Defining Sharpness

Topics Covered:

  • Defining sharpness
  • Visual Acuity
  • The Snellen Chart

Dr. Optoglass opened a drawer, and removed a scalpel. “Okay, Bob. How sharp is this knife?”

Bob examines the scalpel. “Pretty sharp.”

“Would you say it is sharp enough to cut your finger?”

“Sure.”

“I’ve got a quotation for you:”

I believe there’s nothing more disturbing than a sharp image of a fuzzy concept – Ansel Adams (The Camera)

“Unfortunately, sharpness itself is a fuzzy concept. If you hold this scalpel far enough, it blurs in our vision, and we can no longer see the edges. If we look at it closely, or under a microscope, the sharp edge becomes a smooth surface. What does that tell you, Bob?”

“Sharpness is in the eye of the beholder.”

Sharpness

“Well said. There is no absolute sharpness in objects, only in our perception of them. Do you understand, Paul?”

“Sort of.”

“In the olden days, people sharpened their swords and carried them into battle. Their lives depended on their definition of sharpness. Today, we carry out innumerable computer tests on mathematical charts and then what? We still have to worry about the sharpness of our swords! Ultimately, it is impossible to satisfy the whims of human subjectivity with controlled testing.”

“So does that mean there’s no way to measure sharpness?”

“Not directly. No. But you could use a few tricks.”

Paul was worked up. “Surely doctor, you don’t know about cameras and lenses. It’s very different from testing eyes. It’s complicated.”

“I’m quite aware of the differences, Paul. But the science is reasonably the same. Haven’t you ever wondered why an important organ such as your eye is tested on a small chart with alphabets on it? Surely your eyes are more important than your camera, especially since you need them to look through your viewfinder.”

“I hadn’t thought of that. So why do we look at a chart with a few alphabets on them?”

“The alphabet part is easy. It is the most identifiable bunch of symbols in a culture. One would hope to be able to serve the largest number through the use of alphabets.”

“That’s logical. But who came up with all those sizes for the alphabets?”

“The chart most commonly used today is called the Snellen chart.”

“It’s a simple chart that needs to be read at 20 feet. If you can read the last line, you have twenty-twenty vision. What that means is that an average human with good eyesight can read the last line at twenty feet, and if you can too, your eyesight is average. If you can only read the last line at ten feet, then you have 20/40 vision. If you can read it at 40 feet, you have 20/10 vision. In my line of work, the sharpness of the eye is called Visual Acuity.”

“Is there anybody alive who can read it at 40 feet?”

“Let’s find out.”

Takeaways:

  • There is no absolute sharpness in objects, only in our perception of them.
  • Visual Acuity is the general term for clearness or sharpness of vision.

Links for further study:

Next: Image Resolution
Previous: The Eye Checkup