RT AEA RIED TE Te RS ES RÉ RENE ASE EM Pn CSA EE 
provided a new FL were derived for the granularity case. This propo- 
sition will be maintained in this paper though there is no further 
evidence available as confirmation. A Selwyn granularity figure of 
1.3um was adopted for the film. 
A current research programme on granularity is aimed at analysing 
the variations of granularity with scanning aperture diameter and 
density. It is anticipated that techniques developed by Zweig (1956), 
Doerner (1962) using the Wiener spectrum will be of significant 
application in this study, especially with regard to the combination 
of effects of granularity with MTF's. Reports on this work will be 
given later. 
Until further information is available on effects of granularity 
the approach of Hempenius will be adopted. Since relative variances 
of coordinate observations are being determined for this research the 
inaccuracies introduced by the assumptions should be minimized. 
  
2.5 Target Contrasts and Sizes 
Target contrasts of 5.6:1, 2.5:1 and 1.2:1 (the same as in the 
previous studies) will be considered in this paper (equivalent to 
modulations (B2,- B1)/(B2 + B1) o£ 0.70, 0.44 and 0.10). Contrasts 
falling on the emulsion at different flying heights were investigated 
in Trinder (1974). It was shown that contrasts of approximately 5:1 
or higher are possible for normal aerial films provided artificial 
targets are used with artificial backgrounds. They may drop to as low 
as 2.5:1 if the target is placed on the natural surface, depending on 
the surface material, and may be substantially lower than 2.5:1 if 
natural details are used as control points. The importance of the 
placement of artificial targets therefore lies not only with the ease 
of identification, but also in the resulting better pointing precisions 
obtained. 
As pointing precisions depend on target contrast, target size and 
sharpness, all 3 factors must be included in this study. The target 
sizes adopted will be those derived in Trinder (1973) as the optimum 
sizes of ground targets. This step is important, since unless 
restrictions are placed on the target sizes, it is impossible to relate 
precisions to image quality characteristics. Optimum target sizes are 
dependent on the image quality of the system, and therefore sizes of 
targets chosen will increase slightly across the image plane as the 
ray inclination increases. 
2.6 Estimates of Pointing Precisions and Variances of Coordinate 
Observations 
The frequency limit (FL) for the 6 sets of combined system MTF's 
were derived for ray inclinations varying from 0 to 35° for wide-angle 
cameras and 0 to 55° for the super wide angle camera at the specified 
target contrasts and optical magnifications of 5, 10 and 20 X. 
eT