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Imatest Documentation For this purpose we use line widths per picture height (LW/PH). LW/PH is equal to 2 * lp/mm * (picture height in mm). Where total picture height is involved, line widths is customarily used instead of pairs (where one line pair equals two line widths). The use of picture height gives a slight advantage to compact digital cameras, which have an aspect ratio (width:height) of 4:3, compared to 3:2 for digital SLRs. Compact digital cameras have slightly more vertical pixels for a given number of total pixels. For example, a 5.33 megapixel compact digital camera would have 2000 vertical pixels— as many as a 6 megapixel DSLR. Another measure of spatial frequency used with digital cameras is cycles or line pairs per pixel (c/p or lp/p). This gives an indication of how well individual pixels perform. There is no need to use actual distances (millimeters or inches) to evaluate digital camera image quality, although such measurements are available in Imatest SFR. Imatest SFR program output contains results on the left and input data on the right (a thumbnail of the entire image, the region of interest (ROI), and selected EXIF data). (top) A narrow image that illustrates the tones of the averaged edge. It is aligned with the edge profile (spatial domain) plot, immediately below. (middle) Spatial domain plot: The average edge profile (linearized, i.e., proportional to light energy). The key result is the 10-90% edge rise distance, shown in pixels and in the number of rise distances per picture height. The red values are for standardized sharpening. Other parameters include overshoot and undershoot (if applicable). This plot can optionally display the line spread function (LSF: the derivative of the edge), or the edge in pixels (gamma-encoded). (bottom) Frequency domain plot: The Spatial Frequency Response (MTF), shown to twice the Nyquist frequency. The key result is MTF50, the 50% MTF frequency, which corresponds to perceived image sharpness. It is given in cycles per pixel (c/p) and line widths per picture height (LW/PH). Other results include MTF at NYQ, the MTF at the Nyquist frequency (0.5 cycles/pixel; sampling rate/2), which indicates the probable severity of aliasing. The Nyquist frequency is displayed as a vertical blue line. For this camera, which is moderately sharpened, MTF50P (only shown when Standardized sharpening display is unchecked) is identical to MTF50. SFR Results: MTF (sharpness) plot describes this Figure in detail. Interpreting MTF50 Results from the ISO 1233 image for the Canon EOS-10D. This section was written before the addition of SQF (Subjective Quality Factor) to Imatest (Ver. 2.1, November 2006). SQF allows a more refined estimate of perceived print sharpness. What MTF50 do you need? It depends on print size. If you plan to print gigantic posters (20x30 inches or over), the more the merrier. Any high quality 4+ megapixel digital camera (one that produces good test results; MTF50(corr) > 0.3 cycles/pixel) is capable of producing excellent 8.5x11 inch (letter-size; A4) prints. At that size a fine DSLR wouldn't offer a large advantage in MTF. With fine lenses and careful technique (a different RAW converter from Canon's and a little extra sharpening), my 6.3 megapixel Canon EOS-10D (corrected MTF50 = 1340 LW/PH) makes very good 12x18 inch prints (excellent if you don't view them too closely). Prints are sharp from normal viewing distances, but pixels are visible under a magnifier or loupe; the prints are not as sharp as the Epson 2200 printer is capable of producing. Softness or pixellation would be visible on 16x24 inch enlargements. The EOS-20D has a slight edge at 12x18 inches; it's about as sharp as I could ask for. There's little reason go go to a 12+ megapixel camera lie the EOS 5D, unless you plan to print larger. Sharpness comparisons contains tables, derived from images downloaded from two well-known websites, that compare a number of digital cameras. Several outperform the 10D. The table below is an approximate guide to quality requirements. The equation for the left column is 8 of 451
Imatest Documentation MTF50 in Line Widths ⁄ inch on the print 150 110 80 MTF50(Line Widths ⁄ inch on the print) = MTF50(LW ⁄ PH) Print height in inches Quality level— after post-processing, which may include some additional sharpening Excellent— Extremely sharp at any viewing distance. About as sharp as most inkjet printers can print. Very good— Large prints (A3 or 13x19 inch) look excellent, though they won't look perfect under a magnifier. Small prints still look very good. Good— Large prints look OK when viewed from normal distances, but somewhat soft when examined closely. Small prints look soft— adequate, perhaps, for the "average" consumer, but definitely not "crisp." Example of using the table: My Canon EOS-10D has MTF50 = 1335 LW/PH (corrected; with standardized sharpening). When I make a 12.3 inch high print on 13x19 inch paper, MTF50 is 1335/12.3 = 108 LW/in: "very good" quality; fine for a print that size. Prints look excellent at normal viewing distances for a print this size. This approach is more accurate than tables based on pixel count (PPI) alone (though less refined than SQF, below). Pixel count is scaled differently; the numbers are around double the MTF50 numbers. The EOS-10D has 2048/12.3 = 167 pixels per inch (PPI) at this magnification. This table should not be taken as gospel: it was first published in October 2004, bandit may be adjusted in the future. Subjective Quality Factor (SQF) MTF is a measure of device or system sharpness, only indirectly related to the sharpness perceived when viewing a print. A more refined estimate of perceived print sharpness must include assumptions about viewing distance (typically proportional to the square root of print height) and the human visual system (the human eye's Contrast Sensitivity Function (CSF)). Such an formula, called Subjective Quality Factor (SQF) was developed by Eastman Kodak scientists in 1972. It has been verified and used inside Kodak and Polaroid, but it has remained obscure until now because it was difficult to calculate. Its only significant public exposure has been in Popular Photography lens tests. SQF was added to Imatest in October 2006. A portion of the Imatest SFR SQF figure for the EOS-10D is shown on the right. SQF is plotted as a function of print size. Viewing distance (pale blue dashes, with scale on the right) is assumed to be proportional to the square root of picture height. SQF is shown with and without standardized sharpening. (They are very close, which is somewhat unusual.) SQF is extremely sensitive to sharpening, as you would expect since sharpening is applied to improve perceptual sharpness. The table below compares SQF for the EOS-10D with the MTF50 from the table above. MTF50 in Line Widths ⁄ inch on the print Corresponding print height for the EOS-10D (MTF50 = 1335 LW/PH) SQF at this print height Quality level— after post-processing, which may include some additional sharpening. Overall impression from viewing images at normal distances as well as close up. 150 8.9 inches = 22.6 cm 93 Excellent— Extremely sharp. 110 12.1 inches = 30.8 cm 90 Very good. 80 16.7 inches = 42.4 cm 86 Good— Very good at normal viewing distance for a print of this size, but visibly soft on close examination. An interpretation of SQF is give here. Generally, 90-100 is considered excellent, 80-90 is very good, 70-80 is good, and 60-70 is fair. These numbers (which may be changed as more data becomes available) are the result of "normal" observers viewing prints at normal distances (e.g.., 30-34 cm (12-13 inches) for 10 cm (4 inch) high prints). The judgments in the table above are a bit more 9 of 451
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