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Imatest Documentation Noise reduction, applied in the absence of contrasty features. Attenuates response at high spatial frequencies, i.e., removes fine, low contrast detail (texture), which is interpreted as noise. Many cameras increase noise reduction at high ISO speeds. The signal processing algorithms are proprietary; they are a part of a manufacturer's "secret sauce" for producing pleasing images. Though they vary a great deal, some generalizations can be made. Most cameras do NOT apply noise reduction and sharpening uniformly throughout an image. Contrasty edges tend to have better (more extended) MTF than low contrast edges. For this reason it may be a good idea to photograph both a relatively contrasty edge (though not so high that it causes clipping) as well as a relatively low contrast edge. The SVG charts (above) are excellent choices. Both types of edge can also be produced using Imatest Test Charts. An estimate of chart contrast derived from the average light and dark pixel levels (away from the transition) and gamma is displayed in several places in SFR and Rescharts Slanted-edge SFR. (Estimated chart contrast = (avg. pixel level of light area/avg. pixel level of dark area)^(1/gamma) ). Nonlinearities are analyzed in depth in the Log F-Contrast module. Print the test image (either downloaded or created by Test Charts) from an image editor using a high quality inkjet photo printer on glossy, semigloss, or luster paper. Alternatively, send it to a lab to be printed. Dry Creek Photo has an excellent listing. I recommend printing at least two copies: one for measuring center sharpness and one for edge sharpness. The printed image size should be 8x10 inches (20.3x25.4 cm) or smaller— size is not critical. Be sure the edges look clean and sharp to your eyes; examine them with a good magnifier or loupe. Chart quality is discussed in detail in Chart quality and distance. The charts should be tilted approximately 5.7 degrees (anywhere between 2 and 7 degrees is OK) when they are photographed. It is tilted 5.7 degrees when the tick marks, located near the edges, are vertically or horizontally aligned with the center. This is illustrated by the red horizontal and vertical lines on the right. Imatest Master can analyze edges of nearly any angle; exact horizontal, vertical, and 45° edges should be avoided for best results. The use of guide marks for tilting the chart The charts are printed straight and physically tilted because the edges print sharper that way. If they were printed at an angle, the printer dot pattern could cause some jaggedness. A 5.71° angle (tan -1 (0.1)) is an offset of one part in 10. Assemble the prints into a target to be photographed. The following target is described in detail in The Imatest Test Lab. It can be used to measure lens performance near the center, part-way out, and near all four corners. The Log F-Contrast and Star charts to the left and right of center are not necessary for lens testing; they provide information about signal processing. Slanted-edges or other charts (the Kodak Q-14 step chart or the Gretag Macbeth ColorChecker) may be substituted if your primary interest is lens testing. 252 of 451
Imatest Documentation Sharpness target (30x40 inch) for cameras up to 13 megapixels The printed charts are mounted on a 32x40 inch sheet of 1/2 inch thick black foam board with spray adhesive or double-sided tape. If a ColorChecker is substituted, Velcro is recommended so it can be removed for dark storage. 1/2 inch foam board stays flatter than standard 1/4 or 3/8 inch board. Black board results in less flare light than white board. (Flare light is light that bounces between lens elements and off the inside of the lens barrel, reducing image contrast). An image of a horizontal or vertical edge on an LCD monitor (desktop or laptop) can also be used as a target. The camera should be tilted with respect to the monitor. The image dark gray and white rather than black and white to minimize clipping. This page contains a description and an image, as well as a description of the Screen Patterns module, which can be used to create an image. The disadvantage of this technique is that you have only one edge to work with; you can't easily create a map of lens performance. Photograph the target. Framing Frame the chart so there are usable edges near the center, part-way out, and near the corners. Take care that the chart is properly aligned. A number of useful alignment techniques and tricks are presented in The Imatest Test Lab. For a given lens/focal length combination, I recommend testing the lens over a range of f-stops from wide open to fully stopped down. Digital SLR lenses are typically sharpest around f/8. Sharpness is lost to lens aberrations at large apertures and diffraction at small apertures. With Imatest SFR you can find the precise optimum aperture. Lighting The chart below summarizes lighting considerations. The goal is even, glare-free illumination. Lighting angles between 30 and 45 degrees are ideal in most cases. At least two lights (one on each side) is recommended; four or six is better. Avoid lighting behind 253 of 451
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Imatest Documentation © 2009 Imate
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Imatest Documentation Using SFRplus
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