Eye Exam Mysteries Explained

6:00 PM

I had a great email question in my inbox this month so I decided to devote an entire post to it (keep those questions coming!).

Why does my doctor turn the lights low when checking my distance vision when I want to see sharply in daylight?  And then when they check my reading vision they use a bright light, while my problem is that I have trouble seeing to read in dark places, like in restaurants to look at menus?  

This is a question that a lot of my own patients ask, so I know there are a ton of people out there that have been wondering the same thing.  Here's your chance to have this mystery explained!

Checking Distance Vision in Low Lighting Creates the Most Challenging Environment

Is it easier to drive in daylight or during the night?  What about on a bright day or when it is raining?  Most everyone will agree that low lighting is very difficult on their eyes' distance vision.  That's one simple reason why your doctor typically uses a dimly lit room.  They want to challenge your visual system to find a prescription that works for all of life's difficult visual environments.  There are some technical reasons that make low lighting important too, however:

Phoropter via

In dim lighting your pupil dilates, and your eye muscles that control focusing relax.  When you are behind the phoropter, your doctor needs to keep you from squinting and over focusing to get a true reading, and lowering the lights is one way to achieve that.  Now your doctor is going to use several other tactics to prevent your eyes from over focusing too.  When I perform a refraction (getting your prescription) I always make sure I  go through a series of blurring and then clearing steps to make sure the eyes aren't over focusing.  There is a reason your doctor spends four years in optometry school -- they are being educated on how to make sure they always give you the best prescription without overworking your eyes, even if you don't answer 1 or 2  "correctly" if you will.

Checking Reading Vision in Good Lighting Gives the Truest Prescription

In bright light, or in near focusing conditions,
the pupil constricts (letting less light into the eye)
Pupil constriction is tied with the eye's ability to
focus at near.  That's why we read better with light than
in the dark, no matter the prescription of
glasses or contact lenses.
Have you ever tried to read in dim lighting?  It's hard, right.  The simple answer is that we check reading vision with light because it is next to impossible to achieve good vision to read in the dark, even if you are a kid.  The results just won't be very good, no matter what prescription we give you in glasses.

There is a technical reason to explain this, of course.  Everyone sees more poorly at night; that's because in dim lighting your pupil is larger. The pupil opening is a muscle, and when it is smaller that means it is contracting; and this contraction is inherently tied with your eye muscles focusing.  When we focus on something up close, we naturally have a constriction reaction to the pupil too.  If I want to determine your true near focusing ability, I need to create pupil constriction, and that's why lighting on the reading chart is important.  I want to create an environment where I can test your eye's natural focusing ability, and then see what prescription you need to aid that ability.  If I have your focusing system relaxed (as in dim or no light), everyone is going to have difficulty seeing to read and prefer stronger reading glasses. Remember, we use low lighting to test distance vision because we don't want to initiate your eye muscles to work.   When I check reading vision, I DO want your eyes muscles to work, so I can tell how strong they are.  That is why I need light!  As we age, our eyes are naturally losing their focusing ability (called presbyopia) so it is ALWAYS better to read with light if you want to be able to see what you are reading.  There is no way with any pair of glasses to get you to read as well in the dark as you do in light, because your pupil is not going to constrict as much, and your focusing system is not going to kick in as much!  We check reading vision with a light on because for most people, that is the only way they are going to see the 20/20 line (or anything close to it).

In a dim restaurant and need to read something, but having more trouble than reading glasses can alleviate in full?   Try using the flashlight app on your smart phone for a quick fix!

Feel free to post more questions regarding this here, and I will do my best to provide more technical answers if desired!

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    1. Thank you! If you ever have any suggestions on topics you would like to read about, I will be happy to oblige!

  2. I understand the concept on the lighting but why is it that after an eye exam and new glasses, my daylight vision is not as good, even blurred. Road signs can be hard to see or license plates.