Midterm 2: Ophthalmoscopy Flashcards
During Direct O, if the patient is myopic, what kind of lens would the doctor need to use if the doctor is emmetropic to view the retina?
Minus. Since eye has stronger plus, rays will be converging before doctor’s eye
During Direct O, if the patient is hyperopic, what kind of lens would the doctor need to use if the doctor is emmetropic to view the retina?
Plus. Rays will be diverging before reaching doctors eye.
When do you need to vertex adjust when using Direct ophthalmoscopy?
Vertex adjust when you get further away from patient.
During direct, if both patient and doctor are myopic, how much minus does the lens have to be to work?
equivalent to their refractive errors. Then vertex depending on where doctor is positioned.
The further away the doctor gets during Direct, would the doctor need to add more or less minus if the patient is myopic and the doctor is emmetropic?
More minus.
If the patient is hyperopic during Direct, would the emmetropic doctor need more or less plus as they move further away?
More plus
What must the lens power in the direct ophthalmoscope be equal to?
Equal to the sum of the doctor and patient’s prescription. But then may slightly change due to vertex distance.
Why do you find equivalent power when adding lenses to direct ophthalmoscope?
You find equivalent power because using two lenses changes the magnification. Find equivalent power by…..
Fe= F1 + F2 - d(F1)(F2)
F1= power of eye F2= lens added d= distance separating them
The average angular mag for both emmetropic patient and doctor during DO
15x at 25 cm
If the patient is myopic in DO, do you have more or less magnification?
More magnification
If the patient is hyperopic in DO, do you have more or less magnification?
less magnification
Describe differences in Direct ophthalmoscope magnification between axial myope and refractive myope.
Refractive myopes will have more magnification since the power of the eye is actually changing. Axial myope mag still changes, just not as much.
Describe differences in Direct ophthalmoscope magnification between axial hyperope and refractive hyperope.
Refractive hyperopes will have less magnification since the power of the eye is actually changing. Axial hyperope mag still changes, just not as much.
How large is normal FOV for Direct?
3mm or 1.5 disc diameters
The rays that make it to your retina during Direct have to go through what? What do you call this?
Rays have to go through the exit pupil of the patient and through the entrance pupil of the doctor’s eye to make it to the image plane.
CascadeOptical system
What limits field of view in DO?
Pupils not being able to be on the same plane.
In other words….
Because exit pupil of patient and entrance pupil of doctor are not lined up
How to find angular mag in DO?
Angular mag = power of eye / 4 (being at standard distance, 25 cm)
Is IO or DO more commonly used?
Indirect
Describe what “the cartoon” shows for indirect ophthalmoscope features
- image inverted
- pupil dilated
- light shined into the eye
- doctor eye aligned with patient pupil plane
Advantages of Indirect
Much larger FOV than Direct (8 disc diameters)
Doctor able to use steropsis
Describe image of indirect
upside down, inverted
Where do we perceive the image to be when doing Indirect? Where is it actually?
We perceive it to be inside the condensing lens. It is actually floating in space between you and condensing lens.
For indirect ophthalmoscope, the closer the doctor is to the image, the more the doctor will have to _________.
Accommodate.
When could you use a monocular indirect opthalmoscope? What advantage does the monocular have?
When patient pupils are smaller.
Instead of it being inverted, it re-inverts and is upright while maintaining the same FOV and magnification.
Where does the light of Indirect focus?
Focuses on patients pupil plane.
How do you find lateral mag of indirect?
Lateral Mag= (-) Patients eye power/ condensing lens power
or
image height/ object height
or
image distance / obeject distance
What has a higher magnification in indirect, 20D or 30D condensing lens?
20D because later mag is -60/ 20 = -3x compared to -60/ 30 = -2x
What does axial magnification measure in Indirect? How do you find it?
Depth of image. Perceived depth is exagerrated.
Axial mag = (lateral mag)^2
Why do we not perceive axial magnification as much as we should in indirect?
Are pupil distance is made smaller using the headset for indirect.
Smaller PD results in less stereo acuity so we cant perceive as much depth.
How to find total angular mag for indirect
Angular mag = Lateral mag x RDM
-assume 25 cm (reference distance) and 40 cm (from doc to image) for RDM
If you have 8 times more less magnification, how much more area can you see?
64x more
What limits FOV for indirect?
Amount of rays that are caught in the condensing lens
Where is condensing lens held for indirect?
About 1 focal distance away from exit pupil of patient
