Measuring Digital Dynamic Range

Knowing your camera's range helps you make crucial exposure decisions

By Timothy Edberg Back to

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Do you know the dynamic range of your digital camera—the tonal range of lights to darks it can usefully capture? It can guide the way you shoot; the smaller the dynamic range (DR), the more careful you must be that exposures don’t blow out highlights or fill in shadows. I’ve heard “experts” make conflicting blanket claims of anywhere from a four-stop to 12-stop DR for digital cameras, and manufacturer’s specs might be marketing hype—so why not just measure your DR for yourself? It’s easy to do; I’ll show you how. In the process you’ll learn at what exposure highlights saturate to pure white, and gain insight into how your camera assigns contrast.

You can determine the DR of your digital camera if it has manual expo- sure control, or exposure compensation that runs to ± 5 EV. You’ll need a computer with Photoshop or equivalent, two desk lamps, a sheet of 8.5×11-inch white paper, and a couple of hours. I’ll explain just what DR is for a digital camera; then I’ll describe how to set up and shoot the images you’ll need, and how to measure DR from your image files. Finally, I’ll show the results I got when I tested a Canon Powershot G6, a Nikon D2X, a Canon EOS 5D, and a Fuji S3 Pro.

What is digital dynamic range?

The dynamic range is the number of exposure stops between the lightest usable white and the darkest useable black. For digital cameras, determining the darkest useable black is a little tricky because of digital noise, a random speckle pattern produced by image sensors. A little bit of noise in bright tones isn’t noticeable, but it really stands out in dark tones. Noise limits the dark end of the DR: a camera may dutifully record extremely dark tones, but if noise makes the darkest tones virtually indistinguishable from each other, then, in practice, those tones can’t be used. (This is why I defined DR as extending down to the darkest useable black.)

Since noise sets the lower bound of the DR, measuring DR is a little subjective. A dark tone that’s too noisy for you may be just fine with me, and therefore we’d determine different DRs for the same camera. This subjectivity might be part of why expert opinion varies so widely.

It’s also helpful to know the exposure range the camera allows be- tween medium gray and pure white, which I call the “highlight latitude.” Greater highlight latitude decreases the chances of blown out highlights.

Measurement set-up

The DR measurement is straightforward: shoot a blank piece of evenly illuminated paper at several different exposures. Using Photoshop to read the pixel values for each of these shots gives you the camera’s response to different exposures. You will find the DR from these numbers.

Place the sheet of paper at the edge of a table and use two desk lamps or photofloods to illuminate it (any lamps will do). The lamps should be on either side of the paper, aiming down at 45o to provide uniform illumination, or pointed up to bounce off the ceiling. Ideally there should be less than a 0.1 stop brightness difference between paper center and corners, and certainly no worse than 0.3 stops. Check the illumination with a spotmeter and futz with the lights until the lighting is even. Lacking a spotmeter, you can take a picture of the paper and load it into Photoshop to measure the light distribution. Use the Info palette to examine pixel values at the center and corners. Adjust the lights as necessary and repeat until you’re satisfied. My test setup is pictured in figure 1.

Once the lighting is even, place your camera within a foot of the paper, being careful not to cast a shadow on it; a tripod is a great help. Select the widest lens aperture and manually focus to infinity. (Autofocus will likely default to infinity given the featureless image.) The point of these steps is to throw the paper texture completely out of focus so it doesn’t add to the noise.

Shooting the images

First select the ISO you want to test. ISO matters: the higher the ISO, the worse the noise, hence the smaller the DR. Happily, neither the color of the illumination nor the camera’s white balance had any effect on the DR in my tests.

With your evenly lit paper filling the frame, meter to find the shutter speed for medium gray. Then set the shutter speed to give five stops greater expo- sure. Shoot a frame. Then change the shutter speed in 1⁄2- or 1⁄3-stop increments to take successively darker images, going at least seven stops below medium gray.

Processing JPEG or TIFF output
Now load your camera’s pictures onto your computer and open them in Photoshop. Get some paper to record the results, and put on some good music because this may take a little time. For each frame you shot:

1 Use the rectangular marquee selection tool to select a box in the middle of the frame, about 10% or so of the total image size. Make your measurement only in this small box so that any residual non-uniformity of lighting will be negligible.

2 Open Image > Adjustments > Hue/ Saturation and slide the Saturation slider all the way to the left to completely desaturate the selection. This forces each pixel to have equal R, G, and B values: color information is removed, leaving just a single number to record as the pixel value.

3 Use Filter>Blur>Average to average the selected pixels together.

4 Use the Info palette to measure the pixel value inside the selection.

5 Write this number down, along with the shutter speed used for the image.

There’s one more step. At some point, noise will start to render the darkest shots useless, so you need to find the darkest useable exposure. To find this exposure, make a new Photoshop document and paste into it, in order and overlapping, rectangular selections taken from the center of several of your darkest exposures. Zoom in and look for the edges between rectangles. Moving from lighter to darker samples, the first edge that is indistinct because of excessive noise marks the darkest useable stop, as illustrated in figure 2. This means of finding the dark end of the DR is subjective, but I found that it gave me the same results as an objective criterion I tried out (based on measuring the signal-to-noise ratio in each rectangle), so it should work just fine.

Reading from your notes, the number of stops from the darkest useable exposure to the first pure-white exposure (RGB = 255) is your DR. Count the number of stops between the medium- gray exposure and the first pure white to determine the highlight latitude. Congratulations—you’re done.

For more insight into your camera’s behavior, graph your measurements, as in figure 3. The curve’s shape reveals the tones in which your camera emphasizes contrast—on semilog paper, the steeper the curve, the higher the contrast.

Dynamic range and RAW

RAW should offer more DR than JPEG or TIFF captures. If you want to measure what your camera provides in RAW, convert your RAW files using your standard conversion method; DR measurement then proceeds as above. Another RAW conversion to consider is one that minimizes contrast, which will maximize DR; this will inform you what DR your camera is ultimately capable of. For the record, I used Adobe Camera RAW in Photo- shop CS2 for my RAW conversions, using T = 2700, Tint = 0, Exposure = –2.5, Shadows = 0, Brightness = 75, Contrast = –50, Saturation = –100, and a linear Tone Curve, with all other settings at default.

Results for four cameras

To get some idea of how much DR varies between cameras, I tested four: I used my Canon Powershot G6, and thanks to a loan from my friends at Penn Camera in the Washington, DC, area (www.penncamera.com), I was able to test a Nikon D2X, a Canon EOS 5D, and a Fuji S3 Pro. The G6 represents a high-end point-and- shoot; the others are high-end DSLRs. For each camera, I measured the DR and highlight latitude for JPEGs at the camera’s native ISO (50 for the G6 and 100 for the others), as well as at ISO 400, and for RAW output at the lowest ISO. The reduction of DR at higher ISOs is apparent in the following graphs, but for high-end DSLRs it’s not so bad. The conventional wisdom that RAW capture gives about two extra stops of DR is borne out for the Powershot G6, but in my tests I found that higher-end cameras only gain about one stop with RAW.

The Fuji S3 Pro is unique among these cameras in that it has an adjustable DR. This stems from its sensor design, which has small pixels (for bright light) interspersed with larger pixels (for general photography). Turning up the DR setting combines information from the small pixels with at from the others, extending the useful range. This explains why the S3 is capable of the largest DR and has the most highlight latitude of all the cameras I studied.

Figures 4 through 7 show results for each camera in turn. Horizontal bars placed at the height of the darkest useable exposure indicate the DR. The small arrows denote the medium- gray exposures. As in figure 3, I plot exposure decreasing to the left. There is remarkable agreement between the tonal response of low ISO and ISO 400 JPEGs, it’s just that the low end of the DR generally cuts off sooner at higher ISO due to higher noise levels. For all cameras, the RAW response (as I’ve processed it) is steeper than the JPEGs’ at the high end, meaning there will be more highlight separation. RAW is flatter in the midtones and below, which is necessary to dig deep into shadow detail. RAW also has more highlight latitude than JPEGs.

In figure 8, I compare all cameras’ low-ISO JPEGs. You can see that the Fuji S3 has flatter highlights than the others in its extended DR mode, and the contrastiest highlights in its normal mode. The Fuji maintains roughly equal contrast throughout its ISO range, while the others give contrastier shadows.

If you don’t like graphs, here are my results summarized in a table. Given the subjectivity of any DR test, don’t take my results as absolute truth— your results may vary slightly.

Summary

Based only on the four cameras I tested, it seems that an eight-stop DR is about the best to expect in JPEGS, except for the 10-plus-stop range of the S3 Pro. In general, the DR decreases with higher ISO due to digital noise, but high-end DSLRs suffer much less from this than point-and- shoots. Capturing images in RAW mode will add at least a stop to dynamic range, maybe two, over the best JPEG, and will add a stop of highlight exposure latitude.

While there’s vastly more to camera quality than dynamic range and highlight latitude, they’re useful to know to guide your shooting technique. I’ve shown you how to measure them, and by sharing my results for four cameras, I hope I’ve illustrated the insights you can gain from the process.


About the Author

Timothy Edberg
timothy edberg
Timothy Edberg is an award-winning photographer and teacher whose photographs have been published in numerous photography and nature magazines. Before he heard the siren song of professional nature photography, Timothy Edberg was a Ph.D. research physicist. Now he searches the nation for spiritually uplifting photographs of the natural world.