The Furnival Press, (Geo. Barber & Son, Ltd.), Furnival Street, London, E.C.4 (7144) 
15 years has thrown light on many aspects of image quality 
and has advantages in design, but this approach has not 
produced any simple test to replace resolving power for 
general use, and from its nature is most unlikely to do so. 
Moreover, some of the conclusions to which it leads, such as 
the unsuitability of resolving-power for predicting perform 
ance on isolated details, can be deduced without using 
Fourier ideas. 4 It is conceivable that emphasis may shift 
away from sine-wave analysis during the next few years, as 
apparently happened several years ago in television. 
The single bar test lacks the fundamental qualities and 
some of the advantages of sine-wave analysis, but being in the 
space domain, has the advantage of presenting its results in 
an immediately comprehensible form and thus avoids 
confusion. At the same time it can act as a bridge to the 
frequency domain. Its correlation to subjective quality 
remains to be proved, but the same is true of the transfer 
function. 
The resolving power test will probably continue by virtue 
of its simplicity and familiarity. If properly understood it 
can still be of value. Even a high contrast test, though 
utterly remote from the conditions of aerial photography, can 
be satisfactory for controlling the production of lenses where 
the variation of transfer function shape is small. If all 
transfer functions had the same shape it would also be 
satisfactory for ranking systems, but since they do not it can 
be highly misleading in certain circumstances. 
The low contrast resolving power test is not, strictly 
speaking, valuable because it matches the conditions of aerial 
photography, but rather because it discriminates against 
those systems which have a wide bandwidth but poor modula 
tion transfer. For most practical purposes this test will rank 
systems in their correct order and for that purpose the author 
has yet to see it proved wrong or surpassed by any other test. 
It will not predict what size of isolated detail can be seen, 
but at present there is no simple and proven test which will 
do this. 
In conclusion, studies during the past few years have not 
made it easier to obtain a full evaluation of a system or the 
quality of an image. They have greatly improved our 
understanding of the elements of image quality, so that we 
know what questions a given evaluation technique can or 
cannot answer. 
Acknowledgment 
Acknowledgment is made of Itek Corporation’s per 
mission to publish this paper. The opinions expressed are 
the sole responsibility of the author, but their development 
during the past few years has inevitably been influenced by 
numerous discussions with colleagues at Itek and (formerly) 
at the Royal Aircraft Establishment, and with Professor 
Erik Ingelstam ; also by research programmes sponsored by 
Itek Corporation and by the United States Air Force under 
Contract No. AF 33 (647)-9158. To all of these grateful 
tribute is paid. 
REFERENCES 
1. Photogrammetria, XVI, 1959-1960 : 3. Special Congress Number B. 
2. An excellent comprehensive account is given by Perrin. Perrin, F. H. 
Journal of the Society of Motion Picture Engineers, 69, 151, 239 (1960). 
3. Lewis, N. W., and Hauser, T. V. Journal of Photographic Science, 10, 
No. 5. September/October, 1960. 
4. Contract No. AF 33 (657)-9158. Fourth Engineering Report. Itek 
Corporation. Performed for United States Air Force.