Prentice' rule

[va_optikal]

Please help. I'm not understanding . :o This and the Vertex formula.

[Fezz]

What part of it are you not understanding?

[HarryChiling]

Prentices Rule - is used to find how much prism in present in a lens of a given power, it can also be rearraged to find the power of a lens with a given amount of power and decentration, or the decentration of a lens given the power and prism.

Prism = Power (D) * decentration (cm)

Vertex Formula - If we were to label the visual axis as the (x axis) with the increments as mm I will walk you through an example. The doctor prescribes a 4.00 D lens to the patient refracted at 12mm.

So lets make the lens sit at point 0 on our x axis and the focal ditance of the lens is given by the following equation:

focal length (m) = 1 / Power (D)
focal length (m) = 1 / 4.00
focal length (m) = 0.25m = 250mm

So now we have our givens:

Lens at 0
Cornea at 12
Prescribed Focal Point at 250

No we have a frame that is fitted at 10mm instead the prescribed at vertex of 12mm, so now the scenario changes right. To make the lens sit at 10mm from the cornea our new scenario is:

Lens at 2
Cornea at 12
Prescribed Focal Point at 250

Now the lens to focal point has changed right, it was 0 to 250 = 250, now it's 2 to 250 = 248 so what's the power of a lens with a focal length of 248

focal length (m) = 1 / Power (D)
Power (D) = 1 / focal length (m)
Power (D) = 1 / 0.248m
Power (D) = 4.03D

You can see how this change effects the lens power now hopefully.

[AdmiralKnight]

Prentices' rule should be fairly simple. It's a standard plug in the values equation. P=dD as we learned it. ...or was it Dd? Whatever, same thing. Harry described it way better though, so go with him. :)

[va_optikal]

Thanks for the help. :) I've been studying from the 'Golden Eagle training' as well and thats been a great help to me. I've had the most trouble learning the prism concept. I'll get it just fine eventually.

[edKENdance]

I always thought that prentice rule was the most rudimentary to learn and also the only one that seems to make sense.

/almost there.

[va_optikal]

Well, everyones brain doesn't work the same. :D I'm really learning quite a bit from the Golden Eagle videos and all the study I've been doing these last three months. I can go at my own pace and am finally learning these ' rudimentary' optics. NOW... I'm getting a bit more excited about my job. I'm really 'using' what I'm learning, instead of relying solely on the lab or the clinic techs. There are some folks that have slightly made a bit of sport about my ignorance, but thats okay. I'll make it. :)

[FloridaEyes]

I just do not get it!

Prentices Rule - is used to find how much prism in present in a lens of a given power, it can also be rearraged to find the power of a lens with a given amount of power and decentration, or the decentration of a lens given the power and prism.

Prism = Power (D) * decentration (cm)

Vertex Formula - If we were to label the visual axis as the (x axis) with the increments as mm I will walk you through an example. The doctor prescribes a 4.00 D lens to the patient refracted at 12mm.

So lets make the lens sit at point 0 on our x axis and the focal ditance of the lens is given by the following equation:

focal length (m) = 1 / Power (D)
focal length (m) = 1 / 4.00
focal length (m) = 0.25m = 250mm

So now we have our givens:

Lens at 0
Cornea at 12
Prescribed Focal Point at 250

No we have a frame that is fitted at 10mm instead the prescribed at vertex of 12mm, so now the scenario changes right. To make the lens sit at 10mm from the cornea our new scenario is:

Lens at 2
Cornea at 12
Prescribed Focal Point at 250

Now the lens to focal point has changed right, it was 0 to 250 = 250, now it's 2 to 250 = 248 so what's the power of a lens with a focal length of 248

focal length (m) = 1 / Power (D)
Power (D) = 1 / focal length (m)
Power (D) = 1 / 0.248m
Power (D) = 4.03D

You can see how this change effects the lens power now hopefully.

[FloridaEyes]

http://opticalinstructor.homestead.c...prentices.html

Good Luck to you!

Please help. I'm not understanding . :o This and the Vertex formula.

[va_optikal]

http://opticalinstructor.homestead.c...prentices.html

Good Luck to you!

Hey thanks so much. ;)