Optics. A Greatly Simplified Overview. At a distance of 20 feet, light rays become parallel. Placing a lens in the path of the light results in bending of all but 1 light ray.
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A Greatly Simplified Overview
Placing a lens in the path of the light results in bending of all but 1 light ray.
work in feet
person has 20/200
person needs 20/40
will require 5X magnification
work in meters
person has 0.4/1.0
person needs 0.4/0.5
will require 2X magnification
But…alas, it’s not that simple.
~The standard for near is 40cm (just like the standard for distance is 20 feet).
~Distance the student used/X = 40cm/# of the smallest line read on the near card.
~Remember though…you need to keep the numbers on the right hand side all in the same value. In other words, if you are working in meters for print size, your distance measurements need to also be in meters. So, we must divide the 40 by 100 to get 0.4.
~Then you set up the ratio. For this example we are saying the student read 1.0M and he wants to read 0.5M.
~The fractions should read:
0.4/1.0 = 2 …so,heneedsa 2X(8
0.4/0.5 diopters) magnifier to read 0.5M print.
~Keep in mind, you can use whatever distance the student needs.
~Suppose your student read 2.4M at 15cm…and he needs to be able to read 1.0M at 15 cm. How much magnification should you give him?
-.15/2.4 is what currently reading.
-.15/1.0 is what he needs to read.
-It is a difference of 2.4 so a 2.5X (10 diopters) magnifier should work.
Feq = F1+F2-cF1F2
F1 = spectacle; F2 = magnifier;
c = the distance between the two in meters. (meters, meters, meters.)
Feq = F1+F2-cF1F2
Feq = equivalent lens power
F1 = magnifier
F2 = spectacle add
c = the distance between the two in meters.
~The glasses Rx is no longer correct. Providing magnification at this time is only magnifying “blur”.
~Sometimes only partial correction may be given. This may be done in an effort to maintain a better working distance or to allow the person an opportunity to adapt to new Rx.
~You need to know this… (Lensometer vs. Refraction!) (What does the student need vs. what does student have in current glasses.).
~Partial corrections, the use of an add and viewing distance will influence the focal distance of magnifiers.
~Some of the power of the magnifier may be “eaten up” to make up for refractive error.Tricky Part (cont.)
~If the student hold the magnifier up to the spectacle add:
Feq = F1+F2-c(F1F2).
Feq = +3.00 + 7.00 – 0(3x7) = +10.00D.
You essentially combined the powers of the add with the magnifier.
Feq = +3.00 + 7.00 – .14(3x7).
Feq = 10.00 - .14(21).
Feq = 10.00– 3(actual 2.94)=+7.00D.
Reminding you to change 14cm to .14M before multiplying.
Add +5 diopter lens.
2 diopters used up to bring eye to 60 so there is only a +3 gain in power.
Total power +3 Focal distance (100/3) = 33cm.
Add +10 magnifier.
Total power = +10.
Focal distance (100/10) = 10cm.
Feq =10 + 3 = 13.00D.
Total power +13 Focal distance (100/13) = 7+cm approximately.
Feq = 10 +3 -0.5(10X3) = +11.50D.
Total power +11.50 Focal distance (100/11.50) = 8+ cm approximately.