Base curve question

**bandk** · 08-07-2001, 03:43 PM

Here's an amateur for you guys but here it goes.

When a prescription calls for a base curve of 4.75 and the glasses were made with a 4.00, is that going to largley effect the prescription and how the consumer sees out of them?

Thanks so much for your help.

RT · 08-07-2001, 05:56 PM

According to the ANSI standard, if the base curve is within 0.75 diopters of the requested base, the lens is within tolerance. Therefore, in the example you cite, there is no problem. The difference is probably due to which lens manufacturer your lab happens to have in stock, and except in rare cases shouldn't represent a problem for the wearer. UNLESS, of course, the lens in question is an aspheric or progressive where the 0.75 diopter difference actually means that you've gone to another base (which implies subtle differences in the design on modern products).

Another situation in which that big a difference could be problematic would be if the order was for a single eye. In that case, there could be cosmetic as well as visual problems with the job if there was 0.75 diopters difference in curve between eyes that had essentially the same Rx.

Of course, one has to wonder what's going on when the prescription calls for a 4.75 diopter base curve. Sounds suspicious to me.

RT

**Texas Ranger** · 08-08-2001, 08:14 AM

bandk, I routinely request base curves on most of my surfaced uncut orders;I check what base curves are available, it's those gap rx's that cause the problem, but if you have a patient with several pairs of glasses, base curves can be a major issue. base curves run oddly, some lens designs run even numbers like 2,4,6,8 while others run 3,5,7 etc. there ar sometimes reasons to flip-flop base curves on a job, but I don't think .75 +- would be a problem in any way. it's not like you have that sort of control, since the base curve is a lens manufacturing issue.

**Pete Hanlin** · 08-08-2001, 09:43 AM

Usually, you should be fine within 1.00 diopters of the specified base curve. Naturally, there are those patients (you know, the mechanical engineer who has his axis numbers specified in 1/2 degree increments) who seem to be able to spot any oddity in the lens parameters...

I don't think the ANSI standard is actually talking about adherence to a specified base curve, except to say that the lens cannot be warped beyond a particular amount. I could be wrong.

In any case, if your patient is having a hard time tolerating the new spectacles, I would use the base curve discrepency diagnosis on an exclusionary basis (exclude every other possibility, such as tilt, wrap, v.d., lens thickness, index, etc. first).

Good luck!

Pete

**Jim G** · 08-08-2001, 10:14 AM

ANSI Z-80.1. "5.1.6 Base Curve When Specified, the base curve shall be supplied within +/- 0.75 D. The base curve shall be measured using the method described in 6.5. The base curve shall be given using an assumed index of refraction of 1.530."

Now, anyone want to discuss a dispenser insisting on keeping a 4 base on a myope whose Rx is now -6.00? Happens all the time.

**Jeff Trail** · 08-08-2001, 10:56 AM

Some "small" things to consider that are not really covered..such as what if it is a product other than glass (1.523).. or if it is an aspherical to spherical design of curve? ...there a lone you need to drop around 2^ of curve if you are going from spherical to aspherical.. What about index? ...and trying to atleast attempt to matching ocular curves when you have an RX change... change the index from say a CR39 (1.49) to a 1.60 index can change the base and cross upto and over a full ^ of curve....
I kind of have an advatage I guess being a "smaller" lab and having the same accounts for years and that is the OD's and opticians I work with will supply me the old RX and the new with the old base curve and leave it up to me when we start moving the curves around... plus they often call about RX's, especially when it comes to things like lash crash and those stinking wrap frames that -5 myop just "has to have"..
I think it's more important to actually understand what is happening "optically" when you start to tinker with curves.. knowing spectacle magnification formula's sure comes in handy when you start wanting to tinker with the base curves and RX's...also knowing what is happening along the optical cross and what you are doing to off axis powers and the amount of periphral abberation ...
Gee, sometimes "knowing" stuff helps :-) ... that, or having a lab you can ask the questions and get the answers...I spoil my accounts I guess :-)

Jeff "-9 in a wrap?..sure, if they want to see around corners" Trail

**Pete Hanlin** · 08-09-2001, 09:25 AM

Well, I got out my copy of ANSI standards and pow, there it was in black and white just as Jim described. Now there are two portions of ANSI which I shall disregard as spurious. (I know, you're thinking "Gasp, disregard ANSI"... Hey, they are only standards, and in two cases they are neither practical nor reasonable.)

Pete "Rebel with a cause" Hanlin