Does the “which is better, one or two?” part of the exam actually do anything? 

The refraction is the most well known part of the eye exam. If you’ve ever gotten a professionally done eye exam, they likely did a refraction. In optometry terms, what we’re trying to do is find the correct optical power to place the image directly onto your retina. In layman’s terms, we are just trying to make everything as clear as humanly possible.

I’ve had many people in my career tell me that this part of the exam gives them anxiety because they aren’t sure if they are going to “give the right answers”. Bottom line is, even if you tell me the “wrong” answer once or twice, I’m going to check it anywhere between three and ten times to make sure it’s the correct power for you. The youngest patient I’ve ever given a prescription to was a one year old. Most optometrists will know what you need before they walk into the room.

During an eye exam most people go through an auto-refractor. This is an instrument that gives me an estimate of your glasses prescription. Most newer auto-refractors are really good at getting me in the ballpark of what your glasses prescription will be. I will typically start with what the auto-refractor tells me it thinks your prescription is. From there, I am testing a few lenses to find out which power you prefer. After we have settled on one prescription for the right eye, I’ll do the left, then both eyes together to make sure both eyes are balanced. By the time I’m done, you will have chosen and rechosen your final prescription multiple times.
 

Okay, but why do they look the same sometimes?!?

There are a couple of reasons behind that one. The first being that eyes that haven’t had cataract surgery yet have what is called a lens and a muscle that controls the lens. This muscle allows you to bring things into focus.  Ideally, we want this muscle completely relaxed during an eye exam so that the power in the glasses does all the work bringing things into focus; however, unless you are dilated during your refraction it can be difficult to achieve. Unfortunately, we do not have control over this muscle and it will attempt to bring things into focus even when we don’t want it to. If you have plenty of power left in this muscle, it can allow you to focus through a lens that is too strong for you. This will leave you thinking that the overpowered lens has a similar level of clarity as a lens that’s a more appropriate strength for you.

The second reason is a little more physics-ey. The entire point of the refraction is to put the image directly onto the retina. The problem is - glasses can only be so customized. The glasses’ power is referred to as a diopter. The smallest we can change a diopter is by 0.25.  Hypothetically, if your retina requires glasses of 1.37 diopters, there is no actual way for us to give that to you in glasses. In which case, the 1.25 setting and the 1.50 setting are going to look about the exact same.

Either way, no need to be worried about this part of the eye exam. There are a lot of built-in protocols that ultimately lead both you and your eye doctor to the right prescription in the end.