Checking PALs

[David Wilson]

Lovely to hear your comments, thanks..... I can't believe I've been doing it wrong all these years, ( i.e. fitting at the pupil centre!) and our patients are 'SUPER FUSSY"


I'm going to try out a few fits at pupil base and see what the reaction is

Thanks again

You haven't, Clive. All of the lens manufactureres and their designers (and Darryl) are saying to fit at pupil centre. The vast majority of practitioners around the world fit at pupil centre and all textbooks say fit at pupil centre. I for one don't plan to second guess the experts. Also, as Darryl suggests, the focimeter (vert) is not human and does not perform in the same way as the human eye. That is why we have these little windows of simplicity on the progressive to satisfy the basic instrument.

Among other things, the designers are considering the effects of aberrations, notably oblique astigmatism. If we don't fit how they suggest we will induce aberrations, albeit usually minimal. This is why we have to fit aspherics 1 mm below pupi for every 2 gergrees of pantoscopic tilt. many practitioners don't do that and their clients don't complain That doesn't mean they are getting the optimal result, though.
Regards
David

[Andrew Weiss]

A couple of responses to things posted on this thread:

As Uncle Fester noted in his post, we're finding hyperopes and myopes respond very differently to corridor height. These days I tend to fit myopes at 2mm below pupil center; I measure that way because I see most of our patients while their eyes are being dilated, so finding the bottom of the "normal" daytime pupil is, well, challenging. :o Hyperopes I still fit at the pupil center.

I agree with Darryl that pantoscopic tilt is the key to getting the best vision. Most lenses, in my experience, have an optimal panto. That said, the myopes and emerging presbyopes I work with want nothing to interfere with their limitless distance vision, and for them even panto tweaking doesn't help if I fit at pupil center. What Uncle Fester didn't tell you is that our office is the only one I've worked in where virtually everyone wearing a FT is fit at 5mm below the lower lid rather than even with the lower lid. Interesting.

I also measure just about everyone sitting down and standing up, to get posture. This is how I was taught to measure FTs in "ye olde days". It still works. I like the mirror idea, especially for people who are taller than I am. ;)

Also, Darryl, I have to tell you that my personal experience with the Zeiss Brevity is that the intermediate and reading areas are wider than in the Zeiss Top, not narrower. Our Zeiss rep verified that this is part of the lens design. How'd they do that?

[Darryl Meister]

Also, Darryl, I have to tell you that my personal experience with the Zeiss Brevity is that the intermediate and reading areas are wider than in the Zeiss Top, not narrower.

I'm not really in a position to speak on behalf of Zeiss, but I will say that the Brevity samples we measured at SOLA R&D do indeed have a wider near zone that is also located higher on the lens. Of course, Brevity sacrifices some distance vision to achieve this increased near performance, though not to a degree that I would consider unreasonable (Gradal Top had a rather large distance zone). This high, wide near zone configuration, will work well for near-intermediate distances (say, out to a computer monitor), since the progressive corridor opens up high enough in the lens -- and at a point of adequate add power -- for computer viewing. However, overall intermediate performance between Gradal Top and Brevity (including vision at far-intermediate distances) was similar for the samples we measured.

Several short-corridor progressive lenses, including Zeiss Brevity, AO Compact, and even Shamir Piccolo, deliver relatively wide near zones. However, there are also several on the market that deliver relatively narrow near zones in order to maximize other optical features.

Best regards,
Darryl