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3 
usual collimator arrangement, it is necessary to prepare elliptical test objects so 
that they are equivalent to circular test objects on a distant plane perpendicular 
to the camera axis. A sufficient variety of targets shall be prepared to ensure 
that the major and minor diameters on this virtual object plane do not differ from 
one another by more than 5%. 
  
NOTE: Difference of log luminance has been specified for all test objects. 
This is numerically equal to difference in optical density only if the geome- 
try of the illuminating and viewing system in the collimator is sufficiently 
similar to the geometry of the densitometer used to obtain optical densities. 
  
  
  
Each target shall consist of a suitable range of sizes of one or more types of 
test object. Each size of object shall occur at least twice, orientations being 
chosen to provide at least radial and tangential lines, where applicable. Sizes 
shall be in progression with the common ratio either 2'« or the almost identical 
10/2, 
Illuminant 
The illuminant shall be what is commonly called “mean noon sunlight”, that 
is it shall be equivalent to the light source defined in the International Organi- 
zation for Standardization Recommendation R6 Method for Determining Photo- 
graphic Speed and Exposure Index (latest revision), modified by whatever filter 
is usual on the camera. The filter used shall be specified fully in the report. 
Preferably its spectral transmission curve should be given. 
Collimator 
The target shall be placed at the focus of a collimator. The collimator shall 
be such as to have no adverse effect on the apparent performance of the lens 
under test. 
The collimator aperture and the positions of the collimator relative to the 
camera shall be such that the entrance pupil of the camera lens is filled with 
light from every part of the target. 
Effects of stray light shall be kept to a minimum by illuminating only the 
target and keeping its surround dark. To achieve this, attention should be given 
to baffles and low reflection finishes in the collimator tube. 
  
NOTE: It seems desirable to avoid mixing the effects of veiling glare into 
the resolving power tests. The effects of veiling glare on resolving power 
have been measured. *) Introduction of glare corresponds to a decrease in 
target contrast. Hence, from measurements of veiling glare, reduction of 
resolving power can usually be estimated with adequate accuracy. 
Veiling glare also is very largely controllable by camera baffling and 
lens coating. Consequently, it seems desirable to measure veiling glare 
separately and reduce it if it exceeds permissible limits. This approach is 
similar to those generally used in testing camera mounts and film flatness. 
  
  
  
Number of Exposures 
The target shall be photographed on axis and at a sufficient number of off- 
*) K. M. Baird, “Veiling Glare in Aerial Photography”, Canadian Journal of 
Research A. 27. 130, 1945.