it is now possible to purchase commercial OTF test equipment from Beck in United 
Kingdom and De Oude Delft in Holland. This equipment is primarily for testing 
lenses and cameras, but can be adapted for testing other components, cathode ray 
tube screens, fluorescent screens, etc. 
Alternative optical test methods for specialised vise have been reported. In 
the United Kingdom, a "spot diagram" equipment has been built by BSIRA. It 
yields a "spot" diagram of an optical system for comparison with design ray traces 
and is particularly valuable for systems having appreciable aberrations. The 
National Bureau of Standards in USA have developed a T-bench for measuring 
optical path differences in wavefronts emerging from the lens under test. 
With the aid of high speed computors the lens designer is now able to compute 
the performance of a lens from its design data in any of the above teims, i. e. 
0. T. F., spot diagram, wavefront o.p.d. or in terns of resolving power. These are 
obviously powerful tools in design or production. 
Methods of measuring OTF of films are not yet out of the research laboratory. 
We shall hear more of them at Lausanne from workers in many countries. 
The calibration of air Survey cameras in the laboratory is now well-known. 
Usually calibration is not done under practical working conditions, for example 
the actual film magazine is seldom used. Satellite cameras have in the past been 
calibrated in sites by photographing stars. This has the advantage of calibrating 
the camera simultaneously for a large number of points and by repetition of the 
photography enables the variation from frame to frame to be determined. 
The Coast and Geodelic Survey in USA have developed a method using 400 stars 
to measure all the elements of interior orientation and lens defects with a 
statistical validity of 2 microns. 
For non-Survey cameras, particularly those used with non-infite conjugates, 
the 3 dimensional test field at Stockholm University gives very similar information. 
This will be the subject of an Invited Paper at the Congress. 
Use of colour film has high-lighted the difficulties of specifying the speed 
of film used in Air Survey. The ASA and DIN systems, derived primarily for 
photography on the ground, have long been known to be misleading when used for 
high altitude air photography. For black and white film, the United Kingdom and 
I believe other countries, have used a speed point of 0.4 density above fog when 
developed out in high contrast developer. Other criteria have been used. In any 
system, the speed figure will depend upon the developer used, the development, 
time, and the temperature. There is therefore an obvious need for International 
Standardisation of these conditions. Black and white film has however a reasonable 
exposure latitude, Colour film has a much smaller latitude and it is therefore 
even more essential for the speed quoted for any colour film to be based upon a 
standard, well understood criterion. This is a subject which should be discussed 
at the Lausanne Congress end which our Working Group 3 should consider as a matter 
of priority. 
(e) Research While much work has been done upon the actual measurement 
of 0.T.F. of lenses, films, eto., less is known about the effect of various O.T.F. 
values upon the "image quality" as assessed by the human eye. Since the "perfect" 
optical system is still a long way off, the lens designer needs to know fhe 
effect on the image of the residual aberrations, so that he can optimise the 
design. Biederaann, of Federal Germany, has investigated the relationship between 
/O.T.F.