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| Click photo to read the step-by-step tutorial on How to Read a Camera Test. |
Image quality depends most of all on a camera's resolution, color accuracy, and noise levels at various ISO settings. To determine these factors, we photograph and analyze several industry-standard test targets, each illuminated by a pair of daylight-balanced Dedolight HMI lights. The lights are set at 45-degree angles on either side of the target to minimize glare and reflections. Priced at about $7,500 a pair (street), these lights have spectral qualities and brightness that are as close to daylight as any commercially available light source, far exceeding daylight fluorescent, tungsten, or halogen lights. DSLRs are manually focused on the targets using the highest-quality 50mm lens available from the manufacturer, and exposures are bracketed to ensure proper contrast. With EVF and compact digital cameras, we set the camera to 50mm equivalent and rely on the autofocus system for proper focus.
1) Resolution. The IT-10 test target helps us determine the maximum number of individual horizontal, vertical, and diagonal lines that can be resolved by a camera. It also reveals the extent of color moiré problems in fine details, and provides the first indication of a camera's contrast and noise levels. We average the three line counts and use this number to give the camera a resolution rating based on the scale shown below. As of this month, our resolution ratings have been updated to reflect today's higher-megapixel sensors, making it tougher for a camera to earn top honors. Under our new scoring system, an average of 1700 lines or higher now earns an Excellent resolution rating; previously it was 1500 lines or higher. This means that the 8MP Canon EOS Rebel XT maintains its Excellent rating, while most 6MP DSLRs (including the Nikon D70s) and even the 8MP Olympus Evolt E-300 drop to Extremely High resolution.
2) Color accuracy. We use the GretagMacbeth ColorChecker DC test target to determine color accuracy. The camera's white balance is set manually to match the color temperature of the Dedolight HMI lights (usually 5600K), and we perform separate field tests to help us determine the accuracy of the camera's white-balance system. Software analysis of the 177 color patches on the ColorChecker DC target results in an average Delta E for all the patches.
Delta E is an international standard measurement of color difference, corresponding to the perceptual color sensitivity of the human visual system. Generally, a Delta E value of 1 corresponds to a color difference between two samples that would be noticed only by an expert under strictly controlled lighting, while a Delta E of 3 can just barely be seen by most viewers with good color vision in a brightly lit room. Our ratings are based on the scale shown above.
In addition to determining color accuracy, readings from the same target can be used to create a 3D gamut map, which gives us another indication of the camera's color reproduction capabilities.
3) Noise. We determine a camera's noise level at various ISO settings by shooting an FBI SIQT v.1.0 target and analyzing a 100x100-pixel square in the center of the large middletone gray patch. Using Adobe Photoshop's histogram function, we average the standard deviations of the luminosity, R, G, and B channels, and base our rating on the scale at right.
4) Image quality. Our final image-quality rating is based on a formula that considers resolution, color accuracy, noise levels, highlight/shadow detail, and contrast (weighted in that order). For example, to earn an Excellent image-quality rating, resolution and color accuracy must be Excellent, and the noise rating must be Low, Very Low, or Extremely Low, with High shadow/highlight detail and Normal or Adjustable contrast.
5) AF speed. All 35mm SLR and DSLR reports include the results of autofocus speed tests for light levels ranging from EV 12 (at ISO 100) down to the AF systems limit (usually EV 1, 0, or –1). These can help you determine how one camera's AF system compares to another when shooting a high-contrast object in bright to low light. In addition, we perform field tests to see how well a multizon e AF system works on a variety of subjects, including those with low contrast or fewer details. We usually report the field test results in the main body of the article for a given camera rather than in the Certified Test Results box. For DSLRs, the AF-zone sensitivity and size are also measured in the lab, resulting in a chart that describes the actual location of AF zones and the type of sensor (see AF-Zone Sensitivity diagram below).
6) Battery life. Ratings for battery life are now based on CIPA (Camera & Imaging Products Association) test reports when available. The Japan-based CIPA sets industry standards through research and testing projects, among other things. (For more information, go to www.cipa.jp.)
7) Viewfinder. We measure viewfinder accuracy and magnification for all SLRs and higher-end EVFs. The accuracy measurement tells you how much of the actual captured scene is visible through the viewfinder. Any accuracy measurement above 94% earns an Excellent rating (100% being a perfect score). The magnification rating describes the size of objects in the viewfinder compared to objects seen by the naked eye (a 1X rating would be ideal). Our magnification ratings are based on the scale shown at left.
This story originally ran in the December 2005 issue of Popular Photography & Imaging.
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