Misinformation 101

[Darryl Meister]

Not infrequently, I happen upon articles and such that contain statements of questionable accuracy. Now, I'm sure that these authors generally have the best intentions, and may have even done a bit of "homework." I'm not trying to pick on them. But some "myths," "half-truths," and "rules of thumb" in this industry are perpetuated to the point that they now obscure the very facts upon which they were (perhaps loosely) based. And it seems like I see these more and more with some of the new technologies out there today.

So, I'm going to post a few statements, rules of thumb, and/or observations in this thread for debate. These may simply be misleading or equivocal, or perhaps entirely wrong. (For those of you have come across similar examples, feel free to post your own for discussion.) I'm not concerned with the source here, just the validity of the statement. And don't worry if you may have been under the same impression; this is an opportunity to argue the point either way.

The First, and one I see often, is one I ran across again earlier today:

Using 8 degrees of pantoscopic tilt drops the optical center 4 mm from the pupil. (Or some variation of this.)

True of False? Think about what is really being said here...

[HarryChiling]

True of False? Think about what is really being said here...

Absolutely false!

The proof:

if the visual axis were to be tilted you would get

sin(t)=oc drop/center thickness

sin(8)=oc drop/2.2mm

0.139=oc drop/2.2mm

oc drop=0.139*2.2mm

oc drop=0.306mm

This is just from a trig standpoint, I am sure their are other factors like prism induced that would play a role. I will look into it further at home.

PS - I have got to admit, I have myself spread what I thought to be truths around. I think this forum is a great resource for that. I am reminded of my last post with shanbaum on plano lenses where I ate crow. :)

[shanbaum]

I am sure their are other factors

...like where the pupil is positioned in the frame?

[FVCCHRIS]

Crow is good if sufficiently cooked and properly presented:bbg:

[HarryChiling]

Here is an example of what I think is why the rule of thumb seems to be around, and why I think it is inaccurate.

[OPTIDONN]

If 8 to 10 degrees of pantoscopic tilt are present the oc should be dropped 4-5 mm below the pupil center. Inducing 8 to 10 degress does not automatically drop the oc.

[HarryChiling]

If 8 to 10 degrees of pantoscopic tilt are present the oc should be dropped 4-5 mm below the pupil center. Inducing 8 to 10 degress does not automatically drop the oc.

Donn,

I don't think that is totally correct. I have always had a sort of problem with that concept. If this case were to be correct than all progressives would need to be fit 4-5mm below the center of the pupil. I agree that the OC should drop, but I think what bothers me is the amount 4-5mm in the scheme of things seems like alot.

[jherman]

1/8 or a 1/12

[EyeFitWell]

Half of a quarter is 1/8

1/4 divided by two = (1/4)times(1/2)=1/8

[Darryl Meister]

?? Is that misinformation? Or information?

It's a very broad generalization that may be right in some cases, and wrong in others, depending upon factors such as refractive index, density, minimum center thickness, prescription, blank size, etcetera.

[Robert Martellaro]

I recently learned about how poly can be much lighter than high index plastic (depending on the Rx) even though high index can be considerably thinner.

Right.

Trivex, Finalite, the newer Seiko 1.6, and poly are lighter in weight than 1.67 and 1.74, even in very high powers. The key is the lower specific gravity- low 1.20's for poly and some of the 1.6 materials, and 1.11g/cm3 for trivex. The only exceptions might be the atoric designs that have some asphericity on the ocular surface like the Vizio and Optima, (1.74 atorics are still(?) unavailable in the US), and with large eye sizes, although if you're trying to minimize weight I doubt you'll put a -10 in a 60mm eye size.

[Cherry Optical]

Right.

Trivex, Finalite, the newer Seiko 1.6, and poly are lighter in weight than 1.67 and 1.74, even in very high powers. The key is the lower specific gravity- low 1.20's for poly and some of the 1.6 materials, and 1.11g/cm3 for trivex. The only exceptions might be the atoric designs that have some asphericity on the ocular surface like the Vizio and Optima, (1.74 atorics are still(?) unavailable in the US), and with large eye sizes, although if you're trying to minimize weight I doubt you'll put a -10 in a 60mm eye size.

I can't seem to get my mind to undestand this whole concept. Does there ever come a point where a lighter/thicker material will have a total weight higher than heavier/thinner materials? I've played around with some numbers and just can't seem to convince myself one way or another.

For example: Would a -15.00 Polycarbonate weigh less than a -15.00 1.70 HOYA lens?

Any help on this would help to put my mind at ease.

Regards,



Adam