Aspects of Vision 2: Detail
This is the one most people think of first. Is what I’m looking at big enough to see? Second question — is it big enough to see comfortably?
- When you’re looking at a bacterium, it’s simply too small to see, by anyone.
- When you’re looking at legal fine print, or getting a tiny splinter out of your finger, those are things that healthy eyes can see, but they are uncomfortably small.
Our ability to discriminate fine detail is measured by Visual Acuity (VA). Whenever you see a notation like 20/20, 6/6, 20/400, 6/36 and so on, this is a measure of visual acuity.
It’s important to understand that visual acuity not a test score. 20/20 is not like getting 20 out of 20 on an exam — it doesn’t mean you have ’20 out of 20′ vision. All it means is that you can see at 20 feet (the first number) letters of a size that a ‘normal’ eye can see at 20 feet (the second number).
Most of the world uses the metric system, so the equivalent is 6/6 (seeing at 6 meters what you should be able to see at 6 meters). I’ll be using the metric system throughout this website, because it’s the standard, but I’ll try to give some imperial measures as well, to keep readers in countries that use the imperial system (Liberia, Myanmar and the USA) happy.
If your ability to see fine detail is something other than normal, the second number will be different. For instance, a person with VA 6/24 (which is the same as 20/80) isn’t seeing very well, because at 6 meters they can only just see letters of a size that people with normal vision could see much further away, at 24 meters.
Visual Acuity tells us about just one aspect of our vision
So, what exactly does VA tell us? It tells us how good the very best bit of vision in your eye is, when given beautiful black and white single letters in a well-lit room. That’s it. It doesn’t tell us anything else — and there’s a lot more we need to know.
VA is actually a pretty good measure of vision in a healthy eye. If you just have optical blur (you’re short-sighted or have some astigmatism), your VA will be reduced, and how much it’s reduced will be a reliable indication of how strong your glasses will need to be. VA is the main measure used in determining whether you have to wear glasses for driving, and is quite valid for that, for healthy eyes.
If you have healthy eyes, VA is a good measure of how well you see.
But, let’s get one thing clear right now. In many eye conditions, high-contrast single-letter well-illuminated detail vision is one of the last things to be affected. If you’ve got an eye condition/disease and your VA is poor, you definitely have an impairment. But having a good VA does NOT mean you have good vision. You can have a severe vision impairment and still have a good VA.
Much more on that later.
If your eye isn't healthy, having a good VA is NOT a reliable indicator that you have good vision.
Many of my patients express frustration that they go to their eye care professional for help with what feels like very poor vision, but they are told they still have very good vision, based purely on the fact that they can still read a long way down the chart. Sometimes it almost feels like gaslighting. When I use tests that demonstrate there are problems with other aspects of their vision, it can be a relief.
Which Part of the Eye Determines VA?
Since visual acuity is a measure of how good is your very best bit of vision, that bit is almost always the fovea. But if your fovea has been damaged, the next-best bit of vision will come from part of the macula — or, if the macula has been damaged, the point on the retina that’s closest to the macula.