In the quest for high-quality digital capture in the uncontrolled setting of the outdoors, we’re always seeking ways to overcome ambient conditions that pose obstacles to our photographic vision. Chief among these limitations is light—or, more precisely, its insufficiency or absence. Now one of our old allies in overcoming the challenge of low-light natural environments—increased ISO—is taking center stage with the introduction of new digital SLRs offering ISOs as high as 25,600! What do these advances really mean for outdoor and nature photographers working in the digital realm?
Why ISO Matters
ISO (International Standards Organization) ratings represent the sensitivity, and thus the speed, of photographic films. Higher ISO films are more sensitive to light and record an image faster than lower ISO films, but the quality of the image is compromised by the resulting increased grain, loss of sharpness and degradation of color. In digital capture, ISO equivalents represent the sensitivity and recording speed of the digital sensor. As larger sensors packed with more megapixels are coupled with faster and more efficient processors, digital cameras can achieve clean capture in extremely low-light conditions, realizing at high-ISO equivalents results that are impossible to duplicate with silver-based film. Our tests show there’s little image degradation at ISOs as high as 1600, and acceptable results can be obtained at even higher ISOs.
With film capture, changing ISOs meant changing a roll of film or, for some of us, carrying two cameras equipped with different films. But the twist of a dial or push of a button adjusts the ISO on digital cameras from frame to frame. Flexible ISO capability can resolve a number of photographic problems related to light, such as the need for a faster shutter speed to stop action, a smaller aperture to increase depth of field or even a slower shutter speed to suggest movement.
To alleviate low-light conditions, we typically add electronic flash, or floods and reflectors in a studio. In outdoor/nature photography, projected flash and reflectors might work, but not at great distance.
To use one of our favorite, and most elusive, examples, consider the moose. Active and more easily found at dawn or in cool, overcast conditions, a moose doesn’t welcome a close approach, has a light-absorbing dark coat and is constantly moving as it feeds. What’s needed in this situation is a fast shutter speed to stop the animal’s movement and enough depth of field to sharply render the entire large body. Even with a fast telephoto lens, the optimum ISO of 100-200 won’t offer sufficient light-gathering capability to capture a usable image. Digital ISO to the rescue! By “dialing” up ISO 400, or even ISO 800, your D-SLR will allow you to choose the faster shutter speed and smaller aperture needed without a significant loss of quality.
Landscapes captured before sunrise and after sunset can yield a unique mood and unusual palette of colors that sometimes can be captured by a simple long exposure taken from a tripod. The problem can be the subtle movement of wind-stirred foliage or the clouds, moon or stars in the dark sky. If a sharp image is desired, the only solution to this problem is the increased ISO that will enable a shorter light-gathering exposure.
So why would you want to slow down the exposure time? Some cameras offer ISO settings as low as 50. This one- or two-stop desensitizing of the sensor, coupled with a polarizing filter, reduces light and lengthens the exposure to accomplish an effect, such as emphasizing the silky flow of water in a brightly lit stream or emphasizing the movement of a subject. Significantly longer exposure capabilities can be achieved with the lower ISO in combination with neutral-density filters.
The Downside Of Higher ISO
Here’s where the compromise comes in. While the newest D-SLRs offer greatly improved results at remarkably high ISOs, we’re still sacrificing some image quality every time we click that dial upward. Today, maximum image quality is achieved at ISO 100-200 on a camera with a full-frame sensor and the latest in-camera processor. The result is a sharp, finely detailed image. As you increase the ISO setting, the sensor becomes more sensitive, not only amplifying the light from your subject, but also random signals from other sources. The effect has been likened to increasing the volume of a radio with poor reception—both the program and the static get louder. The smaller and more tightly packed the megapixels, the more the sensor itself generates noise. The in-camera processor’s job is to filter out background noise while accurately rendering the image information to the recording media. The less efficiently the noise is filtered, the more it remains in, and degrades, the image.
There are two kinds of noise found in a digital image. One is luminance, colorless, granular variations in lightness that give an effect like film grain. Luminance is the most predominant type of noise in a digital image and the source of some textural detail. The second, more damaging kind of noise is chrominance, an array of green, magenta and blue specks. Chrominance is most evident in un-detailed expanses of a single tone, such as the sky. There’s nothing good to say about chrominance noise in an image. Excessive noise masks detail and sharpness in an image and causes color aberrations.
|Excellent results are seen in the blown-up insets even at very high ISO settings.|
It’s best to control noise before it becomes a nuisance. Resist the temptation to use ISO settings beyond the threshold of your camera’s abilities. Point-and-shoots with pixel-packed small sensors are pretty much limited to the lower ISOs, while recent full-frame D-SLRs have much greater capabilities in higher ISO ranges. Captures at up to ISO 800 have produced relatively noise-free images in our tests with a prototype of the new Canon EOS 5D Mark II.
Noise is most prominent in underexposed, dark areas of an image. Using multiple-capture HDR (High Dynamic Range) techniques can minimize, or even eliminate, noise in dark areas. Whenever insufficient light is an issue, use a tripod and carefully balance your ISO, shutter speed and aperture to achieve the highest-quality image possible.
When conditions demand the higher ISOs that undoubtedly will produce noise, postcapture software programs can save the day. These programs do more than simply smooth the mosaic of your noisy image; they allow you to choose the level of detail you want to retain (the textural grain) while restoring sharpness and resolving color anomalies to your preferences. Four of the best-known are Dfine 2 (www.niksoftware.com), Noiseware (www.imagenomic.com), Neat Image (www.neatimage.com) and Noise Ninja (www.picturecode.com), but there are many others.
The most important aspect of this type of program is control—of both the amount and the specific placement of reduction of each type of noise. A simple example of an appropriate application is a landscape with noise obviously present in the sky, but not as apparent in the foreground. In fact, the software is most effective in expanses of monochromatic or out-of-focus background. There’s always some loss of sharpness with the application of noise-reduction software, so you need to carefully balance the removal of noise with the smoothing of the image by the program. This means you should work on your image at a higher magnification than that at which it ultimately will be viewed. Competent noise-reduction software can frequently do an acceptable job on images taken at ISOs from 1600-3200. Beyond that, it can help, but don’t expect miracles. The result is likely to turn a very noisy image into a very smooth image, with neither containing the kind of information you need for a quality photograph.
Plan Ahead For Best Results
Digital ISO capabilities have clearly improved our ability to capture quality images and fast-action sequences in imperfect conditions. But when considering image-degrading higher ISOs, you have to make some compromises. Balance the importance of getting the shot and its intended use with the limitations of the technology, get the best quality capture you possibly can at the outset and learn how to improve the image in post-capture software. Actually, that’s a workflow that good outdoor/nature photographers should be practicing every time they press that button!
ISO Test: What’s Your Threshold?
When you’re pushing your ISO to shoot in low light or to stop action, how far is too far? Consider the following factors:
1 Your camera’s abilities. The larger your sensor, the better your processor, the more capably your camera will perform high-ISO captures. Some cameras have built-in noise-reduction software that can help, but it may render your image less sharp.
2 Potential use. You can get away with more noise in images that are intended only for projection or posting on the Internet, or small prints. Larger prints will magnify the problems.
3 Postcapture work. Are you willing to put in the time to correct noise-related image anomalies in a postcapture software program? In this process, be careful not to remove detail and sharpness from your image, and don’t expect perfection. The quality of the results will depend on the quality of the original image.
4 Individual standards. Everyone has different limits and tolerances for imperfection in their images. One person’s Picasso is another’s trash-bin fodder.
To find out what your equipment will do and to give you confidence about using increased ISO in the field, test your camera under controlled conditions.
5 Set up your test with the intention of obtaining the best images possible. Use a tripod and a cable release, and select the optimum lens aperture (ƒ/8 to ƒ/11). Turn off autofocus, and if the lens has image stabilization, be sure the IS or VR function is turned off or your images will be consistently un-sharp. Work with manual exposure settings, basing the exposure on the exposure scale in or on the camera.
6 Select a subject that will remain stationary long enough for you to accomplish a series of long exposures.
7 Shoot the subject under low-light conditions. Lepp accomplished all of the images in a landscape test before sunrise with only the first light in the sky on the subject (not shown). Be aware that the quality of that light will change constantly until the sun breaks the horizon, so work quickly. The meerkat was photographed in full shade.
8 For the landscape, when set at ISO 100 and ƒ/11, the first exposure was 30 seconds. At ISO 200, the exposure was 15 seconds at ƒ/11; at ISO 400, 8 seconds. Lepp continued to double the ISO (one stop) while halving the shutter speed (one stop) until the camera’s maximum ISO of 25,600 at 1⁄4 sec. was reached. With the meerkat, the fastest shutter speed possible at ISO 100 was 1⁄45 sec. As the ISO increased, Lepp was able to use faster shutter speeds and smaller apertures for depth of field.
9 There are several ways to evaluate the test. Bring the image up to full-screen in your image-editing software and examine it closely. Then enlarge the image to 100% (in the camera’s native resolution). At 100% you should be able to evaluate the amount of luminance and chrominance noise. Then make an 8-1⁄2x11 print of the entire image. If you plan to make larger prints, blow up the image to the intended size and print an 8-1⁄2x11 section for comparison purposes.
10 Look for granularity (luminance noise). It looks a lot like excessive film grain. Also look for specks of magenta, green and blue (chrominance noise) in areas where they don’t belong. You’ll see this most predominantly in skies and monochromatic expanses. Be alert to a lack of sharpness within the image due to excessive noise.
11 After you’ve reviewed your test series, you may want to run some of the higher-ISO images with unacceptable noise levels through noise-reduction software to determine how much improvement you can achieve with postcapture processing.
Doing your own tests is worth the time. Once you really understand the relationships between high ISOs and noise in your photography, you’ll be ready to use this new digital technology to its full advantage.