6 Flashcards
Number one complaint of a pt presenting for LV services?
-Most popular LV device?
READING
-Hand magnifier! (HH mag)
In order to all 1M reading (standard), what power must be greater than the other?
F LVD > F eq
-LVD’s power must be greater than the equivalent power necessary for the pt to read 1M (20/50) print
Three uses for the term “Mag” - X-RATED!!
1) RSM - relative size mag - make object physically larger (i.e. large print books)
2) RDM - relative distance mag - bringing object (or patient closer - both will increase retinal image size
3) Angular mag
Formula for relative dist mag?
RDM = orig dist/new obj dist (40cm orig, moved to 20cm = 2X)
Effective mag compares the angular sizes of what two things? What’s the formula?
- Assumes the object is also WHERE w/ respect to the HHmagnifier?
- Which LVD brand uses this formula?
Meff = F/4 –> more commonly used
- angular size of image to angular size of object assuming reference distance of 25cm - object must be held @ 25cm from spectacle plane
- Eschenbach
What is the magnification of a 4 diopter lens?
1X - because that’s assumed. F/4 = 4/4 =1
Conventional mag formula? Why the extra step?
-Which popular company uses the conventional mag formula?
(F/4) + 1
- accounts for accommodation…it just adds one. period.
- Optolec
Define: angular mag. Formula?
- which two LVDs is this mainly used with?
- what must be given when angular mag is used with a hand magnifier?**
magnification of the image created completely by the optics of the LVD being used
Angular Mag = 1 + hF; h=dist from mag to spect. plane (m), F=lens power
1) HH mags, 2) telescopes
- the DISTANCE the magnifier is held from the spectacle plane
Prism half-eyes are what type of LVD? Advantage? Disadv?
-Prism incorporated is in what direction? Purpose?
-Without any prism, it becomes hard to converge past how many diopters of convergence demand?
- Microscope - large FOV; but NEAR WD (smell the text)
- BI; reduces BO demand (convergence demand) at near
~10D of convergence demand (~10cm away)
-pt learns to move text past the lens, rather than moving head/lens
How much BI prism will you Rx in a pair of prism half-eyes with a +4.00 add? +6.00 add? +8.00 add?
- for the above values, is this in EACH eye, or a total??
- Do you take the distance power into account?
+4.00D => 6D BI, EACH eye (so 12D total!)
+6.00D => 8D BI, EACH eye
+8.00D => 10D BI, EACH eye
-note that the near point had NOTHING to do w/ the power of the lens (dist power) - ONLY the near add matters!
-NO - take the ADD POWER ONLY
Cardinal rule with hand magnifiers (how does the SIZE of the lens affect the AMOUNT of magnification?)
-where should you hold the object w/ respect to the magnifier?
the LARGER the hand magnifier, the WEAKER it is!
- but you can only see a word or two if it’s super small/strong…
- hold object at ANTERIOR focal point of the magnifier
T/F: if the object of interest is at the anterior focal point of the handheld magnifier, it does NOT matter how far you hold the magnifier from your glasses
TRUE - Feq is independent of eye to lens distance! (therefore, F1 = Feq)
as the power of the hand magnifier increases, the useable FOV _____
-as the distance of the hand magnifier from the spectacle plane increases, the FOV of the magnifier ____
decreases
descreases
…just think about it ;) makes sense
What type of image is ALWAYS created by a stand magnifier?
-IMPORTANT - THE OBJECT AND IMAGE DISTANCES are always (positive/negative)?
virtual, erect, divergent image - ALWAYS!!!
-b/c its divergent - you ALWAYS need an add to see clearly thru it!
-NEGATIVE! -use them to figure out L+F=L’; usually solving for L’
What is the relative distance mag of a CCTV when viewed from 40cm?
-What to do when you’re viewing the CCTV from a distance CLOSER than the standard 40cm? (What do you multiple together?)
RDM = 1 @ 40cm (just a standard.)
closer? RDM x RSM = total mag
Kindles/E-readers can mag up to _._M print (typically)
iPads can mag up to _._M print
- 5
3. 2
When calculating the near point for a pair of prism half-eyes, what do you use?
the ADD POWER ONLY! Don’t do 1/the whole dist and reading power. ONLY the add.
-recall: the amt of prism is 2prism diopters MORE than the add itself, PER EYE
MAX add possible in a pair of:
- PALs:
- FT - bifocals:
- ROUND bifocals:
- Unilens Stick-on microscopic bifocal:
+4.00
+8.00
+24.00
+40.00
-remember: NEAR PD will need to be calculated depending on whether you want prismatic relief! CAN’T just take the near PD!
-NO relief: NPD = DPD -1.5(add); 2.0 if you want relief
If you’re using a HH magnifier with the object AT the anterior focal point of the lens, should you use the add or distance portion of the lens?
No need for the add - parallel light leaving! Use distance
If a pt is using a 10D HH magnifier and is holding the object at 7cm (inside the focal point of the magnifier), should they use the distance or add portion of their glasses?
ADD REQUIRED - diverging light is leaving the object
If the question reads F1 (the LVD) at AT the spectacle plane, what two values do you need to solve the equation?
F1 and F2 - no need for c; because the distance from the magnifier to the spectacle plane is 0.
What’s the formula to find the linear field of view for a HH mag?
W = (df)/h ==> ALL UNITS MUST MATCH (put them all in cm)
d=lens dia
f=focal length
h=dist HHmag to EYE
A patient is known to have tremors, and also happens to have Glaucoma. What LVD do you suggest? Why?
STAND magnifier - beneficial for tremors, and PERIPHERAL field defects. Also works for RP.
Feq for HH mag equation:
Feq = F1+F2 -cF1F2
Feq equation for STAND mags: How to find ER?
Feq = ER x add
ER = (L’ - F)/L’
Feq equation for CCTVs: How to find Mt?
-RDM X RSM is another way; what’s RDM if working dist = 40cm? 20cm?
Feq = Mt x add
Mt = img linear size/obj linear size
RDM @ 40cm = 1
anything closer than 40cm: RDM X RSM to figure it out (if 20cm working dist, then RDM = 2x, and it’ll tell you what RSM is)
