# Thread: Lens thickness & fitted back vertex distance

1. Originally Posted by Robert Martellaro
“In response to poor fitting in the form of tilt or decentration, lenses with aspheric form surfaces were found to have greater off-axis power errors than best-form lenses with spherical surfaces.” -Atchison, D. A. & Tame

https://www.ncbi.nlm.nih.gov/pubmed/8278198

https://www.opticianonline.net/news/...lar-refraction

https://www.odob.health.nz/wp-conten...f-D-Wilson.pdf
Nailed it, best form base curves can be estimated with vogels rule, thats where plus 6 comes from it's the guesstimated optimal back curve. demo lenses have a curvature to which you can clock, it comes out to being close enough to what the actual back curve is, then as Brent so eloquently put it the power error is negligible from there.

2. Since I'm no longer in the field I can give one of my best vertex measuring device secrets away. Take a simple PR ruler with see through windows (you know the ones that sit on the bridge and have a window that measures the PD's), get a Fresnel prism 25D (or any two Fresnel prisms that add up to 50D, 25 is simple because you only have to use one and cut two pieces, but a 10 and 40, or 20 and 30, etc. will work). Cut the prism into 2 pieces and place them so that the lines are oriented vertically to match the hash marks on the windows on the PD ruler, but only cover the top half of the window leave the bottom of the window clear, place the other piece on the back side of the ruler over the top of the window with their base in the opposite direction. This now adds up to 50D of prism in the top half of the window. Now the difference between top and bottom is 50D of prism. Since prism diopters measure a deviation of 1cm over a distance of 1m and 100cm go into a m you can hold the PD ruler to the front of the frame (with demo lenses out) and bisect the pupil with the top and bottom half of the prism line, the distance in mm between them * 2 is the vertex distance. You would be surprised how accurate this is and the measure is to the front plane of the specs, so if you want to get super nerdy you can caliper the frames for the distance to the bevel from the front and subtract that out and now you have the exact measure to a useful reference point. The bevels can be placed and measured, the back sag can be measured so you can add/subtract away all these measures to come to a accurate measure, of course it's a bit much.

3. MO, what field are you in, now?

4. Originally Posted by MakeOptics
Since I'm no longer in the field.... .
I feel like Darryl just died again...But good luck MO in your new career...

5. I remember seeing a picture of a "prismatic doublet" vertexometer online (from Rodenstock?) that I always coveted but never could obtain.

Now I can build my own!

Much appreciated.

Glad you hang out here, occasionally MOHC.

I'm going to use a corneal reflex. Maybe I'll post a pic.

6. Originally Posted by drk
I remember seeing a picture of a "prismatic doublet" vertexometer online (from Rodenstock?) that I always coveted but never could obtain.
Yes, Rodenstock made two versions. Pictured, with instructions, on page 6 and 7 of the David Wilson link above. Prof. Wilson used to post on Optiboard.

Harry, I hope your having fun and getting paid for it.

Best regards,

Robert Martellaro

7. I didn't open that, before.

That's a killer resource.

8. When I’m faced with a real VD/power compensation in Rxs say, > -11.00D, and a fitted VD out if the norm—say 8-9mm measured with a VD caliper and no known refractive VD—I will order some cheap 1.74 lenses and glaze the chosen frame.

Then I call back the patient, fit these, and over refract to obtain best acuity. Then this becomes the ordered Rx.

Small price and effort to pay to ensure the absolute best vision when I’m fitting \$1k+ premium FF progressives.

And of course, they always say “No one’s ever done that before.”

B

9. Wouldn't it save a lot of time and money to do that in a trial frame?

10. Nope

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