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Century focus and test chart instructions
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Page 2 D101010 REV.A
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
Page 3 D1010010 REV.A
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 opti-cal 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 re-garding the system or assist by calculating values or distances not cov-ered by the tables.
System Contents The System consists of:
1) 25" X 38" wall chart with NBS type resolution targets and Sei-mens 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 D101010 REV.A
2) 8 1/2" X 11" laminated field chart — large NBS type reso-
lution 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 iden-tical 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 compu-ting distances and resolution values for situations not cov-ered by the tables.
RESOLUTION TARGET NBS TYPE
Page 5 D101010 REV.A
Figure A
Resolution: A Note From Schneider Optics, Inc.
There are no absolutes regarding what constitutes acceptable res-olution. 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 calcula-tions: 16mm cinematography: a circle of confusion of .001" is gen-
erally 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 milli-meter*, 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 D101010 REV.A
Page 7
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 record-ed image, over the entire length of the lines and in the correct ori-entation, subject to the provision that no coarser pattern shall be unresolved.
The appearance of resolution in a finer pattern after failure to re-solve 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 re-duction. 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 D101010 REV.A
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
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 D101010 REV.A
Figure 1
Example:
In testing a 50mm lens at 100X reduction pattern E is re-solved. 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
Note 1. On the Century Test Chart the patterns in the corner tar-
gets 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 num-
ber of lines is different. Pattern H always represents the same
resolution value at a given reduction.
Page 9 D101010 REV.A
NUMBER OF FOCAL LENGTHS FROM CHART TO TEST LENSES
Val
ue
of
Pa�
ern
s in
Lin
es P
er M
illim
eter
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
Page 10
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 tre-mendous 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).
D101010 REV.A
Page 11 D101010 REV.A
TA
BL
E B
Dis
tan
ce fr
om
C
ha
rt
2
5X
50X
1
00X
2
00X
4
00X
8
00X
Foca
l Le
ngt
h
(mm
) To
Len
s (m
m)
To Im
age
(mm
) To
Len
s (m
m)
To Im
age
(mm
) To
Len
s (m
m)
To Im
age
(mm
)
To L
ens
or
Imag
e (m
m)
To L
ens
or
Imag
e (m
m)
To L
ens
or
Imag
e (m
m)
1.9
4
8 4
9 9
5 9
7 1
90
192
3
80
760
1
520
3.5
8
8 9
1 1
75
179
3
50
354
7
00
140
0
280
0
5.7
1
43
148
2
85
291
5
70
576
1
140
2
280
4
560
6
150
1
56
300
3
06
600
6
06
120
0
240
0
480
0
8
200
2
08
400
4
08
800
8
08
160
0
320
0
640
0
9.5
2
38
247
4
75
485
9
50
960
1
900
3
800
7
600
10
250
2
60
500
5
10
100
0
101
0
200
0
400
0
800
0
12
300
3
12
600
6
12
120
0
121
2
240
0
480
0
960
0
15
375
3
90
750
7
65
150
0
151
5
300
0
600
0
120
00
16
400
4
16
800
8
16
160
0
161
6
320
0
640
0
128
00
18
450
4
68
900
9
18
180
0
181
8
360
0
720
0
144
00
20
500
5
20
100
0
102
0
200
0
202
0
400
0
800
0
160
00
25
625
6
50
125
0
127
5
250
0
252
5
500
0
100
00
200
00
32
800
8
32
160
0
163
2
320
0
323
2
640
0
128
00
256
00
35
875
9
10
175
0
178
5
350
0
353
5
700
0
140
00
280
00
Page 12 D101010 REV.A
Dis
tan
ce fr
om
C
ha
rt
2
5X
50X
1
00X
2
00X
4
00X
8
00X
Foca
l Le
ngt
h
(mm
) To
Len
s (m
m)
To Im
age
(mm
) To
Len
s (m
m)
To Im
age
(mm
) To
Len
s (m
m)
To Im
age
(mm
)
To L
ens
or
Imag
e (m
m)
To L
ens
or
Imag
e (m
m)
To L
ens
or
Imag
e (m
m)
40
100
0
104
0
200
0
204
0
400
0
404
0
800
0
160
00
320
00
50
125
0
130
0
250
0
255
0
500
0
505
0
100
00
200
00
400
00
55
137
5
143
0
275
0
280
5
550
0
555
5
110
00
220
00
440
00
75
187
5
195
0
375
0
382
5
750
0
757
5
150
00
300
00
600
00
85
212
5
221
0
425
0
433
5
850
0
858
5
170
00
340
00
680
00
90
225
0
234
0
450
0
459
0
900
0
909
0
180
00
360
00
720
00
100
2
500
2
600
5
000
5
100
1
0000
1
0100
2
0000
4
0000
8
0000
120
3
000
3
120
6
000
6
120
1
2000
1
2120
2
4000
4
8000
9
6000
135
3
375
3
510
6
750
6
885
1
3500
1
3635
2
7000
5
4000
1
0800
0
150
3
750
3
900
7
500
7
650
1
5000
1
5150
3
0000
6
0000
1
2000
0
180
4
500
4
680
9
000
9
180
1
8000
1
8180
3
6000
7
2000
1
4400
0
200
5
000
5
200
1
0000
1
0200
2
0000
2
0200
4
0000
8
0000
1
6000
0
240
6
000
6
240
1
2000
1
2240
2
4000
2
4240
4
8000
9
6000
1
9200
0
250
6
250
6
500
1
2500
1
2750
2
5000
2
5250
5
0000
1
0000
0
200
000
300
7
500
7
800
1
5000
1
5300
3
0000
3
0300
6
0000
1
2000
0
240
000
Page 13 D101010 REV.A
Dis
tan
ce fr
om
C
ha
rt
2
5X
50X
1
00X
2
00X
4
00X
8
00X
Foca
l Le
ngt
h
(mm
) To
Len
s (m
m)
To Im
age
(mm
) To
Len
s (m
m)
To Im
age
(mm
) To
Len
s (m
m)
To Im
age
(mm
)
To L
ens
or
Imag
e (m
m)
To L
ens
or
Imag
e (m
m)
To L
ens
or
Imag
e (m
m)
385
9
625
1
0010
1
9250
1
9635
3
8500
3
8885
7
7000
1
5400
0
308
000
400
1
0000
1
0400
2
0000
2
0400
4
0000
4
0400
8
0000
1
6000
0
320
000
500
1
2500
1
3000
2
5000
2
5500
5
0000
5
0500
1
0000
0
200
000
4
0000
0
600
1
5000
1
5600
3
0000
3
0600
6
0000
6
0600
1
2000
0
240
000
4
8000
0
650
1
6250
1
6900
3
2500
3
3150
6
5000
6
5650
1
3000
0
260
000
5
2000
0
800
2
0000
2
0800
4
0000
4
0800
8
0000
8
0800
1
6000
0
320
000
6
4000
0
100
0
250
00
260
00
500
00
510
00
100
000
1
0100
0
200
000
4
0000
0
800
000
Page 14 D101010 REV.A
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 ex-
ceeds 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 re- solving 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 test-ing 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 15 D101010 REV.A
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 re-solving 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 fig-ures are half as much. A study of Table A confirms this relation-ship. Because all changes are proportional, one can find the val-ues 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 resolu-tion (9 L/mm)
Performance varies with the stop. Typically, resolution will be low-est at wide open. To increase resolution closed down one stop or more. Continue to stop down until resolution start to fall off.
Page 16 D101010 REV.A
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 posi-
tions 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 re-solved.
Page 17 D101010 REV.A
Appendix A.
To convert from To Mul�ply
mm inch 2.54
inch mm 1/2.54
inch feet 1/12
7701 Haskell Ave. Van Nuys, CA 91406 818-766-3715 Fax: 818-505-9865 Email: [email protected]
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Email: [email protected]
