I’m researching new lenses, and after reading people’s explanations, I still don’t really understand “base curve”. I take it that a 6 base has a more curved front lens than a 4. But in practical terms, what does that do?
I have a high myopic prescription of about -5 I think. Would a higher base mean that the lens can be made with a thinner outside edge, because it’s curving with the inner edge more? I would count that as an advantage. Does it matter noticeably in the vision of the lens? Some of the Oakley frames I find the most attractive have a 6 base, but at first I was very hesitant to put them on the short list. Thanks!!!
The base curve does not affect the edge thickness, it affects how flat the front surface of the lens is. The power of the lens is determined by the difference between the base (front) curve and back curve, That’s a bit of a simplification, because the index of refraction of the lens material plays a part in it.
Also, you may be getting base curve and index of refraction confused, Base curve is usually described in terms of steeper or flatter. A high index of refraction does allow for a thinner lenses.
Ideally the back curve should match the curvature of the eyes, but sometimes with frames that have a big amount of wrap it’s necessary to make the base curve match the curvature of the frame so the lenses will stay in and look like they belong in the frame.
Wearing lenses with a radically different base curve than the ideal can result in things looking or feeling vaguely weird or even distorted. Some people are more sensitive to the effects than others, and the higher the prescription the more noticeable the effect will be.
For what it’s worth, I wouldn’t put Oakley frames on any list. They’re like the Apple earbuds of spectacle frames.
Maybe more than you want to know, but the tool for measuring base curve is called a lens clock. It used to be common for opticians to be able to calculate the power of a lens by measuring the front and back curves. Not too practical now, thanks to all the different high index materials available.
Can you expand on the Oakley/Apple comment? Do they have a quality issue? I’m wearing a pair I got on ebay five years ago. I replaced the temples because they had a strength problem, and I’ve had no problems until very recently. I just think they make sporty looking frames.
So you’re saying that the higher the base curve, the more likely it is that the wearer might notice a visual distortion?
Oakleys are just a mediocre quality frame at a premium price. They aren’t bad, but you can get the same quality in a much lower price, better for a lower price and much better for a similar price.
It’s more like the stronger the prescription the more likely the wearer is to notice, but the further the base curve is from the ideal (which could be too steep or too flat) the more likely it is to be noticeable as well.
With the proper base curve, the back curve will match the curvature of the eye better. The eye should as close as possible to the same distance from the back of the lens no matter what direction the eye is turned. If the back curve is too steep, the center of the lens will be further from the eye than toward the edges. If too flat, the center of the lens will be closer than toward the edges. This gives an alteration in the apparent magnification, similar to moving a magnifying glass closer or further from what you’re looking at.
Not easily. There are a lot of variables involved, and there are a lot of misperceptions about BC, and it’s pretty common for optometrists to specify curves that are nowhere near appropriate, usually based on guidelines from when glass and CR39 plastic were the only options. It’s best to let the finishing lab figure it out. They have special computers for it.
Here’s a paper (pdf) on the subject written by an ABO certified master optician. More than eye docs and even most experienced opticians know about base curves.
If by “thinner edge” you are concerned with weight, try Zenni Optical. We have ordered from them several times with excellent prices and results. As you build your glasses online, they will offer all the suitable lenses listed by index of refraction, with a recommendation. You can select from maybe a half dozen different glass and plastic lenses.
Fitting eyeglasses properly requires face to face time at the beginning and end of the process. There are measurements (OC heights, seg heights in multi-focals, vertex distances, etc.) that require the optician to see the frame on the patient. This cannot be done online any more than you could cut someone’s hair online.
