# What lens is needed for a retina scan?

I was originally planning to as this on Physics Stack exchange, but found this question while searching, which directed the asker to here.

While my question is not a duplicate of that, it still falls in the same classification, so I believe it's on topic here.

That said, I recently had a Fluorescein angiography and the following thought occurred to me about it.

Assume there's a piece of graph paper in front of me, the job of the lens in my eye is to project a sharp image of the paper on my retina. I have astigmatism, in both eyes as it happens, making glasses with a cylindrical correction necessary to read anything.

So, during the angiogram, the exact process is reversed. The camera involved projects a sharp image of my retina on a photographic sensor. Except I don't have my glasses on, so I would expect the astigmatism would lead to an imperfect image on the sensor.

How then is the camera able to get a sharp image? When reviewing the results afterwards with the doctor, I was quite impressed with the detail and clarity of the images.

• @Rob My apologies, I've updated the question to correctly describe the procedure. From the Wikipedia page, it involves light at 490 nm wavelength and 525 nm wavelength. – dgnuff Aug 3 '18 at 1:31

I recently had a Fluorescein angiography and the following thought occurred to me about it.

I have astigmatism, in both eyes as it happens, making glasses with a cylindrical correction necessary to read anything.

The camera involved projects a sharp image of my retina on a photographic sensor. Except I don't have my glasses on, so I would expect the astigmatism would lead to an imperfect image on the sensor.

How then is the camera able to get a sharp image?

From the American Academy of Ophthalmology: Astigmatism

"Astigmatism is measured in diopters. A perfect eye with no astigmatism has 0 diopters. Most people have between 0.5 to 0.75 diopters of astigmatism. People with a measurement of 1.5 or more typically need contacts or eyeglasses to correct their astigmatism in order to maintain clear vision.".

Most people have some astigmatism.

The Topcon Medical Systems "TRC-50DX Mydriatic Retinal Camera" webpage lists their TRC-50DX Datasheet (.PDF), which says the device has these settings (and autofocus):

"Diopter Compensation Range For Patient’s Eye

 0 Setting:  -10 to +6 Diopter
- Setting:  -9 to -23 Diopter
+ Setting:  +5 to +23 Diopter
+ Setting: +22 to +41 Diopter (Ocular Anterior Photography)".


The machine has its own glasses built-in, which the doctor can adjust to match the eyes of the patient (whether they own glasses or not). Glasses are not worn since there could be smudges, scratches or dirt; it would be more glass in-between the sensor and your eye, and more to adjust for.

• TL;DR: pick a lens which provides a focussed image for the operator or the camera in use :-) . I think the OP is thrashing a bit here. – Carl Witthoft Aug 3 '18 at 14:36
• DR, He is asking "Why is it that he needs glasses to see clearly yet he can take them off and the machine can look in the opposite direction and see clearly?". That's why they use the phoropter, so they know what numbers to use. – Rob Aug 3 '18 at 14:47