Is "plano" (0.00) plus or minus...and why?

[Emmet Ropia]

I was following a discussion of this issue dated 11/06/06. Nearly all of the contributors conceded that a plano lens could actually be classified as a plus lens for various reasons.

I offer this: Real or positive images can be projected on a screen but cannot be "seen" by the natural eye. Virtual or negative images can be "seen" by the natural eye but cannot be projected on a screen.

Plano images can be "seen" by the natural eye. Therefore, a plano lens could actually be classified as a minus lens.

[braheem24]

I would offer something else for a rebuttal...

I surface my (plano sph) to (-0.12 sph) and see no change in image with my naked eye as opposed to a (plano sph) that seems to distort images in the distance.

[lensgrinder]

Or I give you this as a rebuttal.

SM = 1 / (1 - (t/n)D₁)

A plano lens on a 6.00 D base with 2.5mm of thickness in plastic.

SM = 1 / (1 - (0.0025/1.498)6)
SM = 1 / (1 - 0.01001335)
SM = 1 / 0.999
SM = 1.01 or 1% magnification

But braheem's explanation works good too.

[braheem24]

:D

:cheers:

[For-Life]

I gotta show you something

1.00, 0.00, and -1.00

Which one has a negative sign in front of it?

[Barry Santini]

I think I'm of a mind to allow plus or minus for plano...

Now, the *real* question for me is:

"If -0.25D is a *prescription*...and
+0.25D is a *prescription*....

Then, is *plano* a prescription?....

and if so, should it *expire*.... and therefore be subject to an *expiration date? Should *everyone* then, be rechecked for potential eye disease, with a frequency less than the noted expiration date?

Thoughts?


Barry

[For-Life]

and if so, should it *expire*.... and therefore should *everyone* should be rechecked for potential eye disease, and not just "the powers that be"

Thoughts?


Barry

Yes

[Barry Santini]

Excellent!.....an opinion...!

Any other boarders feel that we should make mandatory eye exams the law of the land?

Barry

[optical24/7]

Excellent!.....an opinion...!

Any other boarders feel that we should make mandatory eye exams the law of the land?

Barry

If you fail a DMV vision test then, yes, if you want to renew or get a drivers license. As far as mandatory nope. It's your human right to go around with lousy vision.

[For-Life]

I love the DMV thing. You fail, you go see a doctor, he refracts you to what is needed, you get the form signed and you hand it in. No one in ensuring you update the RX.

Yes, if you have no glasses beforehand the RX forces you to get glasses, but how many people out there that are wearing glasses that do not grant minimum 20/40 in both eyes?

[hipoptical]

but how many people out there that are wearing glasses that do not grant minimum 20/40 in both eyes?

54,205,191

Of course, that's just for the good 'ole USA:

Population: 301,139,947-(% people 20/20)-(% wearers who update Rx)-(% for error in assumptions)

[For-Life]

54,205,191

Of course, that's just for the good 'ole USA:

Population: 301,139,947-(% people 20/20)-(% wearers who update Rx)-(% for error in assumptions)

lol, ever see that South Park that pokes fun at senior drivers? All the seniors keep hitting people, and when they return from Old Country Buffet all at once there is a killing spree. The kids run into a house to hide but one car comes through the window. They run up the stairs and another follows. They hide in a room, turn on the lights, and there is a car just sitting in there.

I can just see that happening with these non-20/20 drivers.

[Emmet Ropia]

Excellent!.....an opinion...!

Any other boarders feel that we should make mandatory eye exams the law of the land?

Barry

No, the requirement for eye exams should not be legislated. But, your logic begs the question, "Should plano sunglasses and safety glasses only be made available with a prescription?"

[Emmet Ropia]

[quote=lensgrinder;198587]Or I give you this as a rebuttal.

SM = 1 / (1 - (t/n)D₁)

A plano lens on a 6.00 D base with 2.5mm of thickness in plastic.

SM = 1 / (1 - (0.0025/1.498)6)
SM = 1 / (1 - 0.01001335)
SM = 1 / 0.999
SM = 1.01 or 1% magnification

But you only performed "half" of the SM equation...the full SM equation is given below:

SM = (1/(1-(t/n)D1)) x (1/(1-hD))
SM = (1/(1 - (0.0025/1.498)6)) x (1/(1 - (0.0015)(0))
SM = (1/0.999) x (1/0)
SM = (1.01)(0)
SM = 0

Therefore, the lens has zero magnification and maintains the characteristics of a virtual, i.e. minus lens! (The image through the lens can be seen with the naked eye.)

[JRS]

Take an reasonably thick PLANO (say a safety thickness lens), put it in a lensometer and move it about 10~15 mm off-center. Which way does the prism, although slight, actually move?

[Emmet Ropia]

"Natural" light enters the eye in only two ways, divergent (minus) or parallel (plano). Natural light never enters the eye in a convergent (plus) manner unless it has first passed through some type of optical system. True plano lenses do not produce convergence and share a virtual focus along with minus lenses.

[lensgrinder]

But you only performed "half" of the SM equation...the full SM equation is given below:

SM = (1/(1-(t/n)D1)) x (1/(1-hD))
SM = (1/(1 - (0.0025/1.498)6)) x (1/(1 - (0.0015)(0))
SM = (1/0.999) x (1/0)
SM = (1.01)(0)
SM = 0

Therefore, the lens has zero magnification and maintains the characteristics of a virtual, i.e. minus lens! (The image through the lens can be seen with the naked eye.)

You have two parts to the SM equation:
The shape factor 1 / (1-(t/n)D₁
t is the thickness
n is the refractive index
D₁ is the base curve

The power factor 1 / (1 - hD)
h is the vertex distance in Meters
D is the power of the lens

Now let us take the power factor and use your 15 mm vertex:
1 / (1 - (0.015 X 0.00)
1 / (1 - 0)
1 / 1 = 1

So the only part of the SM equation that you use is the shape factor when figuring a plano lens.

[LENNY]

I love the DMV thing. You fail, you go see a doctor, he refracts you to what is needed, you get the form signed and you hand it in. No one in ensuring you update the RX.

Yes, if you have no glasses beforehand the RX forces you to get glasses, but how many people out there that are wearing glasses that do not grant minimum 20/40 in both eyes?

No one forces them to have GLASSES just an eye exam!

[For-Life]

No one forces them to have GLASSES just an eye exam!

That is what I said. They are only forced to have glasses if they did not have a pair before. It is written on the drivers license to have corrective lenses.

[Emmet Ropia]

You have two parts to the SM equation:
The shape factor 1 / (1-(t/n)D₁
t is the thickness
n is the refractive index
D₁ is the base curve

The power factor 1 / (1 - hD)
h is the vertex distance in Meters
D is the power of the lens

Now let us take the power factor and use your 15 mm vertex:
1 / (1 - (0.015 X 0.00)
1 / (1 - 0)
1 / 1 = 1

So the only part of the SM equation that you use is the shape factor when figuring a plano lens.

Good point and thanks for discovering my error. However, let's look at what happens to the magnification when we turn your lens around and look through it BACKWARDS? Remember, it's still a plano lens...

SM = (1/(1-(t/n)D1))
SM = 1/(1 - (0.0025/1.498)(-6)
SM = 1/(1 - (-0.01))
SM = 1/1.01
SM = 0.99 magnification or 0.01 minification

If you apply the formula to a plano lens made from plano surfaces, the magnification is 0.

[lensgrinder]

Well you could "flip" the lens around, however this is not the way we look thorough lenses. You can look through any lens from the front surface and achieve a different result. The SM formula does ask for the front surface power of the lens.

[Barry Santini]

No, the requirement for eye exams should not be legislated. But, your logic begs the question, "Should plano sunglasses and safety glasses only be made available with a prescription?"

I guess it depends on whether "plano" is considered a prescription... that's my intentionally-provocative point!

I make no secret of my opinion that eyewear "Rxs" are really *not* akin to a conventional Rx, and therefore not inherently medical in nature. Following this premise, they should not be subject to an expiration date!

If duplicating a present Rx beyond its expiration date is opined as healthy, than encouraging a client to use, or allowing the use of any older, "spare pairs", or any other eyewear they have that DOES NOT have EXACTLY the same Rx in it, is...well

Malpractice??!


It's not the use of the eyewear itself that expires.

Since a person's vision is always fluid, let's say their "Rx" starts out "-0.25D". And, as they age, it progresses to "+0.25D". Are all these powers inherently an "Rx" (-0.25D, 0.00 & -0.25D)? I can make quite a persuasive case that EVERYONE would benefit from an additional -0.25D for night driving (night myopia). Does that mean that "plano" people have no Rxs that *expire* during the day, but when they drive at night, they need to see an eye doctor every two years?

And what about OTC readers....if they're OK, what about OTC -0.25 to -0.75...for "night driving"

I think, for a long time, we have not seriously challenged some of the standards of practice we've adopted.

Your thoughts again, please!

Barry

[Samuel Jong]

In the precription form, do we write PLANO or 0.00?

[For-Life]

In the precription form, do we write PLANO or 0.00?

Plano, because it makes us look fancier

[RGC_man]

Take an reasonably thick PLANO (say a safety thickness lens), put it in a lensometer and move it about 10~15 mm off-center. Which way does the prism, although slight, actually move?

That always confused me that you could move the image on a plano. But as a perfectionist I always used to try and get the image central (not that the patient would ever know the difference).