Optometric Device: How Does it Work?

I went to the eye doctor last week for the first time in years, and they asked me to look into a machine. It had a little picture of a house, that kept shifting in and out of focus. After about 30 seconds, the gal pointed to her computer screen, and said, “This is your prescription, and the axes* on this image show your astigmatism.”

My question is, how did the machine know my 'script?

My guess is that it reflected light off my cornea, recorded where it bounced to, and were able to find both the thickness of the cornea, and the angle it was at, at all points. Then, they bounced light directly off my retina, to find out how “deep” my eyeball was. Knowing those, it would be trivial to calculate where the light is focused, compared to where it should be focused. Am I close?

*Actually, she said “the football shape”, and I wanted to ask why she didn’t say, “the rugby ball shape”. :wink:

Not familiar with the device, but there are several ophthalmic devices that use reflections from the cornea to determine its shape. They don’t all work the same way. There isn’t any need for it to measure the thivckness of your cornea, or to measure down to your retina, My guess is that is calculated your prescription from the shape of your cornea. it’s possible there was a little more to it – like maybe somehow sending a beam in and checking to see if it’s focused.

The reason I thought it would need to know the thickness of your cornea was for the same reason a straw looks bent in a glass of water. I forget what the effect is called.

I like your idea of it checking to see if a beam is focused. It felt like I was constantly refocusing, and the image was getting “farther” and “closer” the whole time. That would make it easier, so you could mount the beam to the image producing device, and not have to articulate it in order to simulate a change in focus.

If that were the case, would the optics in the machine be looking at your retina in order to see if the beam was focused there, and in changing the “distance” of the image, be able to see in which direction it was skewed?

I went yesterday and it was the same old “which lens makes the top line look clearer?” routine. It’s really subjective, sometimes I’m really not sure which is best, and I was wondering at the time why there wasn’t a machine that could simply measure your lens, eyeball shape and size, etc, and work it out from there.

I agree about the subjectiveness. After the gal put me on that machine, I went to another room, and another gal put me on the same ol’ “One, two?”, routine as well. However, when I sat down in the chair, the gal used the numbers to show me three lines, of which I was able to read the middle one, without having to look at any other than those three. Then, she put the lenses on me, and the large dials were already adjusted to within a few “flips” of the small increment changers. She did the “flips” just to fine tune what the machine had originally said.

I think they do this because the digital machine can only get so close, but the mechanical machine can get it spot on. Also, since it is subjective, that might actually be a benefit, because, after all, you’re the subject that’s going to be using said 'script. Also, when she got to the point where I couldn’t really tell the difference, I think she went past it, and then took the middle one of the three or so I couldn’t tell the difference between. It seemed that she was going in one direction, then stopped, then came back.

I’m very familiar with the machine you’re talking about. I’ve been wearing disposable contacts for about 15 years now and so I get an exam every year and this machine is always part of it. I asked the Dr how that thing ‘knows’ when the image is in focus but I don’t remember his answer. Next time you go ask them.

The picture of the little farmhouse at the end of a path is new. It used to be just an image of an enlarged asterisk ‘*’.

Slightly off topic, but just to show that the new, modern machines are not necessarily as “objective” as one might think:

I’m getting laser eye surgery in a couple of weeks, so I’ve become pretty well acquainted with the various pieces of equipment used for determining prescriptions. As long as I have worn glasses/contacts, I have only needed correction in my left eye and it looked like I would only need to have surgery on my left eye.

In a recent visit, the machine described in the OP picked up a significant (but not huge) prescription and similarly a significant astigmatism. Apparently, the prescirption and astigmatism had been working against each other to the point that the old-fashioned machines (“which looks better, 1 or 2?”) were registering my prescription as being very slight.

On the other hand, they could not get my eyes to accept the “correct” prescription according to the newer machines; however, the new machine gave a better starting point for comparison which lead to a prescription which provided better vision than I have had for years without corrective lenses. The new lenses actually got me to 20:15, though it differed pretty greatly from the reading from the newer machine.

Although the new machines provide a detailed map of the eyes, it seems that the old method is still important for determining the actual level of correction necessary. From what the doctor told me, there are still components of how the brain interprets images, etc. which cannot be completely determined from measuring the eye.