The World of Contrasts

Topics Covered:

  • What is Contrast?
  • Contrast Ratio and Dynamic Range

Contrast is what makes photography interesting – Conrad Hall

“You like boundaries and edges, don’t you?” quizzed Dr. Optoglass. He can see through me, thought Bob.

“They are interesting, I guess.”

“More than that, my dear boy. They are absolutely necessary. Did you wonder why we use black and white lines for our measurements? Why not shades of grey?”

Contrast

“It’ll be more difficult to see.”

“Yes, it would, wouldn’t it? So we chose the two colors that are the furthest apart from each other. We call this difference contrast, and a pure black and white image, with no shades of gray, has the highest contrast. We can reduce the amount of white and black to shades of grey on each side, until the two grey sides are exactly the same. Do you understand what this means, Paul?”

Paul wasn’t expecting an oral examination at the eye doctor’s. “Not really, Dr. Optoglass.”

“It’s easy. Boundaries need contrast! Without contrast, we won’t be able to distinguish one object from another.”

“I see.”

“And so you should. Therefore, contrast is the value that tries to explain the difference between the brightest parts and the darkest parts of an image. This difference is called the Contrast Ratio or the Dynamic Range. Unfortunately for us, though, this too is a fuzzy concept.”

“Why, Dr. Optoglass?” Bob’s mind was racing. “We already have the black and white points, don’t we?”

“Extremely sharp observation, Bob! Yes, we have the values of white and black, but scientists disagree on how to use that to find the contrast of an image. Still, we are not too badly off.”

“Say, doctor.” Paul had to show he wasn’t sleeping. “If contrast determines the boundaries, then won’t an image appear sharper if we just darkened the boundaries?”

“We have an answer for that too, Paul. It’s called Acutance.”

Takeaways:

  • Simply put, contrast is the difference between luminance values of an image.
  • Unfortunately, there are many schools of thought on how contrast should be measured.
  • Contrast Ratio or Dynamic Range is the difference between the lightest and darkest parts of an image. Like contrast, this property, too, has various interpretations.

Links for further study:

Next: At the Edge of Acutance
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