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LENS-FILM RESOLVING-POWER 
Ten years ago it was still possible to find those who held that 
there was no need to improve the performance of photographic lenses, ‘and 
that one should look rather to the film. The experimental evidence showed 
clearly, however, that the greatest total information content in the 
negative was to be obtained by using the fastest available type of film 
with the lens at an appropriately small aperture, slower films used with 
the necessarily larger aperture leading to e lower resolution except for 
a narrow zone around the axis. This was quite generally true for the 
lenses made at that time, and for all negative emulsions whose speed was 
sufficiently high for them to be considered for air photography. In more 
recent years, however, some lens manufacturers have made very successful 
efforts to change this situation, and lenses are now available, even 
among wide-angle types, with a performance which fully justifies use of 
relatively slow fine-grain films (e.g. Class C or Class A in Table |). 
The greatly improved marginal illumination in these later wide-angle 
lenses allows the stower film to be used in lighting conditions which 
could not have been contemplated a few years ago, so that the overall 
benefit to air photography has been very marked. Resolution tests with 
one such lens on two different types of panchromatic film are shown in 
Fig.7. It will be noted that the curves for the fine-grain film lie 
above those for the faster material except at the extreme corners of the 
format and do not fall much below them at any point. In the particular 
example chosen the apertures are not exactly balanced for equal exposure 
at a constant shutter speed, but the further improvement on stopping 
down the lens below f/Il is so small that the comparison may be taken as 
valid for all practical purposes. - The question of weighting resolving- 
power figures for area is a somewhat controversial one, but any simple 
method is bound to be somewhat arbitrary, relying as it must on the 
expression of rather intangible things by a number. It seems certain 
that weighting by area alone is not satisfactory, at least for photo- 
grammetry. 
It should perhaps be emphasised that the original views on the use 
of slower film still hold for the majority of the lenses in use and also 
that the potential performance of the newer ones will only be obtained in 
practice if image movements of all kinds are kept at a negligible level. 
Methods of Testing Definition 
Photographic resolution tests, established on an empirical basis 
several years ago, have been widely used, but have aiso been severely 
criticised and alternative methods of specifying performance have been 
investigated and discussed in the literature. |t is therefore felt to 
be worthwhile to express a point of view on this subject. 
The resolution test has been used by the Royal Aircraft Establish- 
ment and by British industrial laboratories ever since 1945, and difficul- 
ties have certainly been encountered in obtaining agreement between 
different laboratories under working conditions. Nevertheless, the 
writer believes that the difficulty of standardising resolution tests 
has been exaggerated. t is suggested that much of the trouble has arisen 
because the tests are usually performed by engineers or optical techni- 
cians, who do not have an instinctive sympathy for the proper treatment