e. Specifications for Checking the Performance of MTF 
Equipment at Large Field Angles using a Gimbal Mounted f/4, 200 
mm doublet lens. 
f. Dimensions, Tolerances, and OTF Characteristics of the 
Sira Group 200 mm Doublet Test Lens. 
g. Significance of OTF Methods in Assessing Lenses to be 
used with Partially Coherent Illumination. 
h. A Workshop Instrument for Testing Binocular and Other 
Sights Using the MIF Criterion. 
i. OTF Standards for Aerial Mapping Lenses. 0 
5.1.2 "Simplified" MIF Study: Work at the Royal Aircraft Establi- 
shment, Farnborough, Hampshire, has concentrated on showing how 
to apply OTF methods in the specification and quality control of 
air camera lenses. Having explored the problems of obtaining accuracy 
and reproducibility of measurement and calculation, they have looked 
for a simplified form of OTF test which is easy to use and rapid 
enough to be suitable as a routine inspection tool. 
MIF value at one or two spatial frequencies appears to satisfy 
the requirements, but further work is being undertaken to check 
that such a simple criterion of lens quality is related sufficiently 
closely to the quality of the photograph the lens will ultimately 
produce, Details of the study are reported by Marchant. 
5.1.3 W. Vinten Ltd. Quality Control: The ELF apparatus for focusing 
aerial cameras by measurement of modulation at one spatial frequency 
has continued in production use. It bas been applied to measuring | 49 
squareness of mounting of lenses in cameras, by checking modulation 
at several azimuths spaced around the format. In using the apparatus 
for testing wide-angle lenses off-axis, pinholes of appreciable 
thickness, e.g. 10 micrometers, have been found unsatisfactory. 
Only pinholes in evaporated metal films, where the metal thickness 
is less than one micrometer, are acceptable. 
5.1.4 Comparison Studies: Experience in comparing the actual MTF 
values with those found by other laboratories has indicated the 
need for careful matching of spectral sensitivities if agreement 
within a few per cent is to be obtained. Careful attention to spec- 
tral response has also been found important when focusing cameras 
for multi-spectral work; for example, all infrared response must 
be removed when focusing cameras with tri-colour green filters, 
since these filters transmit freely in the infrared. 
5.1.5 SIRA Activity: The SIRA Institute is at present working 
on a Standard for T.V. lenses and proposing to work on a standard 
for afocal systems. Reports are available on the following lenses: Í © 
16