lage vue sous l'angle que 
eure. Votre Bureau de 
isposition de toutes per- 
réunion d'un comité res- 
Iq ou six sujets dont je 
provoquer de telles ré- 
de tout simplement de 
ma case, ma case c'est le 
rons au mieux pour vous 
ne inprimé donne moins 
laire que j'ai adressée le 
'ersonnes ayant quelque 
ommission I, cette circu- 
du temps minuté — je l'ai 
n ce qui me concerne — 
our excuse d'avoir com- 
eu de 9 hrs. A partir de 
Attwell, qui est l'officier 
nission I, mon secrétaire 
et moi-méme, nous veil- 
nploi du temps trés stric- 
lus attendre je vais don- 
ur Brock. 
Commission I Invited paper 5 
The Quality of the Photographic Image 
by G. C. BROCK *) 
Royal Aircraft Establishment, 
Farnborough, England. 
1. Introduction. 
The basis of photogrammetry is the negative photographie image and its derived 
positive, which depict more or less faithfully a perspective projection of the original 
scene. For each object in the scene the task is to answer two questions — “What is it?” 
and “Where is it?” This paper is concerned with the first of these, with the qualities 
that make photographic images good for recognition. The quality of very small images 
is particularly important, because objects of metre dimensions are commonly imaged in 
tenth to hundredth millimetre size, i.e. in the region where photographic reproduction 
deteriorates and eventually breaks down. Currently accepted methods for measuring and 
expressing quality in this region are recognised to be imperfect and are still the subject 
of argument and research. Photographic qualities on the macro scale can be described 
fairly well in terms that are agreed and have changed little over many years. But the 
camera and its sensitive film are only part of a long process starting at the scene photo- 
graphed and finishing as a percept in the mind of the user. Easily distinguished factors 
which may influence image quality are indicated in Table 1. 
Here are at least eighteen stages where the image, or the message it conveys, can 
be degraded. These are not listed as a prelude to discussing them all, but so that they 
shall not be overlooked. Air photography is now a highly developed art in which pro- 
gress has been made largely by concentrating on a few things, e.g. lenses and emulsions. 
Both of these have now been so far improved that the other stages of Table 1 cannot be 
ignored if the potential advantages are to be consistently obtained. It may not be out of 
place to suggest, that, whereas this paper will be mainly concerned with purely photo- 
graphic matters, a successor could usefully deal with some other aspect such as the cam- 
era environment in the aircraft. The other thought suggested by the list is that al- 
though this is one process directed to one end, the influence of the eighteen factors is 
described in at least two dozen different technical terms, and there is no single simple 
deseription for the excellence of the result achieved. Many of the descriptions and tests 
used are felt to be unsatisfactory. Thus the quality of photographie reproduction is the- 
oretically described in terms of negative and positive characteristic curves, but in prac- 
tice their application is limited by “dodging” and they inevitably fail to express the 
quality of the micro-image. Resolving-power is an attempt to define the latter but a sta- 
tement of the size at which a diminishing, deteriorating image finally becomes useless is 
by no means all that should be known. The degradations and distortions it undergoes due 
to high-gamma reproduction and lens aberrations may well affect position as well as 
recognition, and have not been sufficiently investigated. There is a need for some degree 
of unification, whereby these factors, or some of them, may be studied and defined in 
*) Crown Copyright Reserved.