How to calculate loupe over glasses focal distances.

[Lutz]

A customer has ordered a pair of Compound Optics Loupe to wear over his glasses in order to have 3.5X magnification at distance of approximately 500mm to 600mm.
Since these are fixed focus magnifiers sourced from China he asked my advice when ordering them and I made the above recommendation according to the suppliers charts. On arrival he brought them in and unpacked the magnifiers in the store to let me see them. I tried them first and found them to work as advertised with a focal distance of about 600mm. When he tried them the focal length increased to 900mm which is beyond his reach and so not useful for him.
These telescopic magnifiers are worn over the glasses with the eyepieces being accessed through the distance portion of the lenses.
I wear progressive lenses with an RX of;
OD -1.50 -0.25 x 135
OS -1.75 -0.25 x 100 Add+2.50
My customer wears ST35 bifocal lenses with an RX of;
OD +1.50
OS +1.50 Add +2.50

Would anyone know if the differences in our prescriptions are causing the differences in focal distances or should I be looking for another reason?

[Dr. Bill Stacy]

A customer has ordered a pair of Compound Optics Loupe to wear over his glasses in order to have 3.5X magnification at distance of approximately 500mm to 600mm.
Since these are fixed focus magnifiers sourced from China he asked my advice when ordering them and I made the above recommendation according to the suppliers charts. On arrival he brought them in and unpacked the magnifiers in the store to let me see them. I tried them first and found them to work as advertised with a focal distance of about 600mm. When he tried them the focal length increased to 900mm which is beyond his reach and so not useful for him.
These telescopic magnifiers are worn over the glasses with the eyepieces being accessed through the distance portion of the lenses.
I wear progressive lenses with an RX of;
OD -1.50 -0.25 x 135
OS -1.75 -0.25 x 100 Add+2.50
My customer wears ST35 bifocal lenses with an RX of;
OD +1.50
OS +1.50 Add +2.50

Would anyone know if the differences in our prescriptions are causing the differences in focal distances or should I be looking for another reason?

It may be that you were looking through a relatively zero power of your PALs and he might have been using his segs, which could do that if they were designed to be used with his distance Rx.

[rbaker]

Trial lens over whatever he is currently wearing.

[drk]

If the working distance is about arm's length (50-60 cm, which is as advertised and ergonomically reasonable) then it has an additional ~+1.75D of converging power to be used as an add.

If the guy can't see at arm's length with a +1.75 add, then he has to be underplussed through his distance portion. If your "spitball" of him having to hold things out at 90 cm is correct, he's underplussed:

1/0.9M = 1.1 D
1/0.55M =1.66 D

about +0.55 D.

However, as was discussed, either refract him again or use loose trial lenses and see if you can push another +0.25 or +0.50D in the distance portion of his glasses.

[Robert Martellaro]

Would anyone know if the differences in our prescriptions are causing the differences in focal distances or should I be looking for another reason?

I'm not an expert with telescopic loupes, but I don't see how the Rx can have any notable effect, certainly not 300mm worth. There should be some depth of focus, maybe 100mm or so, with a 600mm work distance, maybe a little less considering the high magnification.

[uncut]

A customer has ordered a pair of Compound Optics Loupe to wear over his glasses in order to have 3.5X magnification at distance of approximately 500mm to 600mm.
Since these are fixed focus magnifiers sourced from China he asked my advice when ordering them and I made the above recommendation according to the suppliers charts. On arrival he brought them in and unpacked the magnifiers in the store to let me see them. I tried them first and found them to work as advertised with a focal distance of about 600mm. When he tried them the focal length increased to 900mm which is beyond his reach and so not useful for him.
These telescopic magnifiers are worn over the glasses with the eyepieces being accessed through the distance portion of the lenses.
I wear progressive lenses with an RX of;
OD -1.50 -0.25 x 135
OS -1.75 -0.25 x 100 Add+2.50
My customer wears ST35 bifocal lenses with an RX of;
OD +1.50
OS +1.50 Add +2.50

Would anyone know if the differences in our prescriptions are causing the differences in focal distances or should I be looking for another reason?

I would suggest that you have the advantage of wearing a multifocal and may also have a better accommodate ability. He is handicapped with a bifocal and could use a trifocal.............that would make your abilities more equal.