(221) 
Circular 533 entitled “A Method for Determining the Resolving Power of 
Photographic Lenses” 3 ). 
The chart will be available in two contrasts. The high contrast test chart, 
shown in figure 2, is printed in black ink on glossy white paper; the contrast 
between lines and spaces is approximately 1.4. The low contrast chart, shown 
in figure 3, is printed with gray ink on gray paper; the contrast between lines 
and spaces is approximately 0.26. Six test charts of each contrast will accom 
pany the circular. 
The range of resolving power in 
the image plane of the lens under test 
at the standard distance of 26f extends 
from 12 to 80 lines/mm in twelve steps 
forming a geometric progression with 
the ratio between two successive terms 
approximately equal to the fourth root 
of 2. Three lines are used in each pat 
tern to facilitate visual interpretation. 
The use of the ly 2 ratio in size insures 
closer steps between successive values 
that can be read as compared to the 
older V 2 ratio. Perhaps the most strik 
ing feature of the new chart is the rela 
tively great length of the lines forming 
the patterns. They were so made for 
two reasons. First, the ratio of length 
to width of line is sufficiently great 
that the visual resolving power as read 
will not be subject to variations arising 
from end effects and the images will 
continue to look like lines down to the 
limit of resolution. Secondly many labo 
ratories are now equipped with micro 
densitometers and it is desirable to have 
the lines sufficiently long to permit the 
negative images at 25 X reduction to be 
scanned in such instruments without 
the necessity of so far reducing the 
length of the scanning slit that the loss of sensitivity of the instrument and 
effects of granularity of the emulsion impair the validity of measurement. In 
addition, the line patterns are so arranged in the chart that the negative images 
of all patterns can be scanned with only two settings of the test negative in the 
recording microdensitometer. If, however, shorter lines are preferred it is pos 
sible to obtain them by making the lines on the present chart. The standard 
Air Force chart, for example, is of high contrast and consists of 3 lines and 2 
spaces, the lines being 5 times as long as wide, thus forming a square pattern. 
Several such patterns for each frequency can be formed by blocking or masking 
out portions of the lines on the high contrast chart. 
Figure 4. 
Comperative results on axis for a lens 
tested with high and low contrast 
charts. 
The upper curves are microdensitometer 
traces made by scanning the negative 
images formed by a lens of the high and 
low contrast charts. The light area of 
the reproduction of the chart at the 
bottom of the figure shows the portion 
of the image actually scanned. 
3 ) A method for determining the resolving power of photographic lenses. F. E. Washer and 
I. C. Gardner. NBS Circular 533 (In press).