Agfa Library: Technical Terms - R
R
Reciprocity failure
According to the reciprocity law of Bunsen and Roscoe (H = l x t =
constant), it is irrelevant whether the exposure H (the product of
exposure intensity and time) results from a high intensity and short
exposure time (e.g.10 lux x 1/1000 s), or from low intensity and
correspondingly longer exposure time, e.g. 1/1000 lux x 10 s.
In cases of extremely long or short exposure, however, the reciprocity
law falls down (depending on the emulsion used and on the wavelength of
the light). This is known as reciprocity failure.
With color films and color printing materials, the photochemical
process of reciprocity failure basically corresponds to the
circumstances prevailing with black-and-white materials:
With very long exposure times (studio shots with small aperture,
astronomy shots, large-format enlargements), the relative speed of the
materials declines (due to increased recombination of the photo
electrons with defective electrons, building up fewer latent image
nuclei), while at the same time the gradation becomes steeper. Very
short exposure times combined with high intensity (e.g. flash
exposures) also lead to a reduction in the relative speed due to a
different kind of photochemical loss process (disperse latent image), (ultrashort effect).
With color materials, additional problems can arise because the three
individual layers can react differently. Very long exposures lead not
only to a reduction in the general speed, but also to a possible change
in the relative speeds of the individual layers. In the case of
reversal films, these shifts can be corrected with the aid of color
compensating filters, and in the case of color negative films and color
printing materials, corrections can be made by appropriate print
filtering. With very low light intensities (the gradation becomes
steeper) and at very high intensities (the gradation becomes flatter),
the photochemical effect of reciprocity failure can differ from one
layer to the next, resulting in color deviations (cross-cast) in areas of high
and low density. These are too severe to be eliminated by correction
filtering.
Resolving power
The resolving power - or resolution - of a photographic emulsion layer
describes the ability of a material to reproduce the finest adjacent
details (e.g. lines in a line screen). It is thus a purely visual
criterion for the image quality of a photographic material
(nevertheless based on standardized photographic and physical testing
conditions).
The maximum resolving power of a photographic material describes the
number of lines per millimeter which, at a given contrast of e.g. 1.6 :
1, can be depicted as separate from one another. A direct comparison of
the resolving power of various film materials is only possible if the
testing conditions are the same for all the materials.
The resolving power is influenced among other things by the range of
contrast, the diffusion haloing (anti-halation),
which reduces sharpness and contrast, and by the granularity of the
photographic emulsion.
One example of the influence of contrast on resolving power is that the
resolving power of the CN film AGFACOLOR OPTIMA 100 PROFESSIONAL is 140
lines (l/mm) at a contrast of 1000 : 1 or 50 lines/mm at a contrast of
1.6 : 1. Both values represent the same material and the same
outstanding resolving power, but apply to completely different contrast
ratios.
RMS
Method for measuring the granularity
of a photographic material.
Granularity is the term used to describe agglomerations of silver
grains in the developed emulsion layers of black-and-white materials or
agglomerations of dye in the emulsions of color materials. On
enlargement, granularity becomes evident as an uneven structuring of
grey and color densities. The granularity of films can either be
compared visually (usually with the aid of 12x enlargements of
comparative prints) or by measurement, e.g. with the RMS method, which
aims to achieve conformity between the measurement and the visual
perception of film granularity. The abbreviation RMS (root mean square)
indicates that this method measures statistical density fluctuations of
the film granularity and converts them into figures.
Film specimens, exposed and processed under identical conditions, are
made to rotate and are then measured with a microphotometer at a
density of 1.0* behind a visual
filter. The specimens are measured in a very small
range (diameter of measuring area: 48 µm). From this measurement
we get the standard deviation of the local density fluctuations of the
specimen. The measurement is multiplied by 1000 to give the RMS
granularity. The lower the RMS granularity, the lower the grain of the
film.
For example, the medium-speed AGFAPAN APX 100 PROFESSIONAL has an RMS
granularity of 9 (measurement 0.009 x 1000), and the low-speed AGFAPAN
APX 25 PROFESSIONAL has an even better RMS of 7 (measurement 0.007 x
1000).
* For black-and-white film: calculated on
measurements at diffuse density of 1.0. For color film: calculated on
the integrally measured color densities of the emulsion dyes at a
density of 1.0.
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