What Is Dynamic Range?
The light that my eyes saw and the light that my camera was able to record are the same in this image.
I think of myself as reasonably smart, but photography, on a fairly regular basis, challenges that perception. For example, it took me forever to completely understand how light in a photo is managed through aperture, shutter speed and ISO settings. I am happy to say I have gotten over that learning hurdle. I have also made peace with the totally unintuitive system we use to describe the settings of aperture, shutter speed and ISO. It just does not seem right that an aperture setting of f/2.8 describes a lens opening that lets in more light than an aperture setting of f/16 and a shutter speed of 5 lets in more light than a shutter speed of 250. All of these observations serve as an introduction to another stumbling block in photography, one that I have not come to terms with, the concepts of dynamic range and tonal range. I mistakenly use these two terms interchangeably. Let’s start with a formal definition which I found in the DP Review Forum, Trying to understand tonal versus dynamic range.
“Dynamic range is the luminance range the sensor is able to detect, while tonal range is used to describe the actual number of tones captured in the image.”
While that definition is a start, it does not lead me to a practical understanding of what either term means. In fact, what that definition does is open up a whole line of questions for me.
Dynamic Range
First of all, the term dynamic range is used to describe a property of an image capturing device, like a camera or our eyes. All of those times that I commented that a photo had a great dynamic range was a total misuse of the term. Tonal range is used to describe a property of an image. Dynamic range and tonal range measure the same thing but in different contexts.
Dynamic range is measured in stops. Alas, stops are not a constant measurement like inches or ounces. One stop is making the exposure, however dark or light it is, either twice as bright or twice as dark. The brightness, however bright it is or is not, is changed by a factor of 2.
This is the first of three pictures that will demonstrate the effect of adding one stop of light. This picture was taken with ISO 200, Aperture f/2.8, Shutter Speed 1/400 sec. Clearly, this is an underexposed picture. I want to double the light. I can do this by changing the ISO, which in a digital camera is a measure of how much the sensitivity of a camera’s sensor has been amplified. I can also add light by leaving the shutter open longer. In this situation, I cannot add light by opening the lens any more because at f/2.8 it is all ready wide open.
I doubled the light by increasing the ISO one stop to ISO 400. The Aperture stayed at f/2.8 and the Shutter Speed stayed at 1/400 sec.
I doubled the light once more by increasing the ISO by one more stop to ISO 800.
The human eye has a dynamic range of 20 stops. We can see light that ranges from pure white to pure black with 253 in-between gradations of highlights, midtones and shadows. Certainly not everything we look at uses our dynamic range capability of 20 stops. A sunny, evenly lit day can have low dynamic range for our eyes to see because even the shadows are only 3 stops darker than the brights. Also, when the range of tones is severely restricted, there is a low dynamic range. If you look at a red flower against green foliage it will have very few tones and thus use only a portion of the dynamic range our eyes are capable of seeing.
I chose this particular plant as an example because of the limited range of tones. If you look at the histogram in the bottom left hand corner you will see it is very narrow and mostly consists of shadows and the dark mid-tones. The dynamic range was so small that it used less of both my eye’s ability and my camera’s ability to capture stops of light.
Converting this picture to b&w shows just how few tones it really has.
What happens when a camera with, say a dynamic range of 10 stops, records a scene that our eye records with a dynamic range of 20 stops? In a high dynamic range scene, and by that I mean a scene where the human eye sees the maximum number of tones, which is 255, a device that has a dynamic range of 10 will still record a pure black and a pure white, but compress the in-between tones. As an example, Tone 200 in the 20 stop dynamic range might map to Tone 255, which is pure black, in the 10 stop dynamic range. Every tone darker than Tone 200 will also map to Tone 255. A lot of tones, to be exact, 55 of them, will be lost along with the detail that our eyes can see. On the lower end, Tone 45 might map to Tone 0, which is pure white. In addition, every tone brighter than Tone 45 will also map to Tone 0. In this case we say the light is “blown out” and we have no chance of recovering detail in 45 tones where our eyes see detail.
To give you an idea of dynamic range device capability, a high end camera can record 10 to 14 stops of light, a high quality monitor can display 10 stops of light, and a photo print can display about 7.5 stops of light.
Exercise
What do you do when your camera cannot record all of light in a scene?