Page 4 - pm_1951_06

• COLOUR FILTERS FOR PA

These filters correct the extreme sensitiveness of the gelatine bromide
emulsion for ultra violet, violet and blue, in such a way that the tones obtained
on the film will be as near as possible to that seen by the human eye-which is
particularly susceptible to yellow. They obtain a better and more correct
rendering of natural tones or, when the operator so desires, heighten a specific
colour-bearing in mind that coloured filters lighten objects of their own colouring
and darken objects of complementary colours. They soften the former and
increase the contrasts of the second.
Filters transmit certain light rays, the degree of which varies with each
filter. A factor must, therefore, be applied when estimating the exposure in
order to give due allowance for the lowest light values when a filter is used.
For example : If the lens reading should be f 16 without a filter, it must be
increased to f I I with a factor 2 filter and to f8 with a factor 4 filter.
Maintenance. A filter requires the same careful attention as a lens. It

Increase of Colour
Colour Factor lens aperture photographed
C.900 Red/ Orange xs 2! stops Yellow
C.905 R.ed
Green
Blue
C.901 Yellow / Green X4 2 stops Yellow
C.906 Red
Green
Blue

C.902 Red X7 3 stops Yellow
C.907 Red
Green
Blue

C.903 Light Green X2 I stop Yellow
C.908 Red
Green
Blue

C.904 Ultra-Violet X-Nil Nil
C.909
\ Amber X2 I stop Yellow
C.298
Red
Green

C.306 Grey (Neutral X4 2 stops Blue
Density)

Page Four