Quality Vision

Last month I wrote about 20/20 vision and how that was a measurement of the size of an object seen by the human eye at twenty feet. What it doesn’t tell me is how well you can actually see that object.

A good way of thinking about quality of vision would be if you placed a car number plate at the official 20.5 metre distance. On a dry clear day you would normally see that very clearly. Now, just imagine a bank of fog drifting between you and the number plate. You can still make out the numbers but, as the fog gets denser it becomes harder to make out the number. Legally, you would still meet the UK standard for driving because you can read the appropriate size numbers at the requisite distance but it would be a very unpleasant and possibly dangerous experience driving because of the poor quality of your vision.

Comfortable vision is also another factor that is very difficult to measure. There are no tests and no numbers to measure or describe comfort. As eye-care professionals we have to rely on your description of comfort. We typically ask you to rate your experience on a scale of 0 to nine. The problem then is you might rate your discomfort as a three while another person experiencing the same problems and level of discomfort could rate it as a nine.

Frequently I see patients who can easily recognise small letters when reading but are unable to read fluently. They often have been elsewhere and told their vision is fine. Likewise, I often see elderly patients who can read a number plate at 20.5m but who are an absolute liability when driving in poor visibility or at night. When it comes to vision it is not always size that matters.