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Ophthalmoscopes: BIO vs direct
-field of view
-depth of focus
-image orientation

-larger FOV + DOF
-smaller mag

-smaller FOV + DOF
-incr mag


-how direct and BIO work

DO: a series of lenses is used to correct for ametropia in both the pt and dr, thereby maintaining the required conjugacy

BIO: a condensing lens is used to form an intermediate, inverted, real image - another lens or accomm is needed to see the image, which is typically located about an arm’s length away


-what it measures
-how it works (concepts not math)

Back vertex power, prism

One views a target object via telescope system thru both a standard lens (of some known power/distance in the lensometer) and lens of interest
1) to view the target (perpend lines), parallel light must strike the viewers eye (therefore parallel light must strike the TS)
2) this means parallel light is leaving the test lens - how do we make this happen?
3) light from the standard lens must converge at Ƒ, the primary focal point of the lens
4) now we move the target (relative to the lenses) until this happens


-mathematical description

𝑥 = (ƒ^2)(Fv)

Distance that the target is moved (meters) = (focal length of the standard lens (meters) squared)*(back vertex power of the test lens)

Note: when 𝑥 is negative, Fv is negative
-if the target is moved away (further from dr), 𝑥 is negative and the spectacle lens has a negative power (myope)


-steps of how to read a lens power

1) Focus eyepiece
2) Blur by turning the power wheel (+), then slowly step back toward minus until the sphere lines are clear
3) Adjust the axis
4) Continue turning power wheel in minus until cyl lines are clear


-steps of how to read a lens prism

Compare the location of the cross hairs formed by the sphere/cyl lines with the location of the bull’s eye image of concentric circles

Cross hairs will be deviated the same as base direction:
-if to the LEFT in OD = base out
-if to the RIGHT in OD = base in
-if above = base up
-if below = base down


Hand neutralization
-minus lens
-plus lens

(-): like 2 prisms stacked APEX TO APEX -> motion is WITH

(+): like 2 prisms stacked BASE TO BASE -> motion is AGAINST

*think (-) neutralized with plus, which is what you add for ret when you have with motion



Measures radius of curvature of RGPs

Forms a target image at some point (P) b/w the viewer and lens
Light from this image will reflect off the cls and form another image at some point (Q)
The viewer will see a clear image:
1) P is located at the surface of the lens - object distance is 0, so image distance is also 0, henc P and Q are at the same location
2) P is located at the center of curvature of the lens - object distance is at the radius of curvature, so image distance is also at the roc, and m = -1. P and Q are again at the same location
The distance b/w these 2 clear points is the radius of curvature


-take home summary

There are 2 locations at which the lens can sit such that the viewer sees a clear image
We need to move the lens from one such location to the other and measure how far it had to be moved
The distance is the radius of curvature



Measures radius of curvature (therefore effective refractive opwer) of the center of the cornea along certain axes

The cornea acts as a CONVEX MIRROR and creates an image of an object (mire)
One then measures the size of the reflected image, which is used to measure roc


-equation for power of cornea

Assume n = 1.3375 and treat the cornea as an SSRI

We get: F = 1.3375-1/r

Therefore: F = 0.3375/r where r is in meters

Equivalently: F = 337.5/r where r is in mm


Lens clock

Measure the sag of the lens

Adjusting a movable pin
-based on sag and assumed value of n, some CALIBRATED clocks give a power readout


Lens clock
-mathmatical description

FL = (FLC)((nL-1)/(nLC-1))

Power of the lens = (power readout of clock)*((IR of lens-1)/(IR of clock calibration))


Slit lamp biomicrocsope

Compound microscope commonly used for ant seg views

Keplarian TS eyepiece
Inverting prism to correct upside-down image from Keplarian
Galilean TS to further magnify image
Objective lens - allows a system optimized for distance viewing for near objects
Complex illuminating system
Binocular viewing system


Fundus lens
-how it works

High-powered lenses create a REAL, INVERTED image
-this image becomes the object for the objective of the slit lamp biomicroscope


Fundus lens
-higher powered fundus lens = _(2)_; _(why)_

Lower magnification
Increased FOV

Creates a TELESCOPE SYSTEM that is essentially a REVERSE TS
-recall magTS = -(Focular)/(Fobjective)
-with fundus lens, this becomes mag = -(Fdr’seye)/(Ffundus lens)


Fundus lens
-magnification of +60D, +78D, and +90D

Recall: mag = -(Fdr’seye)/(Ffundus lens)
-assume dr’s eye power is +60D

For 60D: mag = -60/60 = -1x (inverted, same size)
For 78D: mag = -78/60 = -0.77x (inverted, minified)
For 90D: mag = -90/60 = -0.67x (inverted, minified)


Fundus lens
-magnification of +20D (BIO), -55D (Hruby)

Recall: mag = -(Fdr’seye)/(Ffundus lens)
-assume dr’s eye power is +60D

For 20D: mag = -20/60 = -3x (inverted, magnified)
For -55D: mag = -(-55)/60 = +1.09x (upright, magnified)*

*Hruby lenses create a Galilean reverse TS