Reading level for Slab off prism on progressive
On most flattop bifocal jobs I use 10mm as reading level on the slab off prism calculator with excellent results.
How is the reading level calculated for progressive?
On this progressive lens the MRP is 4mm below the fitting cross and there are 16 mm from fitting cross to the center of the reading part of the progressive lens. This is a Freeform progressive.
Which should be the Reading level on this job? 16mm or (16-4)=12mm.
In this case use 12mm to determine the Vertical Imbalance at near, if the reading depth below the Prism Reference Point is actually 12mm. There will be VI on the distance gaze, maybe enough to be a concern. If true, one might introduce prism to cancel, recalculating the near slab value to compensate.
In some cases when the anisometropia introduces a fairly low value of VI, one might use a PAL with more "drop" (Zeiss uses 6mm), reducing the VI at near, eliminating the need for a slab.
Make sure that the PAL near zone developes 100% of the add about 3mm to 5mm above the reading level, the latter determined by placing a horizontal line bisecting the pupil when the client is reading, confirming with the "mirror method" or by sitting on the floor.
Hope this helps,
Roberts Optical Ltd.
An expert is a man who has made all the mistakes which can be made, in a narrow field. -Niels Bohr
Pal's and slab go well together like oil and water. Clients are usually disappointed. They now get a line or blurr zone in one eye, even with FF's. They can still have diplopia in given area's along the umbilic. I highly recommend they consider a ft bi or tri.
If you still wish to proceed, figure the power at 90. Don't use prism thinning. Put OC's 1-2mm's below pupil, then figure how many mm's below them you'll induce 1D of imbalance. Put your slab line there. Make sure there is adequate room left for near vision.
Also make sure this patient uses both eyes at near before you even go with a slab. If they have gone with this imbalance for very long in Pal's, they probably have developed a monocular near task style of use.
Thanks for the explanations
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