The Photograph
The photograph should be a record of the “appearance” of the things photographed,
as “seen” from the point of view, and at the scale, used. It should include all the details.
These are made perceptible by the boundaries that distinguish them from their different
surroundings. Unfortunately, these boundaries, which may have no thickness in the
appearance, acquire width in the photograph, and the contrast or difference in the tone
values between detail and surroundings may be reduced below the level of perceptibility.
This latter is a common failing in the print. Detail that is visible in the negative may
be lost in solid black or pure white or in indistinguishable grays in the print. The 21
problem is, then, to obtain maximum sharpness and adequate contrast at each and every
boundary. Correct relative tone-rendering of areas, and less than the usual exaggera-
tion of the vertical scale, may also be desirable, as noted above.
KOD.
DENSITY
The Negative
A. Reproduction of sharpness
Factors affecting the reproduction in the negative of the sharpness at a boundary
in the object include:
(1) Refraction in the atmosphere between the object and the camera lens;
(2) Diffusion in the atmosphere and at the optical surfaces of the port, filter, and
lens, due to light-scattering by dust etc. in or near the line of sight;
(3) Diffraction, due to the nature of light and the limited size of the lens aperture;
(4) The design and quality of the lens and its adjustment in the camera;
(5) The colour sensitivity of the film and the colour of the filter used;
(6) The discontinuity of the recorded image due to the graininess of the photo-
graphic emulsion; | IL
(7) Irradiation, or the lateral spread of light in the emulsion due to its transluscence; 34
(8) Image motion during the time of exposure due to: |
(a) ground speed, scale, and time of exposure, |
(b) rotational vibration of the camera, focal length of the lens, and time of
exposure. |
In the above, the first three, atmospheric refraction, diffusion, and diffraction, may |
be considered as the gifts of God, to be accepted with grace—and investigated. ‘The
N
erem
next three, lenses, films, and filters, are the products of manufacturers, to be tested,
compared and selected. Which leaves only the last two, lateral spread and image
motion, to be modified by the operator in order to improve boundary sharpness and > | :
so increase the information recorded in the negative. ^ | ne
(Colour sensitivity and colour filters are discussed more fully under "B", contrast a |
in the negative). | NS
Lateral spread or resolving power vs. density. Experiments conducted at the
University of Toronto and first reported at the International Congress of Photo-
grammetry at Stockholm in 1956 showed that the resolution in lines per mm obtained
by contact printing resolution targets on Kodak Super XX film reached a maximum
at a density of 0.3 and dropped to approximately half that value at a density of 2.0.
Similar tests have been made on Kodak Plus X Aerographic Film, and on Ilford
H.R. Aerial Film. (The latter with 2 different developers.) The results are shown
in Fig. 1.
The results differ with films and developers. But a significant decrease in
resolving power in densities greater than 1.0 is common to all, and it is interesting to
note in the case of Kodak Plus X that a density of .5 gives the maximum resolution
whether it is produced by short exposure and long development or the reverse.
2
