Student counting down days untill exam, please help :o!!!

[studentdo]

Hello there

i have my final exam coming up in 4 days time and was looking for some advice from some real professionals working in the wonderfield field of dispensing.

Basically, there have been many parties and my brain is currently failing me in the area of bifocals and i cant seem to find my notes anywhere. ARGHHH

Questions:

1) Px needs bifocals but their eyes are 3 mm different in terms of their heights?
My gut instinct directs me towards, putting the heights at the relevant lower limbus areas. However is there anything else i may have to consider apart from the segment encroaching into normal straight forward gaze?

2) As they are a high plus explain what NVEE (near vision effectivity error) they might suffer?
Totally confused here :(

3) What lighting issues might they have with their bifocals?
The segment top may reflect light if it is a flat top is clear, however in terms of lighting i cant really think of any other issues apart from glare, any other opinions?

If you have made it this far thank you for your time, if you can help much love

Student DO

[Darryl Meister]

1) That sounds like a reasonable enough answer. You could also keep the seg heights more equal and then tilt the frame and lenses to line them up with each eye, but this would require compensating the cylinder axis and is probably not the answer they are looking for in this case.

2) Near vision effectivity error is the effective reduction in addition power that occurs in thick, curved spectacle lenses compared to thin, flat trial lenses. The image vergence L' through a lens for a near object vergence L is given by:



where F₁ is the front curve, F₂ is the back curve, n is the refractive index, and t is the center thickness in meters.

If you solve this equation for two lenses of equal back vertex power and the same near object vergence L, but with different front curves and/or center thickness values, you will see that the image vergence L' changes. The near vision effectivity error is associated with this change. You can read more about it in Jalie.

3) Scatter or reflection from the segment ledge is certainly a potential issue. For fused bifocals, you will also have a small amount of reflectance at the boundary between the segment and the major portion. They may also want you to discuss the specular reflections associated with high-plus lenses, although this isn't really specific to bifocals. If anything else comes to mind, I'll let you know; I've never really thought too much about this particular topic.

[uncut]

Hi Darryl.......uuhhhmmmmm I think the exam was a couple of days ago.

[Darryl Meister]

Oh, I think you're right; just missed it!

[uncut]

Oh, well! @ studentdo....We hope the final went as planned.

[CCGREEN]

This is for Darryl and Studentdo. If the pt is NOT currently wearing dissimilar seg higths. Just DON'T go there. You could be opening a very frightening can of worms.....but then again going with Darryl's example you may be the best thing next to sliced bread for the patient and will have a follower for the rest of your career. Yes I know the exam was a few days ago but maybe Studentdo will look back over these post someday.

[Darryl Meister]

If the pt is NOT currently wearing dissimilar seg higths. Just DON'T go there.

I certainly agree the patient's previous eyewear should evaluated before doing anything potentially radical. But we must remember that this question was in the context of a test, so they are probably looking for "theory" and not necessarily "practice."

[EyeFitWell]

?? I would have to look back through my notes, but I seem to remember the rule being different heights of two mm, split the difference. If the heights differ by three mm or more, go with the split segs. In my classes, these questions were frequently designed to be sure you know the cut-off point. For example, here it seems the question is trying to see if you know when to opt for different seg ht's as opposed to splitting the difference.
Again, this is what I remember for the TEST, not in practice.

You guys seem to dislike the idea of different seg ht's, I must ask: what if this is the first bifocal? I don't think I've really ever come across this in practice except once or twice and only in progressives so I wasn't worried about cosmetics. I totally agree with an "if it ain't broke, don't fix it" mentality, but if this is the pt's first bifocal, and the seg ht's are decidedly different, wouldn't it be best to fit as measured? If not, what are the possible issues with doing so?