Prism in Glasses

[Neophyte Optician]

I have a 68-year old who needs 4 prism BU OD total (e.g. 2 BU OD, 2 BD OS) at both distance and near. His spectacle Rx is +2.00-1.75x180 OU +2.50 add. Is it possible for him to have 4 BU at distance and near given Prentice's rule? If it's possible, what do I need to tell the lab? Thanks.

[Lensman11]

Considering the rx has only 0.25 power at axis 90 no prism will be created by looking at different parts of the lens. If the rx was different you could order a slab off to create prism at near.

[Neophyte Optician]

Considering the rx has only 0.25 power at axis 90 no prism will be created by looking at different parts of the lens. If the rx was different you could order a slab off to create prism at near.

Thanks! For my own edification, what would I have told the lab if his Rx had been +2.00 DS OU? Also, what would I have told the lab had his Rx been OD +2.00 OS +4.00? Thanks.

[Lensman11]

Use prentice’s rule and calculate the prism after a 10 mm drop. The slab off is bu. The idea is for new opticians to understand basic concepts not just get an answer.

[wmcdonald]

I have a 68-year old who needs 4 prism BU OD total (e.g. 2 BU OD, 2 BD OS) at both distance and near. His spectacle Rx is +2.00-1.75x180 OU +2.50 add. Is it possible for him to have 4 BU at distance and near given Prentice's rule? If it's possible, what do I need to tell the lab? Thanks.

It is important to fully understand prism, and one of the finest free resources I know of can be found here on OptiBoard! John at Optician Works has a ton of information posted here regularly that is outstanding. One of those in on prism. Take a look at it. Easy to find via the search at the top of the page. Lensman is on the spot as well, and there will be almost no induced prism. Take a look at the video, and you'll also find other helpful items there.

[Robert Martellaro]

Thanks! For my own edification, what would I have told the lab if his Rx had been +2.00 DS OU? Also, what would I have told the lab had his Rx been OD +2.00 OS +4.00? Thanks.

For the for former, order the Rx as written by the prescriber. For the latter, there will be vertical imbalance induced at both distance and near that will need to be addressed by careful positioning of the distance OCs, and a slab, separates, etc. for near.

Hope this helps,

Robert Martellaro

[Uncle Fester]

Also, what would I have told the lab had his Rx been OD +2.00 OS +4.00? Thanks.

Keep the frame small but one that will still fit and hold a progressive to minimize peripheral complaints as well. This occurs because of increased vertex distances in the horizontal.

Remember to ask if any double image complaint vertical or horizontal.

If they insist on today's larger sizes document and manage expectations that a double image may occur when pulling out into traffic.

That tends to get their attention!!!