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Imatest Documentation The dynamic range corresponding SNR = 1 (1 f-stop of noise) corresponds to the intent of the definition of ISO Dynamic range in section 6.3 of the ISO noise measurement standard: ISO 15739: Photography — Electronic still-picture imaging — Noise measurements. The Imatest measurement differs in several details from ISO 15739; hence the results cannot be expected to be identical. Imatest produces more accurate results because it measures DR directly from a sequence of chart images, rather than extrapolating results from a single reflective chart image. F-stop noise The human eye responds to relative luminance differences. That's why we think of exposure in terms of zones, f-stops, or EV (exposure value), where a change of one unit corresponds to halving or doubling the exposure. The eye's relative sensitivity is expressed by the Weber- Fechner law, ∆L ≈ 0.01 L –or– ∆L/L ≈ 0.01 where ∆L is the smallest luminance difference the eye can distinguish. (This equation is approximate; effective ∆L tends to be larger in dark areas of scenes and prints due to visual interference from bright areas.) Expressing noise in relative luminance units, such as f-stops, corresponds more closely to the eye's response than standard pixel or voltage units. Noise in f-stops is obtained by dividing the noise in pixels by the number of pixels per f-stop. (I use "f-stop" rather than "zone" or "EV" out of habit; any of them are OK.) noise in f-stops (EV) = noise in pixels / (d(pixel)/d(f-stop)) = 1/SNR where d(pixel)/d(f-stop) is the derivative of the pixel level with respect to luminance measured in f-stops (log2(luminance) ). SNR is the Signal-to-Noise Ratio. The above-right image illustrates how the pixel spacing between f-stops (and hence d(pixel)/d(f-stop)) decreases with decreasing brightness. This causes f-stop noise to increase with decreasing brightness, visible in the figures above. Since luminance noise (measured in f-stops) is referenced to relative scene luminance, independently of electronic processing or pixel levels, it is a universal measurement that can be used to compare digital sensor quality when sensor RAW data is unavailable. 288 of 451
Imatest Documentation Using Colorcheck What Colorcheck does New in Imatest 3.5 ISO sensitivity is calculated when the incident lux level is entered. New in Imatest 3.1 Highly distorted images can be analyzed if automatic patch location is turned off. Several images can be combined (averaged) to facilitate fixed pattern noise measurement. New in Imatest 3.0 Options and Settings I in the Imatest main window allows you to select between traditional ROI selection, which automatically locates patches, and a Multicharts-like method, with adjustable patch locations that are visible during fine adjustment. This can be helpful with distorted images. Colorcheck analyzes images of the widely-available X-Rite TM (formerly GretagMacbeth TM ) ColorChecker ® (available from Adorama and other sources) for color accuracy, tonal response (using the six grayscale patches), noise or SNR (Signal-to-Noise ratio), and ISO sensitivity. Results for tonal response are similar to Stepchart. It is particularly useful for measuring the effectiveness of White Balance algorithms and settings under a variety of lighting conditions. The ColorChecker can be photographed in isolation or as part of a scene. Algorithms and equations can be found in the Colorcheck Appendix. The X-Rite ColorChecker is an 8x11" inch chart consisting of 24 patches with 18 familiar colors and six grayscale levels with optical densities from 0.05 to 1.50; a range of 4.8 f-stops. The colors are not highly saturated. The quality of the chart is very high— each patch is printed separately using carefully controlled pigments. Patches have a smooth matte surface. It is available from ColorHQ, Adorama and many other dealers. A 2.25"x3.25" mini ColorChecker is also available. It is useful for including in scenes for evaluating white balance. Version comparison Studio: Tonal response and noise analyses for B&W patches; La*b* color error and Color analysis Master: User-supplied reference files; noise analysis for R, G, B, C, M, and Y patches; noise spectrum A simulated ColorChecker is shown below for sRGB color space (the Windows/Internet standard). The optical densities (–log10(reflectivity)) of the grayscale patches on the bottom row are shown in parentheses. One color, patch 18. Cyan, is out of the sRGB gamut; hence it can't be reproduced perfectly on most monitors. Simulated GertagMacbeth ColorChecker Color Rendition chart 1. 2. 3. 4. 5. 6. 289 of 451
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