Base Curve/periphery issues

[Quantrill]

So a patient comes in with a high minus Rx (I should have written it down to share, right? I forgot. ). Total power in each eye is around -5.00.

Patient was previously in polycarbonate lens without AR, 4.25 base.

Patient is now in High Index (1.67) with AR, 7 base. Patient is complaining about peripheral vision being 'blurry'. Vision is otherwise fine, and frame is only slightly wider than previous frame.

Now if I remember right - the higher the index, the base curve should go in the opposite direction if you are switching materials (i.e. high minus in poly would be a flatter curve than high index).

But this rule changes with aspheric lenses, right? I am unsure if the customer was previously in spherical poly or aspheric (customer went somewhere else before coming to us).

So the high index lens should be a steeper curve, correct?

What is the correct call in this situation? Put the customer back into a poly lens, but keep the AR (customer seemed very keen on the benefits of AR but the previous eyeglass hut apparently did not mention this) or maybe do a high index in a different base curve?

I'd really appreciate some feedback on this...

[mshimp]

Was the frame an extreme wrap? If so this would account for the 7.00 base curve. Otherwise it should never be on a 7.00 base curve with a
-5.00.

[Quantrill]

Frame did not have a wrap. So the high index should have been made in the same base curve even though it's a different material than the previous pair?

[braheem24]

Yes, you can use a 4 base for the 1.67. Peripheral optics are just as crappy out of either material so it should solve your dilemma unless you had a big rx change especially in cyl I would then switch to 1.60 for better optics then both poly or 1.67 and match the 4 base.

[jpways]

From personal experience (total power -6.50) the higher the base curve the more distortion I experienced (until I switch to a different lens) for a poly my base curve was between 4 and 4.5 as I moved up to 1.67-1.74, aspheric non-digital. My base curves dropped to 2 for 1.67 and 1-1.5 for 1.70-1.74.
I notice a definite distortion in non-digital lenses at my periphery, as most higher myopes do, as my edge thickness increases. By switching to a higher base curve you are making a thicker lens, hence more distortion. Did your lab pick to 7.0 base curve or did you order it that way?
I do have to note on the digital lenses I have 1 being a Nulux 1.7 (base curve 0.50) and the other an Essilor 360 1.74 (base curve 0.75) I have no peripheral distortion and with the flatter base curves and digital surfacing, and are the thinnest lenses I have in my 3 piece drill mounts.

[snowmonster]

I can't believe somebody ran a -5.00 on a 7-base unless it was for a wrapped sun Rx job.

[LandLord]

I also cannot understand why anyone would put a -5.00 on a 7 base. Especially when the patient was wearing a 4 base previously.

Change back to a 4 base and the patient will see better than ever before.

[YrahG]

Originally Posted by snowmonster

I can't believe somebody ran a -5.00 on a 7-base unless it was for a wrapped sun Rx job.

I'm with you I like the minimum T error design, the 7 base isn't totally off though that would offer minimum oblique astigmatism. If we were to look at the lens in poly in the 4.25 base you'll notice that the design is a minimum T error design so in this case it is the optical design that is the issue.

[deejc9]

You can check with the manufacturer to see what base curves are the flattest. Also, since the materials are different you have to take that into consideration. I would try changing it to a flatter base curve and start from there. I hope this helps.

[Quantrill]

The lab picked the curve. Unfortunately the associate doing the original transaction did not check the base curve, thinking that whatever the lab picked would be fine, since the customer would be getting a better material and the addition of AR.


Thanks to everyone for their replies.

[jpways]

Originally Posted by Quantrill

The lab picked the curve.

I'm scared to ask this question, but did the lab give you a reason for picking such a steep base curve?

[Quantrill]

Originally Posted by jpways

I'm scared to ask this question, but did the lab give you a reason for picking such a steep base curve?

I will be doing some investigating trying to find out why this curve was picked. While we sometimes have trouble with lenses arriving with scratches or power slightly outside of tolerance, I cannot recall a base curve problem like this before.

I will try to keep you all updated on this, as I am not happy that this customer will have to make another trip (though we certainly could have at least checked the base curve of the lens that arrived before edging, thus saving a trip for the customer).

[scriptfiller]

That lab must have wanted to move some slow selling inventory to do something like that. Bad lab! tisk tisk tisk...

1.60 would provide cleaner optics.

[snowmonster]

Originally Posted by scriptfiller

that lab must have wanted to move some slow selling inventory to do something like that. Bad lab! Tisk tisk tisk...

bingo!

[Charvak]

Base curve doesn't have to be matched with an aspheric lens because the whole point of an aspheric lens is to allow a flat base like 0.5 without inducing oblique astigmatism. You could test whether the issue is chromatic abberation by seeing if it goes away when viewing a narrowband light image. I should write instructions for using mine and post it online.

But really, what kind of frames use a +7 base and -12 back curve with an aspheric lens? It must look warped.

I'm looking forward to hearing your next update on the problem.

[drk]

I need to question my assumptions.

1. When choosing a new lens material and design (and lens manufacturer) you are very frequently going to get a different lens form--spherical to aspheric; one aspheric curve flattening rate to another; etc. etc. You can't hold things constant.
2. "Base curve" on aspherics are not measurable...so put your lens clock away.
3. (What is the name for the "base curve" on aspherics, again?)
4. Labs use computer programs to determine base curves/nominal curves (whatever it's called). It's generally not a discretionary parameter.

Thank you.

[Charvak]

1. True. I do wish labs were held to more transparency in terms of publishing the specific equations or design parameters going into their particular lenses so dispensers and customers could make informed comparisons.
2. While the curvature on an aspheric isn't constant throughout (hence the name), you can still tell the difference between a 1.25 and a 3.75 curve. The lens clock can give you a general idea of the approximate base curve.
3. I don't know the answer. Are there lens clocks that can take more localized readings of curvature than the standard ones with pins spaced around 1 cm apart? Maybe it's difficult to make them precise enough to reduce the pitch, given the variation in readings I get on the same lens.
4. The base curve is certainly a discretionary parameter, but labs will pick the most suitable one for your frame and prescription. If it's spherical, they'll choose a "best form" curve regardless of frame. If it's aspheric and a normal frame, they'll go with something as flat as possible to make the lens thinner. If you have a good reason to want a more curved lens, you could request one. For instance, I think there is a Sola lens that sort of wraps around with a very high base curve for the best possible vision, but it won't fit in a normal frame.

[YrahG]

Originally Posted by drk

"Base curve" on aspherics are not measurable...so put your lens clock away.

Actually you would need your lens clock, calipers, and a lensometer.

Start by finding out the power of the lens in the lensometer. Next measure the center thickness using the calipers and the back curve using your lens clock.

The clocked reading must be converted to the material index, once that's accomplished use the back curve power and the thickness to find the front surface power.

F₁ = Front curve
F_v = Lensometer reading
F₂ = Back curve
t = thickness
n = index

F₁ = (F_v-F₂) / [(1 + t/n) * (F_v-F₂)]

Now with a front curve, you can take an additional measure of the front curve of the actual lens with your lens clock and convert that curve into the materials index. Knowing the actual front curve (aspheric) and the calcualted front curve (spherical) you can take those two measures and find the aspheric value. Knowing that our lens clock uses pins at a diameter of 50mm apart then the sag equation for the calculated front curve would be:

sag = r - (r² - 25²)^1/2

subbing out the radius with our diopter value:

sag = [(n-1)*1000/F₁] - ([(n-1)*1000/F₁]² - 25²)^1/2

since all values in the above equation are known the sag value can be use in the aspheric sag equation with the known aspheric surfaces values to solve for the unknown aspheric coefficient:

p = aspheric coefficient
F_1clocked = Clocked front curve
sag = same as the equation from above

sag = 25² / ([(n-1)*1000/F_1clocked] + {[(n-1)*1000/F_1clocked]² - p*25²}^1/2)

I was half tempted to be a jerk and write out the equation without steps for posterities sake, but I would more than likely have made a few mistakes or it would have been extremely difficult to read.

Of course this is for lenses that have an aspheric front and spherical or toric back surface, if the back is aspheric or atoric this equation will not work.

[Quantrill]

Originally Posted by Charvak

I'm looking forward to hearing your next update on the problem.

The new lenses arrived last week. The patient says everything is 'fine' now, so presumably the base curve was the issue.

Nobody has given me an actual answer as to why the steeper curve was used the first time around. "We use whatever the computer calls for" is all I could get. I asked if a 7 base was typical for an Rx like this and was told "that does seem a bit high."

So no real answers on this but I will say this is the first time I've encountered an issue like this from that lab so hopefully it was just an extremely isolated incident.

[drk]

Anyone who can post with superscripts or subscripts is my hero. Nice post.

Originally Posted by YrahG

Actually you would need your lens clock, calipers, and a lensometer.

Start by finding out the power of the lens in the lensometer. Next measure the center thickness using the calipers and the back curve using your lens clock.

The clocked reading must be converted to the material index, once that's accomplished use the back curve power and the thickness to find the front surface power.

F₁ = Front curve
F_v = Lensometer reading
F₂ = Back curve
t = thickness
n = index

F₁ = (F_v-F₂) / [(1 + t/n) * (F_v-F₂)]

Now with a front curve, you can take an additional measure of the front curve of the actual lens with your lens clock and convert that curve into the materials index. Knowing the actual front curve (aspheric) and the calcualted front curve (spherical) you can take those two measures and find the aspheric value. Knowing that our lens clock uses pins at a diameter of 50mm apart then the sag equation for the calculated front curve would be:

sag = r - (r² - 25²)^1/2

subbing out the radius with our diopter value:

sag = [(n-1)*1000/F₁] - ([(n-1)*1000/F₁]² - 25²)^1/2

since all values in the above equation are known the sag value can be use in the aspheric sag equation with the known aspheric surfaces values to solve for the unknown aspheric coefficient:

p = aspheric coefficient
F_1clocked = Clocked front curve
sag = same as the equation from above

sag = 25² / ([(n-1)*1000/F_1clocked] + {[(n-1)*1000/F_1clocked]² - p*25²}^1/2)

I was half tempted to be a jerk and write out the equation without steps for posterities sake, but I would more than likely have made a few mistakes or it would have been extremely difficult to read.

Of course this is for lenses that have an aspheric front and spherical or toric back surface, if the back is aspheric or atoric this equation will not work.

[harry a saake]

yrahg must be related to Darryl Meister

[drk]

What he said...

[braheem24]

Originally Posted by harry a saake

yrahg must be related to Darryl Meister

I though Chilly Harry and D.A.R.Y.L had a baby.