmatism.

[BigEars]

Can you explain this datum mark for the astigmatism bit please ? I've looked at my prescription and can see nothing about astigmatism.

Maybe you don't have it. When i had mine made the optician asked where i would like the mark to be. I opted for 12.00.

[derekm]

Rob, up to you to decide the sphere value for a good foresight. If you don't have astig. or prism problems, try out some ready readers to get what will usually be a +ve value. If you have a negative prescription you'll have to estimate from the "near" or "reading" value. If you have astig. you'll have to estimate the sphere from the near value on your prescription. A pistol shooting lens will be weaker than reading, stronger than distance.

Until my astig. became noticeable (and before the Knobloch frames came along at a giveaway price!) I used a ready reader frame with a lens made by an internet Co. like reglaze.

[derekm]

Can you explain this datum mark for the astigmatism bit please ? I've looked at my prescription and can see nothing about astigmatism.

If there's nothing on your prescription for "cyl" (cylinder) and "axis", you don't have astigmatism.

If you do have astig. your prescription will have something like cyl -2.75, axis 140 as well as the spherical value. The 140 is degrees from the datum. My last lenses had datum at 12 o'clock, these new ones it's at 9.

[RobF]

Rob, up to you to decide the sphere value for a good foresight. If you don't have astig. or prism problems, try out some ready readers to get what will usually be a +ve value. If you have a negative prescription you'll have to estimate from the "near" or "reading" value. If you have astig. you'll have to estimate the sphere from the near value on your prescription. A pistol shooting lens will be weaker than reading, stronger than distance.

Until my astig. became noticeable (and before the Knobloch frames came along at a giveaway price!) I used a ready reader frame with a lens made by an internet Co. like reglaze.

Hi Derek,

Ah I get it, so the sphere is just the focal length adjustment on top of your prescription? ... so say my right eye is +1.25 to start with, then to focus at the foresight say at 1m, then I just add 1D. If it was at 2m away i'd add 1/2m = 0.5D If I want to focus at 0.5m then it would be 1/0.5m which is 2D.

Does that sound right?

I've got an astigmatism. Trouble is some opticians round the angle up to 5 degrees. Mine is actually 92... some write it as 95. But at least with shooting glasses you can fix it easily by rotating the lens.

[derekm]

Rob, you're getting near. The sphere is the main part of your prescription, the magnification if you like. Base your shooting estimation on the "near" or "near/reading addition" of the prescription and make your estimation from that.

My right eye sphere is +0.25 and the near addition is +2.25 (near sphere = +2.5). I ordered lenses to +1.0 and +1.25. with the astig cylinder as prescription. At the moment, the +1.0 is correct.

My last pair of lenses from 2 years back were +0.5 and +0.75. As my eyes degraded I went from the former to the latter.

Whatever your own figures, your shooting lenses will be less strong than your reading lenses and stronger than your distance.

Sorry if I can't put it into better words!

[RobF]

Rob, you're getting near. The sphere is the main part of your prescription, the magnification if you like. Base your shooting estimation on the "near" or "near/reading addition" of the prescription and make your estimation from that.

My right eye sphere is +0.25 and the near addition is +2.25 (near sphere = +2.5). I ordered lenses to +1.0 and +1.25. with the astig cylinder as prescription. At the moment, the +1.0 is correct.

My last pair of lenses from 2 years back were +0.5 and +0.75. As my eyes degraded I went from the former to the latter.

Whatever your own figures, your shooting lenses will be less strong than your reading lenses and stronger than your distance.

Sorry if I can't put it into better words!

No, that makes perfect sense thanks. I never quite twigged the near sphere just altered the distance. That makes sense.

[derekm]

The sphere is the degree of magnification, whether near or distance. Less is required for distance focussing, more for reading and somewhere between for pistol, desktop computer etc..

If you have astigmatism, your eyes produce a "squerged" image, (a square becomes a parallelogram), but for mild cases, the brain corrects this. The cylinder, tilted at an angle, feeds a corrected image to the brain and this correction is the same for both close-up or distance. When you first get astig. corrected glasses, because they over-ride your brain, you tend initially to get a squerged image and may, like me, take the glasses back saying they are wrong! After a couple of days, your brain re-adjusts to the now correct image it is receiving.

Prism, I do not really understand, but I think it is correction for cross-eyes.

[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]

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.

[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.