Describe angular magnification
Result of an incr in retinal image size of an object by introducing an optical system (i.e. LENSES USED) in bw the object and the eye
- hand-held/collimating
- stand/non-collimating
Difference bw lateral and angular magnification
L: compares object size to image size
A: compares original retinal image size to retinal image size when viewed thru an optical system
Hand-held magnifiers
-describe
High plus-powered lens
Puts object of interest at the primary focal point of the magnifier
- this creates an image at infinity by collimating outgoing light
- no need for accommodation when using
Hand-held magnifiers
-an object at the primary focal point
Once there, the HHM-object system can be moved anywhere in front of the eye and total magnification will remain the same
-as lens-eye distance changes, the spectacle mag of the lens changes, however there’s an equal but opposite change in RDM = 0
Hand-held magnifiers
-what changes/doesn’t change when HHM is moved closer/farther from eye
Changes: linear FOV (incr when brought closer to eye)
Does not change: angular/total mag, accommodation (not needed still)
Stand magnifiers
-describe
High plus-powered lens mounted such that object to lens disparity is FIXED
- for most SMs, the object is located INSIDE the primary focal point
- upright, magnified, virtual image
- need for accommodation (light is still diverging as it leaves)
Telescopes
-describe
For magnifying distance objects
Both light in and out is parallel/plane = no need for accommodation
Telescopes
-entrance and exit pupils
EnP = objective lens
ExP = image of objective lens thru ocular lens
Telescopes
-meaning of label A x B
A = magnification B = diameter entrance pupil (dent) in mm
E.g. 4 x 45 = 4x mag, 45mm dent
Telescopes
-field of view dependent on (2)
Diameter of EnP - incr dent will incr exit pupil = incr FOV
Diameter of ExP - will continue to incr FOV by incr dex until dex becomes larger than the pt’s pupil, at which point the pupil is the limiting factor
Telescopes
-Keplarian
Both plus lenses
Inverted, real, magnified image
-need another lens to upright -> longer tube, heavier
AS and EnP are the objective
ExP is behind ocular (outside/in free space) = LARGER FOV bc can align with pupil
-typically very small = DIM image
Telescopes
-Galilean
Positive objective, negative ocular lens
Upright, magnified image
-objective lens forms real image that becomes a virtual object for ocular lens
AS and EnP are the objective
ExP is inside telescope = SMALLER FOV
-typically larger = BRIGHT image
Limited to 4x mag
Telemicroscopes
-describe
Telescope + reading cap
Used for viewing near object
-reading cap acts as a HHM - object located at primary focal point -> plane waves enter TS
Spectacle-mounted telescopes
- center fit
- bioptic fit
C: in LOS = continuous viewing, requires minimal training, shouldn’t walk with
B: in upper portion of spectacle lens, used for spotting, significant training required
-uses include classroom work, driving, traveling, grocery stores
Reverse telescopes
- use
- describe
- alternative
Expand FOV - pts with RP, advanced glaucoma, etc.
Pt looks thru objective lens = minifies objects
- must have good central acuity
- most often hand-held, 2.5-4x Galilean
Minus lenses - esp successful with -5 to -10D
VFD
- most effective method
- prism orientation
Scanning techniques
Base toward defect
Classifying vision
- normal
- near-normal
- moderate LV
- severe LV
- profound LV
- near-blind
Snellen: 12 to 25 30 to 60 70 to 160 200 to 400 500 to 1000 Worse than 1000
M notation
1M
- subtends 5 arcmin at 1m
- equals 20/20
- linearly = 1.45mm (or 1/16th of an inch)
Just noticeable difference
Smallest lens power change the pt can detect
JND = snellen ÷ 100
E.g. 20/400 pt’s JND is 4D
-when doing trial frame use +/- 2.00 D flippers
For a pt that uses eccentric viewing, the point located adjacent to the scotoma that is used for viewing is called
Preferred retinal locus (PRL)
Examples of contrast sensivity charts (3)
Pelli-Robson: large letters, uniform size
Vistech system: sine-wave gratings
Bailey-Lovie: 3 charts, each at diff level contrast
Filters
- neutral density filters
- blue blockers/amber tints
NDF: reduce glare, good for pts with photophobia, all wavelengths are transmitted equally so no effect on contrast
BB/AT: reduce transmission of short-wavelengths -> reduced glare and enhanced contrast
VF testing
-central scotomas are detected most effectively with
Scanning laser ophthalmoscope