Wait, but WHY?!?! Base curve craziness

[Prentice Pro 9000]

So I've always wondered.... What is up with the W. Wollaston base-curve lens series? It gets mentioned in almost any base curve tutorial without any further satisfactory explanation. You mean to tell me that a -10.00 lens will have similar performance on a +17-ish base as it will on a plano base? There has never been (as far as I've seen) something online that said "HEY, YOU'RE NOT CRAZY, WE KNOW THIS WOLLASTON THING GOES COMPLETELY AGAINST EVERYTHING YOU CURRENTLY INTUIT AS AN OPTICIAN, SO LET US LAY IT OUT FOR YOU."

That being said, can somebody please lay it out for us?

[drk]

https://www.optiboard.com/forums/sho...urve-Best-Ever!

It is

AN ENIGMA

[Prentice Pro 9000]

AN ENIGMA

man! here's something interesting from that post




[/QUOTE]

[Prentice Pro 9000]

From the enigma post
"The lower (Ostwalt) branch of Tscherning's ellipse serves as the basis for modern "best form" spectacle lenses. Both branches can eliminate oblique astigmatism, but Wollaston's branch also minimizes other optical effects associated with prism and distortion. Ostwalt's branch, on the other hand, is much easier to fabricate."

Best regards,
Darryl

[Uncle Fester]

Scroll down about 2/3 to find and read about Tshcerning's ellipse under the header "Best Form Lens Design"

http://opticampus.opti.vision/cecour...l=lens_design/

Read the whole course by the late Titan of Optics Darryl Meister- but if you are like me, first be sure the room is well ventilated as smoke coming out of your head is common.

[Prentice Pro 9000]

Scroll down about 2/3 to find and read about Tshcerning's ellipse under the header "Best Form Lens Design"

http://opticampus.opti.vision/cecour...l=lens_design/

Read the whole course by the late Titan of Optics Darryl Meister- but if you are like me, first be sure the room is well ventilated as smoke coming out of your head is common.

Yes, I reference this often and it is where I pulled the ellipse image from. I think what I am looking for is the basis of Ostwalt's work and how it relates to Wollaston's. In this article it just says that his lenses were flatter, but there's no real mention of anything like "So Ostwalt was looking at how steep these lenses were and said "NOPE! THIS WILL NOT DO!" or Something to that effect.

[lensgrinder]

In 1804 Dr. William Wollaston published a study that stated that curved meniscus lenses have slightly better imagery than bi-convex.
In 1898 Dr. Franz Ostwalt calculated each power within a range of, about -22.00 D to +7.00 D, to show that a lens with a certain base curve for each power could be made without oblique astigmatism.
Ostwalt’s calculations were done using thin lens equation, therefore, Dr Marius Tscherning re-calculated Ostwalt’s work using thick lens equations. Since an ellipse has two solutions each branch was named Wallaston(steep) and Ostwalt(flat).
The Wallaston and Ostwalt branches show that a lens power(within a certain range) can be made free from oblique astigmatism using two possible curves.
For example a -4.43 made in CR-39 with a 2.0 CT, can be made on a 16.97 BC or a 4.97 BC.

[Prentice Pro 9000]

In 1804 Dr. William Wollaston published a study that stated that curved meniscus lenses have slightly better imagery than bi-convex.
In 1898 Dr. Franz Ostwalt calculated each power within a range of, about -22.00 D to +7.00 D, to show that a lens with a certain base curve for each power could be made without oblique astigmatism.
Ostwalt’s calculations were done using thin lens equation, therefore, Dr Marius Tscherning re-calculated Ostwalt’s work using thick lens equations. Since an ellipse has two solutions each branch was named Wallaston(steep) and Ostwalt(flat).
The Wallaston and Ostwalt branches show that a lens power(within a certain range) can be made free from oblique astigmatism using two possible curves.
For example a -4.43 made in CR-39 with a 2.0 CT, can be made on a 16.97 BC or a 4.97 BC.

ahhhh. Very interesting. Is there anything that mentions whether one is better (even if just marginally) than the other? Also, is there any chart that says something to the effect of "if you deviate from this prescribed BC then you will create X amount of oblique astigmatism".

Thanks in advance.

[lensgrinder]

ahhhh. Very interesting. Is there anything that mentions whether one is better (even if just marginally) than the other? Also, is there any chart that says something to the effect of "if you deviate from this prescribed BC then you will create X amount of oblique astigmatism".

Thanks in advance.

Both are equal.
The purpose of the ellipse is to show that there are only two curves for every power(see attached) that will produce a lens free from oblique astigmatism, which is simply not possible to produce using spherical lenses. It’s also important to note that this does not account for tilt and wrap.
Free form lenses optimize the surface to account for both oblique astigmatism in its as worn position.