32 
laboratory for nominally the same conditions* This lens 
is not very sensitive to a change in focus, but a large 
fall-off with field angle for tangential lines occurs. 
The MTF for radial lines, on the other hand, is relatively 
high both on and off axis. Differences between the MTF 
measurements for + 15 and - 15 also occurred, but these 
were less than the corresponding spread of measurements 
from the different laboratories for this particular field 
angle. 
General Conclusions 
There is little or nothing to be gained by further 
analysis of the Covogon measurements, but this test, probably 
the first of its kind, may have served its purpose of calling 
attention to the need for proper qualification of apparatus 
and standardisation between laboratories. While the results 
were in one sense disappointing, they were not entirely sur 
prising, since little or nothing had been done to standardise 
OTF measurement during the twenty years or more since the 
first measurements were made. When discrepancies between 
different laboratories are so far out of proportion to the 
accuracies estimated by each, and when no questions of sub 
jective interpretation are involved, unsuspected systematic 
errors must obviously be present in at least some of the 
techniques. There is no reason to assume that accuracies 
cannot be greatly improved by studying sources of error in 
conjunction with a sound programme of inter-laboratory compar 
isons. One such programme has been initiated by the British 
Scientific Instrument Research Association, including measure 
ments on a plano-convex lens, which is easily manufactured as 
a reproducible standard. Early results have shown encouraging 
agreement between a few laboratories. An extension of this kind 
of programme to many laboratories and to cover the specific 
needs of lenses for aerial photogrammetry is highly desirable. 
Table I 
Specification for Transfer Function Measurements of the 12" 
Covogon Lens by Different Laboratories . 
(Drawn up by Chairman of Working Group and 
Professor Ingelstara - IOR, Stockholm) 
1. The lens shall be tested at infinite conjugates 
using a collimator. 
2. The 12" Covogon lens is provided with a paint mark 
on the barrel; the lens shall be mounted for test 
with this mark upwards and facing towards the coll 
imator.