Century focus and test chart instructions

Century focus and test chart instructions

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Table of Contents TABLE OF CONTENTS Introduction Page 4 System Contents Page 4 Resolution: A note from Schneider Optics Page 6 The Definition of Resolving Power: Mil Std. 150A Page 6 Resolution Testing Page 7 Calculating the Distance Required to Test a Lens with a Focal Length Not Listed in Table B Page 14 Things to Remember Page 14 Reading the Film Page 16 Appendix A (Conversion) Page 17 D1010010 REV.A Page 3

Introduction Thank you for purchasing the Schneider Optics, Inc Century Focus and Test Chart System. The Focus and Test Chart System is the result of Schneider s 100 years of experience in the manufacturing, repairing, and testing of optical systems. The charts can be used several ways: 1) Simple focusing aid 2) Subjective comparisons (Does it look sharp? Which lens is better?) 3) Measurement of resolving power Resolving power is not the only measurement of lens performance. It is, however, one of the primary factors governing the ability of a lens to accurately image a subject. Other factors such as distortion, contrast, etc. should be taken into account when evaluating the suitability of a lens for a given purpose. In most cases, the user wants to compare lenses or test a given lens to determine the level of performance. A resolution test is a valid method of obtaining this data. The Century System allows the testing to be done accurately. Previously, most charts required the user to compute the lens-to-chart distances. In many cases, determining the resolution also required computation. The Century System provides this data for the user in most cases. The accompanying tables do the work. The Century method of computation is explained for those working at other reduction ratios or focal lengths not appearing in the tables. Schneider Optics, Inc would be pleased to answer any questions regarding the system or assist by calculating values or distances not covered by the tables. System Contents The System consists of: 1) 25" X 38" wall chart with NBS type resolution targets and Seimens star patterns strategically placed. Resolution Pattern: Set of vertical and horizontal lines and spaces of a given size. Example: "L" is a pattern. Resolution Target: A grouping of resolution patterns. NBS Type Resolution Target: A recognized standard covering a broad range of resolution values. Seimens Star: A focusing aid which is known to "snap" in and out of focus better than other patterns, even under poor lighting conditions Page 4

2) 8 1/2" X 11" laminated field chart large NBS type resolution target on one side, Seimens star on the other, for checking focus and establishing "sharpness" in the field. 3) 8 supplemental NBS resolution targets these are identical to the targets in the corners of the wall chart. They may be separated and placed around the wall chart to test the full field of lenses in situations where the wall chart itself is not sufficient. 4) Instructions: The Key to the System includes tables and allow easy correct positioning of lenses for testing and reading of results. It also provides instructions for computing distances and resolution values for situations not covered by the tables. Figure A RESOLUTION TARGET NBS TYPE Page 5

Resolution: A Note From Schneider Optics, Inc. There are no absolutes regarding what constitutes acceptable resolution. In the final analysis, if the results are satisfactory to all concerned, nothing else really matters. Some manufacturers may provide resolution data on their lenses, but most do not. The variables involved in testing and the desire to avoid disputes with customers are among the reasons most manufacturers only provide resolution data for highly specialized lenses. It is possible to make some general statements based on the accepted circles of confusion used in depth-of-field calculations: 16mm cinematography: a circle of confusion of.001" is generally accepted. This translates into a resolving power of 40 L/mm* and means that in most cases, less than 40 L/mm is not sharp enough for professional work. 35mm cinematography: a.002" circle of confusion (20 L/mm) was accepted for many years. New films and higher standards have resulted in the.001" circle of confusion being used for 35mm as well. 35mm SLR (24mm X 36mm) also uses a.002" circle of confusion. Once again, critical users may find a.001" circle of confusion more satisfactory. In most cases, the edge and corner resolution will be significantly lower than the center, especially with wide-angle lenses. *L/mm: Lines per millimeter The Definition of Resolving Power: Mil Standard 150A The resolving power of a lens is a measure of its ability to image closely spaced objects so that they are recognizable as individual objects. The resolving power shall be expressed in lines per millimeter*, usually in the short conjugate plane. Resolving power is measured by photographing or observing suitable test charts at specified angular distances from the center of the field. Page 6

The test charts shall consist of groups of parallel straight lines and spaces of equal width; the resolving power is the reciprocal of the center-to-center distance of the lines that are just distinguishable in the recorded image. By "just distinguishable" is meant that the observer is able to count the correct number of lines in the recorded image, over the entire length of the lines and in the correct orientation, subject to the provision that no coarser pattern shall be unresolved. The appearance of resolution in a finer pattern after failure to resolve a coarser pattern is an indication of the presence of spurious resolution. Spurious resolution is a phenomenon wherein fine lines are resolved, yet coarse lines are not. Video resolution is sometimes specified in "TV Lines" instead of lines per millimeter: TV lines = L/mm X 2 X image height L/mm = TV lines 2 X image height Image Height for common video formats: ½ Video = 4.8mm 2/3 Video = 6.6mm 1" CCTV = 9.6mm Resolution Testing with the Century Test Chart The use of the Century Test Chart is based on the principle of reduction. The further the lens is placed from the chart, the smaller the resolution patterns in the image plane. By controlling the distance between the test lens and the chart, the amount of reduction and the resulting resolution values are known. Page 7 Page 7

Example: A 50mm f/4 Nikkor is positioned 100 focal lengths (100 X 50mm) from the chart. The patterns will be reduced in size 100 times and assume the values shown in the X = 100 column of Table A. The chart-to-lens distance required to achieve the 100X reduc tion is found in Table B Figure 1 Table A Test results may be found in this table. Once the finest pattern resolved has been determined, the actual resolution in lines per millimeter may be read from the appropriate column. Page 8

Example: In testing a 50mm lens at 100X reduction pattern E is resolved. Follow line E across the page to the 100X column to the number 58. This means 58 lines per millimeter have been resolved. Values of the Targets at "X" Reductions in Lines per Millimeter: TABLE A Value of Pa erns in Lines Per Millimeter NUMBER OF FOCAL LENGTHS FROM CHART TO TEST LENSES X= 25X 50X 100X 200X 400X 800X A 58 117 233 467 933 1867 B 42 83 167 333 667 1320 C 29 58 117 233 467 933 D 21 42 83 167 333 667 E 15 29 58 117 233 467 F 11 21 42 83 167 333 G 7 15 29 58 117 233 H 5 11 21 42 83 167 I 3.5 7 15 29 58 117 J 2.5 5 11 27 42 83 K 1.8 3.5 7 15 29 58 L 1.3 2.5 5 11 21 42 M 0.9 1.8 3.5 7 15 29 Note 1. On the Century Test Chart the patterns in the corner targets correspond to those in the larger central target. Note 2. The width and spacing of the lines in Pattern H is the same in the center and edge patterns. Only the length and number of lines is different. Pattern H always represents the same resolution value at a given reduction. Page 9

Table B Table B is the key to the Century system. Simply find the focal length of the lens being tested in the column marked "Focal Length." The distance to the chart to achieve various reductions is shown in the "Distance From Chart" column printed across the page. Note: For the "25X," "50X," and "100X" columns, two distances are shown. If the location of the Image plane is marked or its location is known, use the "to Image" distance to position the chart the correct distance from the camera. When testing with video cameras or any system where the location of the Image plane is uncertain, use the "to lens" distance, and measure from the chart to the middle of the lens being tested. The 200X, 400X, and 800X columns show only one distance. This is because the relative difference between the distance to the lens and the distance to the Image at these higher reductions is small and can safely be ignored. The resulting error is less than 0.5%. This is the difference between the value of pattern E at 200X being 117 L/ mm or 117.6 L/mm: The error becomes even less at 400X and 800X. The reduction factor for performing a lens test should be chosen based on the focusing capability of the lens being tested and the resolution needed. Example #1: A 12-120mm zoom lens has a minimum focus of 5 [1524mm]. To test the 12m position use the 200X distance: 7'11" [2400mm]. The 100X distance of 4' [1200mm] is less than the minimum focus of the lens. Example #2: A typical 6mm lens has no focusing mount but is permanently set to focus at 6' [1828mm] to 8' [2438mm] because of its tremendous depth of field. The 400X column shows a distance of 7 11" [2400mm] for a 6mm lens. Shoot the test at that distance. Example #3: A 1000mm lens will normally not exceed 100 L/mm resolution on film. Shoot at 100X (64' or 50 meters). Page 10

TABLE B Distance from Chart 25X 50X 100X 200X 400X 800X Focal Length To Image To Image To Image or Image or Image or Image 1.9 48 49 95 97 190 192 380 760 1520 3.5 88 91 175 179 350 354 700 1400 2800 5.7 143 148 285 291 570 576 1140 2280 4560 6 150 156 300 306 600 606 1200 2400 4800 8 200 208 400 408 800 808 1600 3200 6400 9.5 238 247 475 485 950 960 1900 3800 7600 10 250 260 500 510 1000 1010 2000 4000 8000 12 300 312 600 612 1200 1212 2400 4800 9600 15 375 390 750 765 1500 1515 3000 6000 12000 16 400 416 800 816 1600 1616 3200 6400 12800 18 450 468 900 918 1800 1818 3600 7200 14400 20 500 520 1000 1020 2000 2020 4000 8000 16000 25 625 650 1250 1275 2500 2525 5000 10000 20000 32 800 832 1600 1632 3200 3232 6400 12800 25600 35 875 910 1750 1785 3500 3535 7000 14000 28000 Page 11

Distance from Chart 25X 50X 100X 200X 400X 800X Focal Length To Image To Image To Image or Image or Image or Image 40 1000 1040 2000 2040 4000 4040 8000 16000 32000 50 1250 1300 2500 2550 5000 5050 10000 20000 40000 55 1375 1430 2750 2805 5500 5555 11000 22000 44000 75 1875 1950 3750 3825 7500 7575 15000 30000 60000 85 2125 2210 4250 4335 8500 8585 17000 34000 68000 90 2250 2340 4500 4590 9000 9090 18000 36000 72000 100 2500 2600 5000 5100 10000 10100 20000 40000 80000 120 3000 3120 6000 6120 12000 12120 24000 48000 96000 135 3375 3510 6750 6885 13500 13635 27000 54000 108000 150 3750 3900 7500 7650 15000 15150 30000 60000 120000 180 4500 4680 9000 9180 18000 18180 36000 72000 144000 200 5000 5200 10000 10200 20000 20200 40000 80000 160000 240 6000 6240 12000 12240 24000 24240 48000 96000 192000 250 6250 6500 12500 12750 25000 25250 50000 100000 200000 300 7500 7800 15000 15300 30000 30300 60000 120000 240000 Page 12

Distance from Chart 25X 50X 100X 200X 400X 800X Focal Length To Image To Image To Image or Image or Image or Image 385 9625 10010 19250 19635 38500 38885 77000 154000 308000 400 10000 10400 20000 20400 40000 40400 80000 160000 320000 500 12500 13000 25000 25500 50000 50500 100000 200000 400000 600 15000 15600 30000 30600 60000 60600 120000 240000 480000 650 16250 16900 32500 33150 65000 65650 130000 260000 520000 800 20000 20800 40000 40800 80000 80800 160000 320000 640000 1000 25000 26000 50000 51000 100000 101000 200000 400000 800000 Page 13

Calculating the Distance Required to Test a Lens with a Focal Length Not Listed in Table B Example: Test a 32mm focal length film camera lens to determine if it exceeds 80 L/mm resolving power. 1) Choose a reduction ratio from Table A which permits you to measure the required resolution. Shooting at 50X yields measurements up to 117 L/mm. 2) The camera has the film plane marked. Calculate the chart to image distance: 51 focal lengths. 51 focal lengths = 51 X32mm = 1632mm 3) If desired, convert metric distance (1632mm) to English units. 4) Carefully adjust the chart to image distance to 5'4" [1632mm], and shoot the test. 5) Determine the finest pattern resolved by the methods found in "Reading The Film" section of the instructions. 6) Look up that pattern in the 50X column of Table A, and read the resolution. Things To Remember Zoom lenses must be tested at specific focal length settings to actually measure resolution. The lens-to chart distance must be adjusted at each specific focal length just as if the test was for a prime lens of that specific focal length. Light the chart as evenly as possible, avoiding glare and hot spots. For best results, bracket exposures. (It is better to be slightly under-exposed than over-exposed.) Choose film carefully. To determine absolute maximum resolving power, choose a fine-grain film with at least twice the resolution capability as the lens you plan to test. Consult the manufacturer for information about specific emulsions. If testing with the emulsion you normally use, check its resolving power. No matter how good the lens is it cannot exceed the limits of the film. Page 14

Remember that the values of the targets are related to the distances from which they are photographed. The relationship between the patterns making up the target is based on the square root of 2 ( 2= 1.414). The next larger target has a resolving power the 2 less than the one in questions. The next smaller is higher by the 2. Example: 29 L/mm target next smaller; 29 X 1.414 = 42 L/mm next larger; 29 1.414 = 21 L/mm If you photograph from twice the distance listed on the tables, the resolution figures double. Shooting from half the distance the figures are half as much. A study of Table A confirms this relationship. Because all changes are proportional, one can find the values of the targets when recorded from distances other than those listed in the tables. Example: A 75mm lens is tested 10' from the chart Convert 10 to millimeters: 10'X12 = 120" X 25.4mm=3048mm Find the reduction ratio in this example: 3048 75mm EFL = 40.6 focal lengths from the chart (Check; 40.6 X 75mm = 10 feet) The actual ratio from the chart (40.6X) divided by any of the ratios listed in Table A, multiplied by the value shown for that target, at the ratio chosen, will equal the actual resolution 40.6X 50X =.81 (81%) Locate the smallest pattern resolved and multiply by.81 Pattern H is resolved. Look up H in 50X column of Table A. It is listed as 11 L/mm. Multiply 11 X.81 to get the actual resolution (9 L/mm) Performance varies with the stop. Typically, resolution will be lowest at wide open. To increase resolution closed down one stop or more. Continue to stop down until resolution start to fall off. Page 15

Results for a 10mm EFL lens for 16mm film might be: f1.8 : 42 L/mm f2.8 : 58 L/mm f4 : 83 L/mm f5.6 : 83 L/mm f8 : 83 L/mm f11 : 58 L/mm f16 : 42 L/mm As the results in the example show, diffraction actually causes resolution to start to decrease after f8. A lens can be stopped down too far. For the best results, use neutral density filters, when necessary, to keep the lens in mid-range. When testing zoom lenses, shoot at least one series wide open. The depth of focus (the margin for error at the film plane) is the smallest wide open and any loss of resolution at wide angle positions may indicate a lack of collimation between the camera and lens. Reading the Film Resolution should be read directly from the original negative (or reversal stock). The human eye can resolve 8 to 10 L/mm. To read finer resolution, assistance is required: 1) The film can be projected large enough to read the fines pattern resolved. 2) An 8X to 10X loupe or magnifier allows reading resolution values to 100 L/mm. 3) A 20X loupe or magnifier permits reading values 150 to 200 L/mm 4) For higher values a microscope is required. Use the lowest magnification possible to find the finest pattern resolved. Refer back to the Mil Std 150A definition of resolving power for guidelines on determining whether or not a pattern has been resolved. Page 16

Appendix A. To convert from To Mul ply mm inch 2.54 inch mm 1/2.54 inch feet 1/12 Page 17

7701 Haskell Ave. Van Nuys, CA 91406 818-766-3715 Fax: 818-505-9865 Email: century@schneideroptics.com 285 Oser Ave. Hauppauge, NY 11788 631-761-5000 Fax: 631-761-5090 Email: info@schneideroptics.com