Stand Magnifiers And Equivalent Power Flashcards
Plus lens or plus lens combination mounted in a housing that sits on the material being viewed
Stand magnifiers
Stand height
Typically less than the focal length of the lens
Image of the stand magnifier
Forms erect virtual image a finite distance behind the page
Enlargement of the lens
- depends not he Fe of the lens and emerging vergence
- system consists of the magnifier F1 and the pateitns add or accommodation
- magnification is a result of RDM and lenargment by the lens itself M=RDM x E
Magnification of stand mag
M=RDM x E
Where they hold the material
Properties of the stand
Add for stand magnifier
1/(i+h)
Virtual imag distance=i
Dist between lens and eye=h
A stand mag has an enlargement factor of 1.8X and its image is 18cm behind the lens. The patient uses a +3.00 add and has a VA of 0.33/4M with the glasses alone. How far away from these glasses must the patient hold the mag?
+3.00 add=33cm working distance
Working distance=i+h
33cm=18cm +h
H=15cm
A stand magnifier has an enlargement factor of 1.8X and its image is 18cm behind the lens. The patient uses a +2.00 add and has a VA of 0.50/4M with the glasses alone. The patient desires to change to a +3.00 add for fill time wear. What mag is this pateint expectd to achieve
Mag=RDM x E
(.5/.33)x 1.8=2.72x
A stand magnifier has an enlargement factor of 1.8X and its image is 18cm behind the lens. The patient uses a +2.00 add and has a VA of 0.50/4M with the glasses alone. The patient desires to change to a +3.00 add for fill time wear. What M size can be read?
Mag=(what you got)/(what you need)
2.72x=(4M)/(what you need)
4/2.72=(what you need)
What you need=1.47M
A stand magnifier enlarges an object 5X and the image is 20cm behind the lens. The patient has a +2.50 add and reference VA of 0.4/5M. Using his glasses, how far away from the spectacle plane must he hold the magnifier
40cm=20cm + h
h=20cm
A stand magnifier enlarges an object 5X and the image is 20cm behind the lens, the patient has a +2.50 add and reference VA of 0.4/5M. What size print should be be able to read using the magnifier and his glasses?
.4/.4=RDM
5=E
5x1=5
5/5=1M
A stand magnifier enlarges 3X and has an image location 30cm behind the lens. The patient’s reference acuity is 0.25m/8M using a +4.00 add. She prefers to hold the magnifier 14.5cm from the spectacle plane. What add will this absolute presbyope need?
WD=h+i 30=i 14.5=h WD=30+14.5=44.5cm 1/.445=2.25
A stand magnifier enlargers 3X and has an image location 30cm behind the lens. The patient’s reference acuity is 0.25m/8M. She prefers to hold the magnifier 14.5cm from the spectacle plane. With the appropriate adds, what size print would you expect her to read?
WD=h+i=30+14.5=.445m
MStand=RDM x E. (.25/.445)x3=1.68
1.68=8/X
X=4.75M
Enlargement factor
Is sometimes published, but can also be calculated using the following formula
E=(V-Fe)/V
Equivalent power of stand magnifier
Fe=F2xE
What is the equivalent power of a stand magnifier labeled 3X, E=2.3X, i=22cm
Insufficient information
Need the add
Use of stand magnifiers
Really good for better contrast. All stand magnifiers should be touching the material they are using
Equivlanet power
- Fe for a lens or lens system is the power of the thin lens that if positioned at the second principal plane of the system, its posterior focal point cosincides with the posterior focal point of the system
- can use this to compare the magnification between optical systems
- there is considerable disparity between vertex power and equivalent power
Equivlanet power of lenses separated by air
- if a patient accommodates with an add, or uses an add or accomodation in conjunction with a magnifier, a new optical system is created compared of the 2 plus lenses
- Fe=F1-F2-(t)(F1)*(F2)
Fe is > F1
When the magnifier is held less than one focal length from the add
Fe < F1
When the magnifier is held more than one focal length from the add
Equivalent power microscope
F=M/r
Equivalent power of HHM
F=M/r
Equivalent power of stand mag
Fe=F2xE
Equivlanet power of telemicroscope
Fe=F2xMTS
