12 
negligible matter. A desirable aim would be to achieve 
uniformity of presentation as in some other disciplines, 
e.g. in electronics, frequency response curves are almost 
always presented on log/log co-ordinates. In the present 
case there is no clear argument for any exclusive choice 
of co-ordinates; but agreement could probably be reached 
on one as the normal and preferred method. Since differ 
ent applications of the MTF may call for different co 
ordinate systems, presentation of data in tabular form 
would assist the serious user. 
Symbols 
Numerous different symbols are employed, thus 
spatial frequency is denoted by S, N, K,t^etc. A uniform 
symbolism should be agreed upon, in consultation with lead 
ing optical laboratories. 
Averaging over Field 
Averaging the MTF over the field of a lens presents 
just as many difficulties as averaging the resolving power, 
indeed the difficulties are greater inasmuch as the resolving 
power figures are replaced by curves. Also, phase shifts, 
which in effect are taken into account in a resolving power 
figure, are presented separately in an OTF measurement, and 
there is no obvious way of averaging them. The averaging 
problem is bound up with the general problem of the inter 
pretation of MTF's, and it may be that the ultimate solution 
will be found in the translation of MTF data into relative 
contrast data for a specified target as discussed later in 
this report. 
One approach, suggested by Back, is to present maps 
of the camera field showing lines along which the MTF is at 
a constant value for some given spatial frequency. This 
approach has the danger that the contrast information may be 
interpreted as applying to general target detail and not 
merely to sinusoidal targets. 
It may well be that no reasonable method of averaging 
MTF's over the field can be found until a method of reducing 
each MTF to a single figure of merit has been agreed upon. 
Similar remarks apply to phase data. 
Accuracy of Present Measurements 
It is commonly believed that measurement of the MTF of 
a lens is much more accurate than measurement of its resol 
ving power. This is not necessarily true, in practice, 
(though certainly true in principle) and there is very little