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This specification of spectral sensitivity represents a 
compromise which is a reasonable representation of the spectral 
sensitivities of current aerial emulsions. 
When it is not possible to meet this specification, the 
actual spectral sensitivity used shall be stated. In this 
connection it is pointed out that the spectral sensitivities 
of photomultiplier cells frequently depart by significant 
amounts from the catalogue values. 
2.3 Filters 
The lens shall be provided with filters appropriate for 
its use in aerial photography, and the determinations shall be 
made either with a filter of sufficiently good quality to have 
no effect on the MTF, or with the filter inserted at some point 
in the optical path where it has no ill-effect. 
The report of the measurement shall state the filter used 
for each MTF determination and a spectral transmission curve shall 
be provided for each filter. 
2.4 Focus Settings 
The determinations shall be made in a series of focal planes 
adjacent to the plane of nominal best focus, separated by 23 
microns. 
One plane shall be chosen as the reference plane. Prefer 
ably the actual distance of the reference plane from some ref 
erence point on the lens mount shall be stated. 
Sufficient determinations shall be made in different planes 
to ensure accurate location of the plane which gives the best 
average MTF across the field. 
2*5 Field Angles 
The MTF shall be measured on axis and at a sufficient 
number of angles off axis (at least 6) to permit presentation 
of reliable data on the variation of MTF with field angle. 
2.6 Target Orientation 
The target or targets shall be orientated in two directions 
at right angles so that the MTF is measured in effect for radial 
lines and for lines at right angles to the radial direction. The 
report shall state the values obtained for both directions under 
each measuring condition. 
2.7 Aperture 
The apertures used in the measurements shall be stated in 
the report.