Superstition or wasted idea

[OdTech]

Hello there again

Most of the time the optician have the superstition which is "Always start with the right lens" so to check it out if this is true i started with the left lens and did it right but when i started with the right lens i spoiled so in this case it's true i mean when you neutralize or edge.

However, i told my optician but what if the left lens has the greatest power than the right, like anisometropia kind of people, although the optician has a superstition.He told me "always start with the lens that has the strongest power" either to neutralize or edge:drop: :bbg:

Now tell if the two are contradictory: "Always start with the right lens" & "always start with the lens that has the strongest power"

Thanks for reply and viewing:D

[Chris Ryser]

Can you repeat this psoting so I can get it a bit clearer.............
Thge way you sayu it i would say start with the middle lens.

[yzf-r1]

This is a rule that has been drilled into me ever since i had an apparent problem with a patient who was R -1.00 L -5.00

When i was checking the right lens first, i was coming up with differential vertical prism. I nearly sent it back when i decided to get a colleage to check it because she had joined recently and i though it was a good training tool. She started with the left lens, and could not find any vertical prism.

The theory behind why this happened im sure you all know, but if you dont, post back and im sure jeff will give you the full monty of induced prism and P=cF:D

[harry a saake]

:D OD Tech, since glasses are an optical/mechnical device, they are seldom perfect in every aspect. In a perfect situation, each side of the frame being exact, each lens cut exact, with exactly the same bevel, etc, in theory it would not really matter which side you checked first. Thats in theory, once in awhile this will even happen.
.....However whenever you have those type prescriptions with a large difference from one eye to the other, you will always check the stonger one first and then look at the prism imbalance if any.
.....Reason is simple, if the oc is 31 high from the bottom on one lens and 34 high on the other you have an imbalance. Quite obviously the higher the power times the same distance will give you a higher reading, misleading you.
.....In the above example if one eye is -1.00 and the other -5.00 and you looked at the right eye(-1.00) and then went to the left, you would read about 1.5 degrees of prism(3x-5.00)=1.5
.....on the other hand if you read the left(-5.00) first an then the left 3x -1.00 would be only apx .30 well within tolerance.

[OdTech]

:D I thanks everyone who replied and clarified the topic of this discussion even though for me it was a big experience of having the fun out of opticianry. Whatever was replied will be remmembered as the lesson well learned in life!

[Optical Plumber]

I am not really confused, just somewhat puzzled!

(please don't ask me the difference! I just don't like the Smilie for 'confused', and would sometimes prefer to have one for 'puzzled' instead!)

... getting to the point... It seems obvious that if you are checking for vertical imbalance, then computing the prism on the lower power lens would make more sense because it will result in fewer rejections, but what is our ultimate goal here? Is it to reduce the number of rejects, or is it to provide the best possible vision correction for the patient. If your example of -5.00 power was 3mm above or below the designed major reference point, does this all by itself not represent an exception to the ANSI standard regardless of any vertical imbalance that may exist with the other eye?

Granted, vertical imbalance is a much bigger problem than BALANCED vertical prism at the MRP (we do it on purpose all the time with progressive lenses), but why should the patient be expected to accept ANY unwanted prism at the MRP without a good reason (such as cosmetic thinning)?

Keep in mind that I am not an optician, so I could be out in left field here. Please feel free to correct me if I am!

Terry

[shanbaum]

See ANSI Z.80, section 6.1.3.2.1 for a description of how to measure for vertical imbalance (hint: it starts out, somewhat ungrammatically, "select the lens with the strongest power...")

As to why the patient should accept anything less than perfection in his eyewear, there's a very good reason: none of the elements involved in the fabrication of eyewear is perfect; not the lenses, not the machines, not the operators.

Well, the software might be perfect...

[Optical Plumber]

shanbaum said:
... As to why the patient should accept anything less than perfection in his eyewear, there's a very good reason: none of the elements involved in the fabrication of eyewear is perfect; not the lenses, not the machines, not the operators...

I am not disputing the ANSI standard, just trying to understand why 1.5 diopters of UNPLANNED vertical prism would be acceptable at the MRP, irrespective of any vertical imbalance that may or may not exist.


Terry
:)

[shanbaum]

Both ANSI and ISO standards specify maximum amounts of unwanted prism which are qualified by maximum amounts of centration error. The latter over-ride the former on higher-powered lenses.

In the ANSI standard, the allowable centration error is 2.5 mm, so a 3mm error on the -5.00 eye would not qualify.

[shanbaum]

I didn't see where the patient's actual pupil height was disclosed, only that the PRP heights differed by 3mm.

Insufficient information, captain.

[harry a saake]

Perhaps i was not clear enough in what i meant, i was just trying to relate the difference in millimeters. It does not matter where on the lenses the ocs are, above or below the datum line if there 3mm apart vertically, there is prism.

[Optical Plumber]

When a SV Rx is prescribed without a specific OC height, the assumption is that the OC is to be placed on the datum line. If the OC on the -5.00 eye actually ended up 3 mm above or below the datum line, would this not exceed the ANSI tolerance for unwanted Prism regardless of whether you measured the right eye first or the left eye first, or the weaker power first, or the stronger power first or even if it was just a single eye job?

I know that in the case of progressive lenses, this standard is not strictly adhered to in order to allow for prism thinning, but what about SV?

Sorry to be such a pain, but I am just trying to understand how this particular standard is applied in practice.



Terry
:)

[shanbaum]

Harry: without reviewing the sample - my point was this: if the patient's pupil height is 34, and the -5.00 PRP is 34, and the -1.00 PRP is 31, the prism is within tolerance (0.3D imbalance). Reverse the heights & powers, and it's not.

...ignoring pantoscopic angle.

Terry: I would say that the conventional practice is to not pay much attention to PRP heights on single vision lenses unless imbalance is detected, or the centers are off in left field somewhere. (That last bit is an American idiom).

Oh, and there's no standard that dictates that the "OC" on a single vision lens must be on the datum line when unspecified - it's conventional, but not normative. So the answer to your question is, no.

[Optical Plumber]

thanks Robert. It is making sense now.

Terry
:)