Image Resolution

Topics Covered:

  • Resolution and Detail
  • The Optotype
  • The Resolution of the Human Eye at 20/20 vision

“There are people who have been tested at 20/8 vision, and 20/12 vision is quite common. But that’s nothing compared to hawks – they supposedly have 20/2 vision.”

Another long intake of breath by Dr. Optoglass. He was like a whale, Bob thought.

resolution

“There are three properties that define the way we perceive sharpness. The most important, which we’ll start with, is resolution. Before you can understand resolution, you must understand the Optotype. Don’t bother with any thoughts on its rhyming with my name.”

I wouldn’t have if you hadn’t put the thought in my head, thought Paul. It doesn’t rhyme, dummy, thought Bob.

“The letters you see on the Snellen chart are all optotypes. It’s basically a symbol that is easily recognizable and has a measurable size. As I told you earlier, we use alphabets to reach the most individuals. The most famous optotype has to be the letter ‘E’.”

“Why?”

“I’m telling you why, aren’t I? Snellen found, that if a man, or woman, stood at twenty feet from the chart, and he or she had 20/20 vision, the size of the optotype, in other words its length, subtended an angle of five arc minutes.”

“Subtended a what?”

Arc minute

“An angle of one degree is 60 arc minutes. We use arc minutes for angles that are too small to be recorded with ‘degrees’. The letter E has five lines, three black and two white. Let’s assume they are of equal thickness. In that case, if a person with 20/20 vision subtends an angle of 5 arc minutes, each line will be equal to one arc minute. Understood?”

“You mean that’s the most a person can see?”

“If he or she has 20/20 vision.”

“But why do we measure this in degrees or arc minutes? Why not millimeters or something else?”

“Good question. Simply because the size of the optotype, at any given angle, will vary according to distance. If you place the chart at fifty feet, the size of the letters change. But the person with 20/20 vision can still only subtend a maximum of one arc minute. This is how we ophthalmologists measure resolution.”

“So what does have to do with cameras and lenses?” Paul had to ask.

“Engineers of camera and lens equipment use lines like the letter E – one black and one white alternatively – to measure the resolution. One black and one white together make a pair, and is called a cycle. Some people measure it in lines per millimeter or line pairs per millimeter, some measure it in cycles per degree. But you’ll find that the basic idea is still the same. So, Bob, can you guess what resolution is?”

“The smallest detail a person can recognize is resolution.”

“Excellent, Bob. I’m impressed.” Dr. Optoglass beamed. “You are absolutely right. Human beings classify objects and name them. This means there must be boundaries between objects so we can distinguish one object from another. As we discussed earlier, boundaries are a fuzzy concept, and depends entirely on how close or far you are from an object or its edge. From a fixed position, if our brain is capable of defining edges around a detail, and we are able to see the detail clearly, the smallest such detail can be used to calculate the resolution of the eye. You realize now why this property is extremely important in our understanding of sharpness.”

“Yes, doctor. Tell me more.”

Dr. Optoglass had a twinkle in his eye. He took another deep breath. Paul braced.

Takeaways:

  • An optotype is a standardized symbol used to measure resolution in test charts.
  • Resolution is the smallest detail the eye can see from any given position.
  • The resolution of the human eye for 20/20 vision is one arc minute.

Links for further study:

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