Part 13 - "I Can't Take That Picture."

With Mother Nature Productions hard at work directing her annual fall epic of color, millions of photographers will be spending much of their time outdoors documenting the splendor of this year's display. While judging a photo competition recently, I was reminded just how challenging, and often frustrating, landscape photography can be to beginning photographers. So, as we gather our gear and head out to capture that perfect fall image, I thought this would be a good time for a photographic reality check.

Because the fact of the matter is: Even after carefully calculating your exposure, what-you-see may not be what-you-get in some lighting conditions. You might even walk away from a scene saying, "I can't take that picture."

This post lays the ground work in basic terms for Part 14, that will show you how to capture those dramatic landscape pictures containing an extreme range of densities -- just what you're likely to encounter this fall.

In the earlier two-part post on "Exposure is Everything", you learned much about the limitations of capturing and reproducing an image. Let's take some time here to expand on this information.

It all starts with the human eye...

Although our eye does not physically capture the picture that's recorded on a camera's data card, it is the image we "store" in our brains. This leaves open the possibility that the image our brain remembers is not the same picture that's stored as ones and zeros in the camera. As a matter of fact, this discrepancy occurs in nearly every picture we take. The difference between our eye's perception of a scene and the camera's digital version of that same image may be minor and of little consequence, or it may be extreme and of vital significance to our final printed picture.

Our eyes don't take snapshots. The activity of the eye is continuous, and our mind assembles multiple, adapted views to construct the image we remember. One of the most important analyses performed by our eye/brain combination is that of being able to see detail in the brightest and darkest parts of a scene. As the eye scans a scene, it constantly adapts to the available reflected light in every portion of the scene in an attempt to find and recognize the details in all areas of the scene.

This includes detail in the brightest snow as well as individual pieces of coal in a coal bin. (From an evolutionary perspective, this makes perfect sense. Who knows in nature where danger lurks -- in the shadows?-- or where the next meal may be found -- high in a brightly lit tree?) Having found all the perceptual detail, the eye assembles the image we see and remember -- moment by moment. Speaking in photographic terms, the range of brightness to darkness within which we can distinguish detail is called the image's dynamic range. Biologists call it survival.

When on location, the human eye can distinguish details in the snow, as well as in the shadows of the blue spruce tree, in this high dynamic range scene.

The camera is not nearly as fortunate. The lower dynamic range of today's DSLR sensors "see" far less detail in the brightest (highlights) and darkest (shadows) areas of a scene. Falling back on the previous posts on exposure, this difference in dynamic range can be illustrated in terms of a camera's histogram display.

An "Eye" histogram

Here's the histogram for a scene as the eye/brain might "record" it. Notice that all densities in this particular scene fall well within the boundaries of the graph. And we "see" the details in all areas.

The camera's histogram of the same scene

But, the same scene captured by a DSLR might appear as the histogram above. It's obvious that the lower end of the graph (shadows) and the upper end of the graph (highlights) extend beyond the borders of the histogram. These extremes will contain no detail in the final print.

The result in this example is that the eye "sees" more detail in the shadows and highlights than the camera's sensor is capable of recording. The dynamic range of the normal human eye exceeds the dynamic range of a camera's sensor.

Here are the same results in picture form:

Although this may be the picture your eye "saw" and remembers,

The camera "saw" and recorded it quite differently. Notice the clouds and trees are light and without detail, whereas the lake lacks detail in the darker shadow areas.

The difference between the dynamic range of the human eye and the sensor on a modern DSLR can also be thought of in terms of f/stops. Although f/stops are nearly impossible to accurately specify for the eye because of the way it functions, some approximations do provide a clearer understanding of the photographic problem.

• the dynamic range (darkest to lightest part of a scene) found in nature on a bright sunny day is approximately 12 f/stops or more
• the dynamic range of a normal human eye is approximately 10 to 15 f/stops
• the dynamic range of a typical DSLR sensor is approximately 5 to 9 f/stops.

Even if these numbers are not "dead on", they do point out the fact that what you see may not be what you are able to capture with a camera. There is also one piece of the puzzle missing in these numbers:

• the dynamic range found in nature on a cloudy day is 3 to 5 stops.

Aha. Dynamic range changes in nature depending on the type or quality of light striking the scene. That's true. Consequently, many pictures we take may fall within our camera's ability to record accurately. It's the exceptionally high dynamic range landscape-type images found on brighter days that concerns this post.

Jargon Alert: High Dynamic Range is often seen abbreviated to HDR.

So, was the camera's exposure incorrect in the mountain and lake scene above? NO. The exposure was as correct as the camera would allow. The dynamic range of this particular scene exceeded the camera sensor's capability to record. And, YES, the camera's histogram would have revealed that this image was going to be problematic.

But take heart DSLR owners, the same dynamic range issues have faced traditional photography since the day film became available.

Does this mean I can't take pictures when the dynamic range is greater than 5 to 9 f/stops? Not necessarily. Stay tuned for Part 14.