Listen to part of a lecture in an Art History class.

(male professor)

I want to return to a discussion we just started last time about how painters are able to show details in any kind of light, from near total darkness to very bright sunlight. It’s easy to take this ability for granted, but if you’ve ever tried painting, you know this is a real challenge. Painters are trying to do something that the human eye does much more efficiently. Scientists have a term for our ability to see details in all kinds of light. They call it our perceptual dynamic range.

Dynamic range is expressed as a ratio. It’s the number of different distinct levels that our eyes are able to perceive between the extremes of dark and light. Now, in our everyday world, it’s normal for that range to exceed fifty-thousand to one, but when an artist tries to recreate that range using paint and canvas, well, those materials aren’t capable producing a range of more than about three-hundred to one. Susan.

(female student)

But that El Greco painting we were looking at last time, that seemed to have a lot of, I mean it had shadows and sunlight. The range looked pretty wide to me.

(professor)

That’s because El Greco used some painting tricks to make you think you were seeing a greater dynamic range. Remember, the very intense colors he used, uh, they were strong but they were also opposites, like the red and the blue clothing on the people. Those contrasting colors made certain details seem much clearer. Do you remember anything else about that painting?

(female student)

Well, there were those white lines he used to outline some different parts of the background. That made, uh, those trees really stand out in the dark forest.

(professor)

Right. Artists often use heavy white or black lines around the edges of a detail to make it pop out. In fact, let’s look, uh, yes David.

(male student)

Sorry, but, I was just wondering about photography. I’m guessing that photographs can show a greater range, uh, a greater dynamic range that paintings, right?

(professor)

I’m glad you ask that. I borrowed some pictures from Dr. Henson’s photography class to show you. But, first, to answer your question, surprisingly, the dynamic range in a lot of traditional photography isn’t much greater because of the film and paper that’s used. So, photographers developed technique to make details stand out just like painters have. Take Ansel Adams. I know you’ve all seen his photographs. Well, he developed a system in 1941. He called it the zone system.

Imagine a scale from pure black to pure white. Adams divided that scale, the whole range of light intensities, into ten equal zones, and this scale with its ten zones is like a visual rule that photographers can use to measure tonal ranges in a consistent way, and because of its consistency, Adams’ system was widely adopted and used until the 1980s. Of course how photographers used it is a little complicated. I think I’ll leave that explanation for Dr. Henson’s class.

(male student)

Does the system only work for black-and-white film?

(professor)

No, Adams used black and white but the system works just fine with color. Here’s something else that may surprise you, though. You’d think that adding color would help make details clearer in a picture, but it’s just the opposite. You really lose a lot of details at both the dark and light ends of the range, or at least that was true until HDR photography came along.

HDR stands for high dynamic range. High means that there’s a greater level of detail visible over the whole range. With HDR, photographers are taking advantage of the capabilities of digital cameras and computer software to overcome some of the inherent limitations of traditional equipment. Let’s take a look at one example.

Here, you’ve got three pictures, all of the same scene. That first picture, up there on the left, is dark, underexposed. You can see details in the snow pretty well and the clouds in the sky, but there’s not enough light to see much of anything in the darker areas. The second picture is a medium-exposure, which captures the middle zones, and the third picture is overexposed. The camera lets in so much light that you can see some details in the shadowy areas but the snow, the sky, they’re pretty much washed out. What if you combine the photos though? Use computer HDR software to blend details from all of the photos into one picture like this.

(male student)

Woo! That’s really neat.

(female student)

Yeah, if only I could have been using that technique! To think of all the photographs I’ve taken that might have ended up on the wall instead of in a trash can.