For nearly 30 years I've been fitting PALs and in all this time, as the manufacturers request, I've been checking the DV Rx in the semi circle that sits above the fitting cross.
A couple of days ago, during a fitting problem with a Pt, I checked the Rx at the cross and of course it was blurred....... which got me thinking.......
Why do we check the Rx 5mm above the point where the lenses are looked through?
That's why I fit the cross at the bottom of the pupil (very successfully).
While I'm in general agreement (I take a fitting height and subtract 1mm, except for Zeiss), I think one factor is pantoscopic tilt in the "as worn" position lowers the apparent seg height somewhat.
Another may be the larger aperture size of the lensometer, but I'm reaching on that.
Fitting at center pupil always seems to cause issues...fitting at the bottom of the pupil is the only way (at least in the last 10 years) to eliminate most fitting problemsOriginally Posted by Spexvet
As most of you already know, progressive lenses employ surface astigmatism in order to produce a change in add power without lines of demarcation or abrupt changes in vision. This surface astigmatism produces unwanted cylinder power and blur in the periphery of the lens. The change in add power occurs over the length of the progressive corridor, which connects distance zone to the near zone. The progressive corridor is only free (or at least nearly free) from surface astigmatism along the umbilical line of the corridor, which is essentially a path connecting the distance zone to the near zone that has equal horizontal and vertical curvatures at any point along it. Away from the umbilic, the horizontal and vertical curvatures begin to differ more and more, producing increasing levels of surface astigmatism.
In order to produce lower levels of peripheral blur and astigmatism in the distance zone, manufacturers generally design the progressive corridor to start increasing slowly at first. They then locate the fitting cross at the top of, but still slightly within, the corridor. This affords the wearer with more distance vision, without compromising the length of the corridor or the amount of eye depression required to reach the near zone. However, this design strategy has two important consequences for power measurement and verification.
First of all, conventional focimeters, such as the lensometer or vertometer, have rather large measurement apertures. When you measure a lens with this type of device, you are actually "sampling" an area of the lens roughly 8 mm in diameter (the size of the aperture). Consequently, these devices can "pick up" some of the cylinder power produced away from the umbilical line of the lens design, which will affect the power reading.
Secondly, there is generally a very small amount of plus power at the fitting cross, since the lens is actually changing in add power over this point. This plus power is too insignificant to affect the wearer's vision, but may still affect your power measurement to some extent. Moreover, the fact that the add power is changing along the corridor also introduces a small amount of unwanted cylinder power (more as a result of a coma-like effect, not the surface astigmatism described earlier), since the power at the top of measurement aperture will differ from the power at the bottom of the aperture (8 mm below). The wearer's pupil is generally much smaller than this, so this power difference is also insignificant.
Consequently, manufacturers design small regions in the distance and near zones, often referred to as the distance reference point (DRP) and near reference point (NRP), for power measurement and verification. The distance reference point, which is at the center of the distance checking circle, is generally located at the very top of the progressive corridor within a small region designed to be nearly spherical (free from surface astigmatism). This allows the dispenser/technician to verify the power of the lens without as many "optical side-effects" from the progressive optics.
Now, we could just locate the fitting cross 3 or 4 mm higher in the first place, which is equivalent to dropping the fitting height measurement, but this would represent an unnecessary compromise from a visual standpoint. After all, the wearer observes much less power error at the fitting cross than your B&L Model 70. It would also force wearers to raise theirs chins or drop their eyes that much farther in order to read comfortably (and that's assuming that the near zone isn't cut off by the frame). You should find that well-engineered progressive lenses are designed to provide sufficient distance vision at the fitting cross, without compromising near performance.
Best regards,
Darryl
O.K. Darrell you opened the box. What size would you suggest we reduce our arpetures to for accurate measurement?
Chip
I wouldn't. I would suggest that you use that distance checking circle and not worry about it. ;)What size would you suggest we reduce our arpetures to for accurate measurement?
Best regards,
Darryl
Thank you Spexvet, finally another optician that does it the same way I do and finds it very successful. I've had so many optician errors through my old manager because she didn't agree with my process of measuring for PALs (she didn't agree with anything), it was getting redicolous and just to please her I measured at few at pupil center and there we go, pt. problems. Thank you thank you thank you soooo much, you just made me believe in my abilities a little more. The old manager walked out on us about 2 weeks ago but I sure wish I could show her your post.
:cheers: Lets drink to the bottom of the pupil :)
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
Darryl,
Where do you place the fitting cross when you fit a PAL?
I have a question for you guys. Do any of you check the position of the cross when the patient stands up (and you stand up to)? I find that if you take one while standing up and one while sitting down, and then compromise then you get a much better measurement.
Well, "back in the day"... ;) Though there are a few contraindications that might suggest a slightly lower fit, I generally tried for pupil center. After fitting and adjusting the frame, I would then double-check the measurement by having the patient face herself in a wall mirror after dotting the demo lenses in order to verify the height of each dot relative to the pupil. This ensured a more natural posture and eliminated the possibility of parallax error. If the patient was unusually tall, had previously demonstrated a sensitivity to normal PAL fitting heights, or had been wearing progressivess with really low fitting heights already, I might have tweaked the measurement some.Where do you place the fitting cross when you fit a PAL?
Best regards,
Darryl
I've found that most of my "minus" wearers want their fitting at the lower pupil, and most "plus" distance wearers are happier with their fitting at mid or even slightly above mid pupil. Of course, if I can match the fit of their old glasses (providing they were happy with those) then I am happier!
Discussion during the fitting and having them look away and back and observing their posture helps to determine whether to drop the seg or not. Ask about sinus/allergy problems....those people hold their chins up more to help sinuses drain (sorry if that's gross), and they will want their fit a little lower than "standard".
Some of us are too short to make a good determination standing in front of the PAL wearer, but if you are tall enough, go for it but remember to scootch down appropriately if you are facing someone shorter!
Darryl, I always enjoy your answers because you give the scientific reasons for what becomes our "common sense" and "rule of thumb" stuff!
For many years now my compadre (Andrew Weiss) and I have had an ongoing disagreement over how high to fit progressives. He's a hyperope and I'm a presbyope. He fits center pupil, I fit base. We finally called the whole thing off when we realized it was as much patient preparation to accept a progressive as to how high it was fit! Proper patient selection, frame fit and lens decentration are the key factors we've come to realize.
That, come to think of it, opens another can of worms. Ever have to adjust the distant cross fit to accomodate an eye that doesn't converge the way the manufacturer assumes it will?
Ahh-- the joy's of the re-do!!
Fester
Thanks, Jenny.Darryl, I always enjoy your answers because you give the scientific reasons for what becomes our "common sense" and "rule of thumb" stuff
Best regards,
Darryl
Truth be known, how many remakes have you made that raise the fitting height? (The complaint would be "I have to lift my new glasses to read", or "I have to tilt my head funny to read".)
How many have you made that lower it? (The more common "these are blurry, you knucklehead".)
It seems that progressive designers, as wise as they are, get wrapped-up in the corridor-length concept (I think because it's where the technology is), and in reality, most people would rather have better distance vision (assuming full time wear).
So, to summarize, it's not unlike fitting segmented multifocals. Fitting heights of progressives (and designs) can be finessed for maximum patient satisfaction. Can't get that at all opticals!
The great thing about this thread is that it allows us to understand what's going on, and teaches us how to be expert.
"One pair of progressive lenses: $249.00. Comfortable vision: priceless."
It's certainly easier for the wearer to spot a progressive lens that has been fitted too high, since the blur from the add power in the progressive corridor will become immediately apparent. When the lens is fitted too low, it can become just as troublesome for near performance, though this is less likely to be obvious to the wearer -- especially upon delivery of the eyewear.Truth be known, how many remakes have you made that raise the fitting height? ...How many have you made that lower it?
Consequently, the "once bitten, twice shy" dispenser who has heard her patients complain about high fitting heights one too many times may start over-compensating by fitting them low, since this results in more immediate wearer acceptance. Ultimately, these wearers may not be any more satisfied than the ones who threw the high ones down in the office, though it would be more difficult for them to discover and articulate exactly what the problem is with their near utility.
They may not even be truly aware of the fact that they are making uncomfortable postural adjustments or experiencing reduced reading efficiency. Also keep in mind that near complaints for low fitting heights will often be related to a restricted field of view at near, which is a rather common wearer complaint. (Essentially, the wearer is forced to read through the top of the near zone or bottom of the corridor if sufficient postural adjustments aren't made.)
Best regards,
Darryl
Had a guy with a distance pd of 69, and a near pd of 63. He had worn PALs previously with little problem, but his add increased by .50, which must have narrowed the corridor near area. He had a heck of a time reading for any length of time. The answer? Not a remake. I sold him a second pair, FT 28 with intermediate in the top and near in the bottom. He uses them for the computer and when he reads for an extended time. The rest of the time, he wears his PALs, very successfully. When opportunity knocks...
:cheers:
(Darryl, while you're paying attention to this thread, post a new one, please, on the Spazio and EVS and the VooDoo!) (I'm starting to understand why Zeiss wanted Sola: innovation.)
With short corridor PALs, is this as much of an issue? After all, If I fit a Comfort dead center pupil, they have to look down about 18mm to get to the reading strength (or 85%, I forget which). But if I fit an Ellipse 4mm below dead center pupil, they still get the same reading strength 18mm down.
Does that sound reasonable?
People who know nothing about PALs presume that they are like an executive bifocal, with the entire bottom part progressing evenly all the way across the lens, and the distance perfectly clear.
Would someone please make a lens like this??? ;)
Not as much of an issue. The optics of a short-corridor PAL have already been somewhat "squeezed" together. Though I don't know that there would be much point in ordering a short-corridor progressive just for the sake of fitting it low. Keep in mind that many of these lenses don't have particularly large near zones, and most will have reduced intermediate utility. The levels of peripheral blur may also be greater or encroach farther into the distance zone. You don't get somethin' for nothin' in lens design. ;)With short corridor PALs, is this as much of an issue?
As a matter of professional etiquette, I try not to use the 'Board to promote our products, so you'll have to post a question if you want an answer. ;)Darryl, while you're paying attention to this thread, post a new one, please, on the Spazio and EVS and the VooDoo!)
Best regards,
Darryl
drk, why not ask Rinselberg? I think he's already done some of the research.
Or check:
http://www.optiboard.com/forums/show...ghlight=spazio
It may be a little off topic but i will ask anyway!
Do you people feel comfortable duplicating old Progressive lenses that are scratch to the extend that you cant even see any laser markings?
How accurate will be distance and near RX.
Lets say you use B@L70...
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