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Imatest Documentation with a constant times the convolution of the original image and the blur function, where convolution is denoted by *. LUSM(x) = L(x) - kUSM * Blur = L(x) * [δ(x) - kUSM exp(-x 2 /2σx 2 ) / sqrt(2πσx 2 )] / (1- kUSM /sqrt(2π) ) L(x) is the input pixel level and LUSM(x) is the USM-sharpened pixel level. kUSM is the USM sharpening constant (related to the slider setting scanning or editing program). L(x) = L(x) * δ(x), where δ(x) is a delta function. The USM algorithm has its own MTF (the Fourier transform of the portion inside the brackets [...]). Using F(exp(-px 2 ) ) = exp(-α 2 /4p)/sqrt(2p), where F is the Fourier transform, MTFUSM( f ) = [1- kUSM exp(-f 2 σx 2 /2 ) /sqrt(2π) ] / (1- kUSM /sqrt(2π) ) = [1- kUSM exp(-f 2 / 2 fUSM 2 ) /sqrt(2π) ] / (1- kUSM /sqrt(2π) ) where fUSM = 1/σx. This equation boosts response at high spatial frequencies, but unlike sharpening, response doesn't reach a peak then drop. Actual sharpening is a two dimensional operation. 30 of 451
Imatest Documentation Sharpness comparisons for several digital cameras Sharpness results derived from Websites Several websites publish images of ISO 12233 test charts that can be entered into Imatest SFR to obtain detailed sharpness measurements. This page contains sharpness results derived from charts downloaded from two sites: Dpreview.com (DPR) Imaging-resource.com (IR) The measurements in the tables below come from two areas of the ISO 12233 image, illustrated by red rectangles in the image below. First row: near the center. The rectangle just above the center of the chart (about 15% of the distance to the corners). Second row: near the corner. The rectangle in the lower right of the chart (about 78% of the distance to the corner). Results include Chromatic aberration, which cannot be measured near the center. These measurements are limited to One lens. For DSLRs this is usually one of the sharpest available, such as the 50mm f/1.4 prime (non-zoom). One focal length. Sharpness is a strong function of focal length for zoom lenses. The sites aren't very consistent in their choice of focal length for zooms in compact digital cameras; long, short or intermediate focal lengths may be chosen. One aperture, typically around the optimum: f/8-f/11 for DSLRs; f/4-f/5.6 for compact digital cameras. One ISO speed, which strongly affects noise and dynamic range, but has little effect on sharpness. One sharpening radius, typically 2, used in standardized sharpening calculations, indicated by (corr.) and red columns. One or two vertical edges (in most cases), resulting in horizontal MTF measurements. Sometimes vertical MTF (from horizontal edges) is different. One RAW converter (usually the JPEG converter built into the camera) with one setting, usually the default. The choce of converter and sharpness setting has a strong effect on sharpness measurements. Gamma assumed to be 0.5, typical for digital cameras. A 10% gamma error results in about a 2.5% MTF50 error. Though gamma error is rarely severe, gamma should be measured with Stepchart or Colorcheck for greatest accuracy. Exposure and lighting are not always optimum. If the image is too dim or bright there may be some clipping on data (where it goes pure black or white), which reduces the accuracy of the sharpness measurement. Lighting may not be as even as it should be. If you want to compare your own camera or lenses with the published charts, test lenses at various focal lengths and apertures, or observe the effects of ISO speed and signal processing, you'll need to download Imatest. The trial version allows up to twenty runs. 31 of 451
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