Just (two) more questions..

[Hannah Doidge]

Hi again!
What is the magnification in direct ophthalmoscopy, assuming the subject's eye to be 25mm from the sight hole, when the subject has -4.00D myopia?
I assume it is more that 15x but I have 15.56 or 16.67 to choose from and I don't know how to get the correct answer.

What is the reflex speed if using a plane-mirror retinoscope at a working distance of 1/2M, with a patient with a -10DS error?
As i've never heard of the reflex speed being 'quantified' before, I don't know where to start...

Appreciate your time as always...

Hannah

[RIMLESS]

Curious...what test are you taking so I know to avoid it

[Hannah Doidge]

Haha! Just year one Optometry! These are meant to be possible passive OSCE questions, but we have certainly not been taught this stuff. They release past papers from previous years and boom! Panic!

[RIMLESS]

My undereducated guess is 16.67
64/4=16X
Then bump it up for the 25 mm viewing distance

[MakeOptics]

Hi again!
What is the magnification in direct ophthalmoscopy, assuming the subject's eye to be 25mm from the sight hole, when the subject has -4.00D myopia?
I assume it is more that 15x but I have 15.56 or 16.67 to choose from and I don't know how to get the correct answer.

What is the reflex speed if using a plane-mirror retinoscope at a working distance of 1/2M, with a patient with a -10DS error?
As i've never heard of the reflex speed being 'quantified' before, I don't know where to start...

Appreciate your time as always...

Hannah

Question #1

We know that the patient is myopic -4.00, so we can adjust a schematic eye:

Patients eye = schematic eye - (correction)
Patients eye = +60D - (-4D) = +64D

The focal length of the eye is:

focal length = 1000/64 = 15.625mm

Traditionally ophthalmoscopy is performed at the least distance of distinct vision which we assume to be 25cm or 250mm, however, we are holding the ophthalmoscope further away that the focal length so we need to adjust:

adjustment = distance held - focal point
adjustment = 25mm - 15.625mm = 9.375mm
distance adjusted = 250mm + 9.375mm = 259.375mm

Now we're cooking with gas, so we have our patient's eye reduced to a simple magnifier and our holding distance adjusted now we can figure out the magnification:

mag = 259.375mm / 15.625mm = 16.6X

I'll have to look at the second problem a bit more when I have some more time. The speed of the reflex is measured as a ratio of the angle of the light entering and the light leaving the eye, I'll have to dig through texts to find that stuff, I am pretty sure there is a shortcut I just don't have that procedure memorized.

[Hannah Doidge]

Thank you so much! I would not have had a clue but that seems very clear now!😅