28 THE PHOTOGRAPHIC IMAGE, BROCK 
posure, and the uniformity of illumination in the camera. In principle we should use the 
highest possible overall gamma in any situation, expanding the luminance range of the 
optical image so that the negative density range just matches the printing medium's 
useful exposure range. Since the latter has no sharply defined limits, however, a com- 
promise is always necessary. Individual ideas on the right compromise seem to differ con- 
siderably. It is very common to see the subject highlights pushed down the paper char- 
acteristic curve so far that all detail within them is lost due to the overall slope at that 
point falling too low. (This affect is especially pronounced for the finer details because 
of the falling amplitude/frequency response.) The use of a paper characteristic too steep 
for the negative gamma is presumably prompted by the desire for strong separation of 
the middle tones, and though one may deplore the practice its widespread acceptance 
indicates that faithful photographic reproduction is not always a necessity. Moreover, 
the better balance here advocated is as much a compromise as that just criticised, since 
an overall unity gamma reproduction would be unacceptably “flat”. 
The introduction of the Plus X emulsion emphasises the difficulty in choosing and 
modifying gamma. Fully developed, this emulsion's gamma of 2.5 is very valuable in hazy 
atmospheres and for small scales, but naturally gives unprintable negatives in clear air 
and on large scales. Since one cannot reserve the emulsion solely for difficult cases, some 
control of gamma in development becomes necessary, and the inevitable loss of speed is 
then a difficulty. The loss can be minimised by change of developer, but this is often 
highly inconvenient. These problems would not exist if it were possible to control gamma 
and speed independently during development, but this appears to be a luxury fundamen- 
tally out of reach. In practice, photographers have some control of gradient, as distinct 
from gamma, by deliberate variation of exposure, but this has obvious limitations. 
Allied to gamma is curve shape. It is well known that the ideal characteristic curve 
for hazy conditions would be steep at the lower exposures and flatten out with increasing 
exposure. Using such a curve would of course present some trieky problems in matching 
it to the image luminance ranging. Experimental attempts to use such a characteristic 
on hazy scenes are much less convincing than similar tricks played with electronic cam- 
eras (though the latter are not completely satisfying due to increased * noise"). The 
reasons for this are not altogether clear and it would add to knowledge if they could be 
elucidated. 
6.2.4. Spectral sensitivity. 
The new Eastman emulsions show a departure from former practice in that the limit 
of spectral sensitivity has been extended from the usual 660 my to 710 my. This has no 
significant effect on the rendering of green vegetation, presumably because of a fortui- 
tous balance between the integrated response over the chlorophyll absorption band and 
the intercept on the rising curve at the long-wave side of this band. (Fig. 17). There has 
been no sign of an emulsion sensitive far enough into the deep red, say to 750 mu, to 
cause a significant lightening in the reproduction of natural greens, as often suggested 
by the writer. 
The slight shift in balance of sensitivity longer wavelengths could in principle have 
other results viz: 
a. A darker rendering of water. 
b. A small improvement in haze penetration. 
c. A slight easing of the lens designer's task, if used in conjunction with an orange or 
red filter. 
The writer has not yet observed any significant effect of a. or b. It has been pointed 
out to him that any move of the wavelength balance towards the infra-red if not ac- 
companied by a broadening of the waveband accepted, makes colour correction easier by 
virtue o 
sion. Al 
vour a | 
sitivity 
typical 
red. 
No 
in then 
infra-re 
to have 
62.5. C 
So 
aerial 
been coa 
Motion 
ture ca: 
sions ar 
grey ( 
base, to 
preserve 
detail. P 
mum det 
grammet 
grey bas 
lenses we 
of the gr 
have ver 
Expe 
base hav 
small but 
exposure 
An i 
with the 
not obvio 
Althe 
trials by 
air photo 
exactly m 
successive 
amount o 
(37) lens 
exposure, 
swamped 
than its c 
It is 
halo back 
Grey 
plate patt 
vacuum f 
light fron 
perimenta