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Imatest Documentation How to Test Lenses with SFR (old page: Imatest SFRplus recommended) Testing lenses: introduction Note: This page has been largely replaced by Testing Lenses with SFRplus. It is being kept for reference only. Lens quality has always been of great interest to photographers. It's what we spend the big bucks for— if we have them. And we look for it in a lens at any price point. Traditionally, lens testing has been highly tedious, best left to professionals and large publications. With Imatest, that has changed forever. All you need to do is photograph a simple target (with careful technique, of course), run Imatest SFR, the similar but more interactive Rescharts Slanted-edge SFR, or the highly automted SFRplus, and interpret the results. SFR and SFRplus measure the most important lens quality factor: sharpness, which is characterized by spatial frequency response (also called MTF for Modulation Transfer Function). Although the complete MTF response curve is of interest, the spatial frequencies where contrast falls to half its low frequency and peak values, MTF50 and MTF50P, are simple and useful indicators of both image and lens sharpness. Their relationship to print quality is discussed in Interpretation of MTF50 and SQF. SFR and SFRplus also measure lateral Chromatic aberration, which can appear as color fringing toward the edges of the frame. Other lens quality factors are described here. RAW files In addition to standard image files, SFR and SFRplus can analyze Bayer raw files: standard files (TIFF, etc.) that contain undemosaiced data. RAW files are not very useful for measurring MTF because the pixel spacing in the individual image planes is twice that of the image as a whole; hence MTF is lower than for demosaiced files. But Chromatic aberration can be severely distorted by demosaicing, and is best measured in Bayer RAW files (and corrected during RAW conversion). Details of RAW files can be found here. Imatest is straightforward to use and produces clear numeric results, but careful technique is vital. To fully characterize a lens you should test it with a variety of settings. Aperture: most lenses are relatively soft wide open and sharpest around the middle of their range (the "optimum" aperture): f/5.6 to f/11 for the 35mm and digital SLRs; f/4-f/5.6 for compact digital cameras. Focal length in zoom lenses also strongly affects sharpness, but there are no general guidelines about which are best. Distance from the image center: Lenses tend to be softer toward the edges. For this reason you should test sharpness at several locations in the frame (easy with SFR; easier with SFRplus). We recommend 9 regions to fully characterize a lens: 1 near the center, 4 part-way out (near the top, bottom, and sides), and 4 near the corners. In a well-manufactured lens, sharpness will be symmetrical about the center of the image, but we live in an era when manufacturers are constantly striving to reduce production costs, often at the expense of manufacturing quality. Even premium lenses may be poorly centered. You'll need to purchase a chart or print it on a photographic quality inkjet printer ,and you'll need a flat surface for mounting the target (thick foam board from any art supply store is perfect), an environment with even, glare-free lighting, and a sturdy tripod: items most photographers have or can easily locate. You can learn a lot by testing your own lenses, but you must be aware of one essential fact. You cannot measure a lens in isolation. It is a part of an imaging system that includes the camera's image sensor and RAW converter (which may sharpen the image), or film, scanner, and scanner software. Hence, Measurements are relative. It's difficult to determine an absolute number for the lens alone. But you can accurately compare lenses on similar cameras. Camera and RAW converter settings, especially those that affect sharpening, noise reduction, and gamma, strongly affect the results. Record them, and be consistent. RAW files often produce the best results. The dcraw RAW conversion software used by Imatest employs no sharpening or noise reduction and has an ideal transfer curve with a (default) gamma of 0.45. The basic steps in testing the lens are: 250 of 451
Imatest Documentation Purchase or download, print, and assemble the test target. Photograph it. Run Imatest SFR or Rescharts Slanted-edge SFR. Interpret the results. The test target These instructions are excerpted from Using Imatest SFR, Part 1. Several alternatives are available for obtaining test targets. Purchase a target from the Imatest store. We recommend the SFRplus target. Download one of the test images shown below and print it on a high quality inkjet printer or send the file to a lab for printing, for example, one of the labs listed by Dry Creek Photo. Create and print a test image with the Imatest Test Charts module. A variety of contrast ratios and highlight colors are available. Several Structured vector graphics (SVG) charts are available in Imatest Master. SVG charts with m x n grid patterns (example on right) are well suited for printing large and for automated testing, and have numerous advantages over the ISO 12233 chart. Purchase a chart such as the ISO 12233 (QA-72) or QA-77 (available from Applied Image) or as a less expensive alternative from Danes-Picta in the Czech Republic (the DCR3 chart on their Digital Imaging page). The Applied Image charts are printed with much higher resolution than an inkjet printer can achieve, hence they can be used at much closer distances. Their disadvantage is that they have very high contrasts (>=40:1), which often results in reduced accuracy, and they have a great deal of wasted real estate: usable edges may not be found in regions of greatest interest such as the four corners. A printable vector-graphics version of the ISO 12233 chart is available courtesy of Stephen H. Westin of the Cornell University Computer Graphics Department. It should be printed as large as possible (17x22 inches (A2) minimum; 24x36 inches (A1) if possible for 8+ megapixel cameras) so measured sharpness is limited by the camera and lens, not the printer. Because of the limitations of the ISO chart (described above), the Imatest SVG charts are generally preferable. You can download printable test charts by right-clicking on the thumbnails below. The bitmap chart, which is included in the samples folder of the Imatest installation, has pixel levels 47 and 255, has a contrast ratio of (255/47) 2.2 = 41 when printed at gamma = 2.2 (the normal setting). This is close to the minimum recommended by the ISO 12233 standard (but higher than optimum; the revised ISO 12233 standard under development will recommend lower contrast). The two SVG charts have contrast ratios of 20 and 2. Instructions for their use can be found in SVG Test Charts. The recommended print size is letter or A4, but these charts print at maximum quality for any size. SVG charts — can be viewed, edited, and printed from Inkscape. Right-click on the links to save the file. Download size: 304 KB (SVG); 500 KB (PDF). Printing instructions are here. Bitmap chart Contrast = 20, light, gamma = 2.2, white highlights (SVG) | Contrast = 20 (PDF) High enough contrast to ensure maximum sharpening in most cameras but low enough to avoid clipping most of the time. Edge_chart_low.png (reduced contrast; pixel levels 47, 255) Nonlinear signal processing and chart contrast Contrast = 2, middle, gamma = 2.2, white highlights (SVG) | Contrast = 2 (PDF) Relatively low contrast: little, if any, sharpening would be expected. The two charts should have the same MTF if no nonlinear processing is applied, for example, with RAW files with dcraw conversion. Although Imatest SFR is relatively insensitive to chart contrast (MTF is normalized to 100% at low spatial frequencies), measured SFR is often affected by chart contrast due to nonlinear signal processing in cameras, i.e., processing that depends on the contents of neighboring pixels, and hence may vary throughout an image. Nonlinear processing is almost universal in digital cameras (though you can avoid it by using RAW images with dcraw). It improves pictorial quality but complicates measurements. It takes two primary forms. Sharpening, applied in the proximity of contrasty features like edges. Boosts response at high spatial frequencies. 251 of 451
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