THE EXPERIMENTAL EVALUATION OF LENS PERFORMANCE 
by 
Irvine C. Gardner. 
A resolving power test as applied at a standardizing laboratory has as its 
purpose the establishment of a quantitative measure of the excellence of a lens. 
The conventional procedure is to photograph a test chart by means of the lens 
to be tested. Unfortunately, the resolving power of the lens and that of the 
photographic emulsion lie within or near the same range of frequencies. In 
other words the emulsion, which is the sensing element, does not respond 
linearly within the range of frequencies to which it is applied. This is contrary 
to the recognized principles of testing. The sensitivity of the test as a measure 
of quality is dulled; the test method offers no greater sensitivity than is 
available in the actual use of the lens; the results are not amenable to analysis 
by which they can be extended to cases other than the one actually tested; and 
the test negative is characteristic of the lens and emulsion, perhaps to equal 
extent. 
This joint effect of lens and emulsion is generally recognized. In fact, it 
has been argued that because of this individual character of a lens-emulsion test, 
it gives precisely the information desired. It is quite true that knowledge of the 
resolution of a lens-emulsion combination is valuable and should be available. 
Or, speaking more precisely, it is highly desirable to know the value of resolving 
power or of some equally suitable measure for the performance of a lens- 
emulsion- ground target photographed from plane combination as distinguished 
from the lens-emulsion-specified test chart combination that is now used as a 
test for lenses. The determination of the resolving power to be achieved when 
photographing from a plane is an expensive procedure and such a resolving 
power represents a complex concept in that it involves the scattering of the light 
by the atmosphere, the effect of the transient variations of index of refraction 
of the air within the light-path and the vibration of the airplane, the latter effect 
probably very significantly different for jet-propelled planes and those equipped 
with piston engines. The resolving power test, as proposed for adoption at this 
meeting, attempts to establish a procedure which will approximate the lens- 
emulsion-ground target viewed from plane combination as nearly as it can be 
readily simulated in the usual laboratory but it omits important elements of 
correspondence and surely no one seriously proposes that the value of the 
resolving power obtained by this test will be the same as that which will be 
attained during a flight. A quantitative determination of the differences in 
resolving power to which reference has been made in this paragraph would 
require an extensive program of airplane photography as well as an extensive 
laboratory program. Such a program is probably only possible to a military 
agency. 
Before embarking on such a program, however, it should be noted that 
it is by no means axiomatic that resolving power is the best criterion for 
grading the excellence of a lens-emulsion combination. It is a matter of general 
knowledge, for example, that a resolving power test, by itself, is not entirely