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Imatest Documentation The large plot on top shows SQF without ( —— ) and with ( - - - ) standardized sharpening for print heights from 5 to 60 cm (~2 to 24 inches), assuming viewing distance (cm) = 30 √(picture height/10). For this mildly oversharpened camera, standardized sharpening decreases the sharpness (removes the strong overshoot), and hence reduces the SQF, which is highly sensitive to sharpening. The large plot also shows viewing distance ( - - - ). The plot on the lower right is the MTF without and with standardized sharpening, displayed in the main SFR/MTF figure, but repeated here to make this figure self-contained. The lower middle contains a thumbnail of the image showing the selected ROI in red. The text on the left contains calculation details and image properties. What do the SQF numbers mean? Ed Granger developed SQF to be linearly proportional to perceived sharpness. A change in SQF of 5 corresponds to a perceptible change in in sharpness— somewhat more than one "Just Noticeable Difference" (JND). Since SQF is a new feature of Imatest (version 2.0.3, Oct. 2006), we haven't had time to build our own database of SQF impressions. So we'll draw on the experience of others. Popular Photography has been using SQF for testing lenses for years. They've developed the only generally-available SQF ranking system. Their scale isn't quite linear. {C, C+} takes up 20 SQF units; twice as many as {A, A+}, {B, B+}, or D. But it seems to be a good starting point for interpreting the numbers. A+ A B+ B C+ C D F 94-100 89-94 84-89 79-84 69-79 59-69 49-59 Under 49 We encourage readers to examine Popular Photography's lens test results and to search its site for SQF. Imatest results should correlate with Pop Photo's results, but there are a few significant differences. Pop Photo measures SQF for lenses alone, while Imatest measures SQF for the entire imaging system. This means that Imatest results are sensitive to signal processing (sharpening and noise reduction; often applied nonlinearly) in the camera and 12 of 451
Imatest Documentation RAW converter. This makes is difficult to compare lenses from measurements taken on different cameras. On the other hand, it means that you know what your camera/lens combination can achieve, and it's excellent for comparing lenses measured on one camera type (with consistent settings). Pop Photo's algorithm for calculating SQF (the SQF equation and the viewing distance assumption; both discussed below) is not known. The scale on the right was developed by Bror Hultgren of Image Integration, based on extensive category scaling tests. According to Bror, the perceived quality level depends on the set of test images (particularly how bad the worst of them is) as well as the task (e.g., a group of cameraphone users would rank images differently from a group of art gallery curators), but the relationships between categories remains relatively stable. These levels are comparable to Popular Photography's scale. Additional considerations in interpreting SQF: SQF has the same interpretation regardless of print size. That means a print with SQF = 92 would have the same quality "feel" for a 4x6 print as for a 24x36 inch print. This is in contrast to MTF measurements, where MTF measured at the print surface is interpreted differently for different sizes of prints: you tend to accept lower MTF for larger prints because you view them from larger distances (though the relationship, described here, is far from linear). Viewing distance is built into SQF. Quality as a function of SQF The SQF calculation omits printer sharpness (for now). It assumes that modern high quality inkjet printers can print as sharp as the unassisted eye can see at normal viewing distances— a fairly safe assumption for large prints (≥ 20 cm high). Measuring SQF SQF is measured as a part of Imatest SFR. All you need to do is check the SQF checkbox in the Imatest SFR input dialog box. Settings will be remembered in succeeding runs. Imatest SFR input dialog, showing SQF Clicking on Options to the right of the SQF checkbox opens this dialog box for setting SQF options. Most of the time you'll want to leave them (except for Maximum print height, which doesn't affect the calculations) at their default values, which you can always restore by pressing Reset . 13 of 451
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