Base Curve Definition

[andr3]

I have an optics question. Maybe @Chris A Hardaway can help out here. How is the base curve defined and what is its unit?
My first thought was: it must be the radius of a sphere, like so:

CC BY-SA 4.0: Radius of curvature - Wikipedia

This article supports that for contact lenses at least (BCOR): Base curve radius - Wikipedia . That would mean though: the bigger the base curve radius, the flatter the curvature and vice versa.

Now, for (sunglass) lenses (?) the definition seems to be different - the bigger the base curve, the bigger the curvature (wrap).

Quite confusing. The base curve definition in this case can't be the base curve radius or the opening angle (as shown above). It could be something like the inverse.

Can you shine some light on my little pea brain, Chris (or somebody else with optics knowledge)? - Thanks!

 

[Chris A Hardaway]

No problem,

You are right. Radius of curve (contact lenses) basically makes a flatter lens a larger base curve i.e. 8.8 mm, and a more curved contact lens would have a smaller radius, so like 8.4mm.

On glasses/lenses the base curve isn't based on a unit of radius at all (though they are too big really to use mm anyway, maybe cm would have worked)! You are correct. It's confusing when you don't know where to find answers.. Thank goodness for the forum.

The base curve of glasses is based on diopters... Diopters are the same units used to measure RX power. Aka Sphere or cylinder power on your Eye Doctors script.

1 diopter is equal to the amount of a curve/thickness of lens that refracts (bends light) parallel light rays to a focal point at 1 meter.

F (focal length in meters) = 1 / Diopters

An example in a lens that was a +8.00 base curve on front, and a +0.00 base curve on back, would be a +8.00 sphere diopter lens. This lens would have a focal length of .125 meters, or 12.5 centimeters, or about 5 inches.

Because F (m) = 1 / 8 (D)

Long story short is that's about how curved many Oakley lenses are, 8.75 D. But both the front, and backside are 8.75D. front and back of lens are parallel, and have no power, hence the term "plano" lenses which means "level" or "flat"... A flat lens is technically 0.00 D on front, and 0.00 D on back.

Does this help?

 

[Chris A Hardaway]

I have an optics question. Maybe @Chris A Hardaway can help out here. How is the base curve defined and what is its unit?
My first thought was: it must be the radius of a sphere, like so: View attachment 740999
CC BY-SA 4.0: Radius of curvature - Wikipedia

This article supports that for contact lenses at least (BCOR): Base curve radius - Wikipedia . That would mean though: the bigger the base curve radius, the flatter the curvature and vice versa.

Now, for (sunglass) lenses (?) the definition seems to be different - the bigger the base curve, the bigger the curvature (wrap).
View attachment 741000
Quite confusing. The base curve definition in this case can't be the base curve radius or the opening angle (as shown above). It could be something like the inverse.

Can you shine some light on my little pea brain, Chris (or somebody else with optics knowledge)? - Thanks!

Ya that 2nd image is trying to over simplify base curve, which is ok. But you could have a "flat frame" with highly obtuse open angle like Oakley Mars that has "steep" 8.75D base curve lenses. The "open angle" is what is called "degrees of facial form" or "wrap angle in degrees"... Typical eyeglasses have 5° of wrap.... Oakley 8.75 base curve eyewear is above 18-20° of wrap. A 6.00D lens like Holbrook, or Sylas and Conductor 6 would be about 10-12° of wrap or facial form.

 

[SiRacer420]

I heard someone say Mars?

 

[Makten]

Long story short is that's about how curved many Oakley lenses are, 8.75 D. But both the front, and backside are 8.75D. front and back of lens are parallel, and have no power, hence the term "plano" lenses which means "level" or "flat"... A flat lens is technically 0.00 D on front, and 0.00 D on back.

This is a bit confusing. You don't mean that the curved lenses without power are called "plano"-something?
I'm deeply into photo nerdery and there a "plano-convex" lens is flat on one side and convex on the other side. A curved lens with the same radius on both sides is then a "meniscus" without power.

Lens - Wikipedia

en.wikipedia.org

Or am I misinterpreting what you wrote?

 

[Chris A Hardaway]

This is a bit confusing. You don't mean that the curved lenses without power are called "plano"-something?
I'm deeply into photo nerdery and there a "plano-convex" lens is flat on one side and convex on the other side. A curved lens with the same radius on both sides is then a "meniscus" without power.

Lens - Wikipedia

en.wikipedia.org

Or am I misinterpreting what you wrote?

Yes, curved lenses, same curve on front and back .... Or flat lenses with same curve on front and back are called plano lenses.

So all "non RX" lenses, without power are considered plano lenses. This is widely accepted industry standard.

In glasses a "plano-convex" lens would be called a "plus" lens. This magnifiers and has plus power... The plus power would be measured in diopters. So if front curve is 8.75 D, and back is flat (0 D). The total power of that "Plano convex" lens is +8.75 sphere.

It would have a focal length = 1/D of only 11 cm, or about 4.5 inches. I hope that helps....

 

[Chris A Hardaway]

Are you guys testing me? I could do these all day lol.

If they get too complex I won't respond lol

 

[Chris A Hardaway]

This is a bit confusing. You don't mean that the curved lenses without power are called "plano"-something?
I'm deeply into photo nerdery and there a "plano-convex" lens is flat on one side and convex on the other side. A curved lens with the same radius on both sides is then a "meniscus" without power.

Lens - Wikipedia

en.wikipedia.org

Or am I misinterpreting what you wrote?

A miniscus lens, without power, per your definition is a plano lens. Not "Plano" curved, but "Plano" power. See Plano curve means 0 base curve. Plano power means the front and back surfaced are parallel.

 

[andr3]

Are you guys testing me? I could do these all day lol.

If they get too complex I won't respond lol

I am certainly not. Honest question. Exactly the answer I was looking for. You’re a wealth of knowledge. Thanks bro

 

[Chris A Hardaway]

I am certainly not. Honest question. Exactly the answer I was looking for. You’re a wealth of knowledge. Thanks bro

Haha, thanks. I was teasing. It's all good