Prism vs aberrations

[torontood]

Hello,

hoping someone can help with an issue a px is describing.

First time using a near Rx with prism and complains of “convex” distortion. The lenses have a 1.74 index as per the engraving and are from Nikon. The px does report a significant improvement with the quality of the vision but cannot adapt to the warped perception. 🫤

I’ve never heard of a px complaining of this with prism alone, so I’m more inclined to believe it’s the asphericity and the aberrations. The BC of the lenses are similar to the habitual specs.

Rx:
OD +3.75-0.25x090 3BI
OS +3.50DS 3BI

I wasn’t the one who dispensed the glasses and I wouldn’t have chosen such a HI lens to begin with.

Any feedback is appreciated- TIA.

[Robert Martellaro]

Base in prism should cause slanting down to the right and left. It takes about 5 to 10 days for the brain to remap the image, with adaptation occurring almost overnight.

Fitting closer reduces distortion, higher Abbe materials reduce blur and color fringing.

Best regards,

Robert Martellaro

[torontood]

Thank you, Robert.

When I did an in office trial frame fit of the Rx and prism, the px didn’t report any image displacement. This is why I’m a bit stumped. Is the frame size a contributing factor as well (it’s quite large). She was quite adamant about not continuing with the specs bc of distortion, stating the specs are entirely unusable.

Do you think cutting back on the prism by 1-1.5 would make a difference or even decentering the PD instead, I know there is some debate about the optics of ground prism vs decentering.

thank you,
Irene

[Lensman11]

There is no difference between ground prism and decentered prism

[AndyOptom]

What was the previous Rx?

Also, what Lensman said about the difference between prism obtained by grinding vs decentration.

Although having said that, I wonder if all else is equal, that using the 1.74 blank may have resulted in a slightly flatter overall lens, which may itself cause some initial adaptation difficulties. Knowing Nikon also has multiple types of SV lenses (aspheric, double aspheric, etc), do check which was used, since the shift from aspheric to double aspheric may also affect vision initially.

But, as always, my first instinct would be to recheck the old Rx and fitting.

[Robert Martellaro]

Thank you, Robert.

Your welcome.

When I did an in office trial frame fit of the Rx and prism, the px didn’t report any image displacement. This is why I’m a bit stumped. Is the frame size a contributing factor as well (it’s quite large).

Yes.

Do you think cutting back on the prism by 1-1.5 would make a difference

Titrating the prism might be the only solution if they won't allow a more appropriate frame size/shape/lens design.

Hope this helps,

Robert Martellaro

[torontood]

What was the previous Rx?

Also, what Lensman said about the difference between prism obtained by grinding vs decentration.

Although having said that, I wonder if all else is equal, that using the 1.74 blank may have resulted in a slightly flatter overall lens, which may itself cause some initial adaptation difficulties. Knowing Nikon also has multiple types of SV lenses (aspheric, double aspheric, etc), do check which was used, since the shift from aspheric to double aspheric may also affect vision initially.

But, as always, my first instinct would be to recheck the old Rx and fitting.

The Rx is almost pretty much the same except for the prism. The lens used is the AS Viewfit - I don’t think it’s a double aspheric. Her previous lenses are 1.6 as per the optician. She is an elderly px so I think this is one of the biggest factors that’s contributing to the poor adaptation.

Thank you everyone for the input!