Help! Seg height and lens sizing question.

[KyleKessler]

Ok, here goes. I have a patient that has a rather large head, and I am fitting him for FT-28 bi-focals. He is in a drill mount. I have determined 2 things. His lenses should be edged 6 mm larger than they are now to accommodate his head size, and the seg height should be dropped about 2 mm from what he has now for a more comfortable fit. Currently the measurements are A=53 B=34 Seg height=18.

If I have them run 6 mm bigger that makes the measurements A=59 B=40 what should my seg height be?

Thanks in advance!!

[HarryChiling]

A = 53
B = 34
Seg Hgt = 18

If you were to make the A size 6mm bigger that isn't going to make the B size 6mm larger. You could multiply the new A size by the ratio of the B/A = 0.64. So the new A size would be 59 and the B size would be 59*0.64 = 38. Since the seg hgt is a measurement from the bottom of the lens, the increase in B size is equally applied to the top and bottom of the lens, but the bottom of the lens to the seg line is going to change by half the difference between the new B and the old B. A simpler method is that the vertical decentration would remain the same. Of course if you are moving the drill holes then this will change everything.

New seg hgt = 20mm

[KStraker]

I would set up a proportion: 16/34=X/40 so X=18.8
It comes out in a similar spot, 1 or 2 below center.


harry, she said that 18 was 2 too high, so you'll need to refigure with an original ht of 16.

[DragonLensmanWV]

In other words, you will only need to recalculate the horizontal inset. If, for the standard eyesize you have say, 6 in and 5 down, you would keep the 5 down and adjust the inset.

[finefocus]

too complicated. the size will go up 6mm; that's 3 above geo and 3 below geo, so raise the seg ht 3 mm to maintain the statis quo - we don't want statis quo, we want 2mm less, so raise
only 1 mm

[jimrask]

is edging a pair of triel-lenses an option? If we change dimensions on a shape, we first make that shape with trial lenses, mount them in the frame and show it to the client. If OK, then we do the measuring. a bit more work, but my clients appreciate it if they can see what the changes will look like before they decide to purchase.

[finefocus]

If you were to make the A size 6mm bigger that isn't going to make the B size 6mm larger.

Sure it will, unless you change the pattern. If a hard pattern, the difference can't change. If an electronic, vector-based pattern, it can be changed but why would you?

[optical24/7]

Draw a 180 line from the nasal drill holes. Measure mm's below this.

[obxeyeguy]

Pay attention here people. This guy wore a pair of specs 3 sizes too small, and the seg 2mm too high for who knows how long. Fit it 3 below, and be done with it, way too much thought...;)

[finefocus]

Pay attention here people. This guy wore a pair of specs 3 sizes too small, and the seg 2mm too high for who knows how long. Fit it 3 below, and be done with it, way too much thought...;)

Do you mean 3 below geometric center? Currently it's 1 above. If it's 2mm too high, then 1 below would be correct.

[Judy Canty]

Ok, so the original B is 34. The original seg is 18 hi
34 divided by 2 = 17.
Original seg is + 1.

New B is 40.
40 divided by 2 = 20.
New seg needs to be lowered by 2, so new seg is - 1.

Right?
OBX is right...too much thinking.

[finefocus]

Ok, so the original B is 34. The original seg is 18 hi
34 divided by 2 = 17.
Original seg is + 1.

New B is 40.
40 divided by 2 = 20.
New seg needs to be lowered by 2, so new seg is - 1.

Right?
OBX is right...too much thinking.

Except that you disagree with him by 2mm

[Judy Canty]

...about the thinking part. :shiner:


3 below / 1 1/2 in was the "go to" for FT's in our day!

Of course, I'd also recommend cutting a demo out of mylar to make sure that the B won't be too deep cosmetically and for a proper fit. 6mm is a big jump in size and it may not have the appearance that the customer wants.

[KStraker]

Ok, so the original B is 34. The original seg is 18 hi
34 divided by 2 = 17.
Original seg is + 1.

New B is 40.
40 divided by 2 = 20.
New seg needs to be lowered by 2, so new seg is - 1.

Right?
OBX is right...too much thinking.

I agree with the too much thinking. We learned to do these kind of proportions in elementary school.

[Barry Santini]

Sure it will, unless you change the pattern. If a hard pattern, the difference can't change. If an electronic, vector-based pattern, it can be changed but why would you?

Asthetically, correct size-proportioning of the shapes requires a non-linear adjustment in the difference.
Harry C is spot -on here.
That said, what you "don't" realize is that the bridge placement *also* requires adjustment for proper asthetics. Besides, id your doing so much custom work, why not offer superior bride fit?

I do the folllowing:

1. ask the client to try on the simless frame in question. Fit the bridge pads. Then ask and r-eask them to place the "frame" on their nose where they fell most comfortable. Caution: Habitual placement may not be the same as preferred placement.

2. Make the pupil position with this bride fit position - lens shape does not matter.

3. Now, place the actual shape being made in front of the eye, bein careful to place the shape in the desired "brow" position (discuss with client). This shape may not even be a shape you have mounted in your rimless samples. In my office, every shape on the boards is fair game. Mark the pupil.

4. Note if the bottom of the shape needs to be altered for any reason, i.e., lens-too-short, to mask shadow lines, cheek clearance, etc. If needed, note the increase of decrease you require in the *bottom-half" of the B dimension to fit accordingly.

5. Now, superimpose the pupil mark of the bridge-fitted shape against the "brow" placed shape. Quantify the bridge placment adjustment bertween the two to combine the benegits of both.

6. Make any adjustments in pupil position base on size/further b adjustments.

7. Voila! Yer done!

Seems complicated. But its not. Betch everyone here does some variation of this if they're rountinely fitting custom rimless.

The above being said, I do the same as jimrask very often:

is edging a pair of triel-lenses an option? If we change dimensions on a shape, we first make that shape with trial lenses, mount them in the frame and show it to the client. If OK, then we do the measuring. a bit more work, but my clients appreciate it if they can see what the changes will look like before they decide to purchase.

FWIW

Barry

[finefocus]

Years ago, when frames came in 2 or 3 or 4 sizes, American frames generally were a constant difference. A 46 eye Sirmont measured 46 x 39. A 50 eye was 50 x 43, still 7 difference. When European zyls started to arrive in force, we often saw different differences; the A would increase with labeled size, but the B wouldn't, at least not as much. Nonetheless, if I order a Silhouette 3-piece with shape 6682 (the sample for which is a 48 eye) as a 50 eye, the finished lens will be 2mm larger in both meridians. The original example in this post followed this pattern, in that the A and B increased equally. In rimless, it's possible to manipulate and customize the shape as Barry describes above, in which case you add to or subtract from the new seg height in a 1-to-1 ratio to the increase/decrease below the original geometric center.

[HarryChiling]

That said, what you "don't" realize is that the bridge placement *also* requires adjustment for proper asthetics.

This is the one part of the equation that can't be computed. Often the drill holes will move up from their current position, to keep the correct proprotions, however the correct proportions of the drill holes can mean that the top edge of the lenses now sit on the patients brows. Custom work is fun but challenging, it is always a good idea to make a mock up for the patient to approve and charge appropriately.

[HarryChiling]

Sure it will, unless you change the pattern. If a hard pattern, the difference can't change. If an electronic, vector-based pattern, it can be changed but why would you?

In this example "A=53 B=34 " if the difference is 6mm larger and your saying that 6mm all the way around would be correct then the new demensions would be:

A = 59
B = 40

That would mean that the

A increased by (59/53 - 1) * 100 = 11%
B increased by (40/34 - 1) * 100 = 18%

Your pattern suffers from "aniseikonia"

You could set the B so that the increase in percentage remained the same and solve for the new B size:

(x/34 - 1) * 100 = 11%
x/34 - 1 = 0.11
x/34 = 1.11
x = 34 * 1.11 = 37.7 = 38mm

See my above post for a simpler method in which the ratio of A to B size is used.

Years ago, when frames came in 2 or 3 or 4 sizes, American frames generally were a constant difference. A 46 eye Sirmont measured 46 x 39. A 50 eye was 50 x 43, still 7 difference. When European zyls started to arrive in force, we often saw different differences; the A would increase with labeled size, but the B wouldn't, at least not as much. Nonetheless, if I order a Silhouette 3-piece with shape 6682 (the sample for which is a 48 eye) as a 50 eye, the finished lens will be 2mm larger in both meridians. The original example in this post followed this pattern, in that the A and B increased equally. In rimless, it's possible to manipulate and customize the shape as Barry describes above, in which case you add to or subtract from the new seg height in a 1-to-1 ratio to the increase/decrease below the original geometric center.

"The phenomenon of nonproportionality that occurs is more evident the larger the pattern size is than the actual size of the lens to be ground. It is therefore advantageous to utilize a pattern as close in size to the finished lens as possible. Some manufacturers provide different patterns for different groups of eyesizes for this very reason. Others furnish one size pattern only and manufacture frames that differ slightly in proportionality from size to size. In this manner any sized lens edged with the pattern provided still fits the corresponding eyesize frame."

Brooks, CW. (1983). Essentials for Ophthalmic Lens Work. Woburn, MA: Butterworth-Heinemann; pg 99

Frames made in the US accounted for this difference in proportionality, but it was known then that the lens shape distorted that's why a small range of eye sizes was available in every frame model, too great of a difference in eyesizes and the distorted shape became more apparent. Thanks for the walk down memory lane, but customized rimless should not be compared to the patterned days of the past, with computerized patternless edgers keeping the proportionality of the shape is not a difficult thing to do so the old tricks of past are no longer relevent.

Barry,

I too would allow patients a run of the boards when shapes were being customized, I also allowed any images, items or shapes to be used and a few patients did tempt me. Of course I did not have access to a router type edger so the radius of the the edgers wheel was my limiting factor. With todays routing edgers almost any shape is possible.

[Uncle Fester]

To get the height I'd put a piece of tape on the small lenses at the height they want.

Measure it.
Add 3.

That's the height! Half of new larger B 40=20 minus new height=below or above (mechanical) center.

[finefocus]

To get the height I'd put a piece of tape on the small lenses at the height they want.

Measure it.
Add 3.

That's the height! Half of new larger B 40=20 minus new height=below or above (mechanical) center.

Exactly right.

Harry, I get what you're saying about proportionality, but the original post said that the B would be 6mm larger. Whatever the percentage might be, 6mm is 6mm, and unless special steps are taken, half of that would be below geo. Would the drill points be altered for aesthetic reasons? The post to which all this is a reply doesn't say so. Also, one might place the bridge differently, to make the frame and lenses look better together, and then alter the pad adjustment to keep the lenses centered vertically as before, preserving the 3-up-and-3-down character of the size increase.

[HarryChiling]

Exactly right.

Harry, I get what you're saying about proportionality, but the original post said that the B would be 6mm larger.

The original poster did say the B would be 6mm larger, that's why my response was:

If you were to make the A size 6mm bigger that isn't going to make the B size 6mm larger.

I tried to address the issue of distortion. When I saw offices in my area try and emulate my customized eyewear this was the most common complaint from patients the shape didn't match the one they picked out originally. I made an attempt to address this in an example using the same variables that the poster presented, this would allow him to save face if he thought that the difference would be applied equally. Had he come back a2 weeks from know and said the shape looks different when he actually made it the responses might of been, "well duh, it's because the proportions are different".

Whatever the percentage might be, 6mm is 6mm, and unless special steps are taken, half of that would be below geo.

That is why I made sure to mention:

Since the seg hgt is a measurement from the bottom of the lens, the increase in B size is equally applied to the top and bottom of the lens, but the bottom of the lens to the seg line is going to change by half the difference between the new B and the old B.

Would the drill points be altered for aesthetic reasons? The post to which all this is a reply doesn't say so.

The original poster might not know this would be required that's why this line in my reply:

Of course if you are moving the drill holes then this will change everything.

Barry also pointed out this phenomenon:

That said, what you "don't" realize is that the bridge placement *also* requires adjustment for proper asthetics.

Thes phrases come from experience, I had to mess a lot of custom jobs up to get this bit of knowledge and the original poster would get it without even asking for it or knowing it would be an issue. Any office doing custom rimless will realize this if they make any significant change with 6mm being considered significant. Now for those doing 1 to 2mm differences then all this might seem moot, but the larger difference the more evident the distortions and misplacement of seg hgts and drill holes becomes.

Also, one might place the bridge differently, to make the frame and lenses look better together, and then alter the pad adjustment to keep the lenses centered vertically as before, preserving the 3-up-and-3-down character of the size increase

If you are customizing the shape you might as well customize the fit. There is no need to maintain a "3-up-and-3-down character of size increase". Part of my response goes into the distortion created by that particular relationship, why would an optician create a pair of glasses knowing that they would need to fix an error by adjusting the bridge into a non-neutral position to get a proper fit? Part of an initial fitting is to place the nosepads into a neutral position, this allows for some ability to adjust the height higher/lower on final dispense if necessary.

[I-Care]

I agree!