Scientifically speaking, to the human eye yellow appears to be approximately over ten times as bright as blue, three
times as bright as red, and one and a half times as bright as green. The average panchromatic film (page 31),
however, registers blue with a brilliance of about four-fifths that of yellow, green with one-third, and red with two-
thirds of the brightness of yellow.
It is, therefore, evident that in order to obtain a colour rendering which will correspond with some degree of
accuracy to impression of colours as perceived by our eye the photographic sensitivity of the various colours
compared with each other will have to be corrected. This can be achieved by the use of filters.
Filters are intended to control on our negative material the varying degrees of brightness. They lighten objects of
their own colour and darken those of their complementary colour (e.g., a yellow filter will darken the blue of the
sky). They may be used to obtain a colour rendering in our picture which corresponds more closely to the impression
made upon our eye by the object : so we speak of "correction filters". Filters may also be employed to emphasise
certain effects regardless of whether such emphasis is scientifically correct or not; for instance, our picture can be
made to show heavy clouds against a particularly dark sky, whereas the actual landscape revealed only light clouds
in a blue sky. Filters for such ends are termed "effect filters".
All filters cut out some part of the light and thus, as a compensation, an increase in exposure time is necessary
when using them. Exact figures can only be given for each particular case, for the exposure ratio depends not only
on the nature of the filter but also on the colour sensitivity of the film and on the colour of the prevailing light (page
58).
The Retinettes take 32 mm. screw-in filters (32 mm is in this case the outside diameter).
The following list gives a summary of the filters recommended and a short explanation of their use. The Focal Filter
Chart gives all filter data fully and at a glance.
YELLOW FILTERS reduce the actinic effect of blue, rendering it darker and are, therefore, particularly suitable for
landscape photography in order to obtain clearly defined cloud effects on a normal blue sky. In the case of a very
light blue sky, use a darker filter and vice versa.
YELLOW-GREEN FILTERS give an effect similar to that of yellow filters, but also hold back red (render it darker), to
which some panchromatic films are comparatively over-sensitive (photographing it too light).
ULTRA VIOLET FILTERS are only to be employed at heights of 6,500ft. (2,000 m.) above sea level and over to avoid
an unduly dark sky such as would be obtained by using a yellow filter. At the same time they absorb the ultra-violet
rays of high altitudes for which the lens is not corrected and which would reduce the definition.
ORANGE FILTERS give over-correction, and serve, therefore, as an "effect" filter for depicting heavy clouds against a
dark sky, and very clear distances in landscapes, eliminating light haze, etc.
RED FILTERS are of still stronger effect than the orange filter, for extreme contrast, creating black sky with brilliant
clouds, faking sunshine into moonlight effects, etc.
BLUE FILTERS are for panchromatic film in artificial light. They absorb part of the red sensitivity. This results in
better skin-tones.
For filters for colour photography, see page 35.
FILTERS AND FACTORS
Filter
Filter
Code
Factor
Reduce Light
Value Setting
by
Reduced film
speed on
Retinette IIA
by graduations
Light Yellow
F I
1½x
½
2
Medium yellow
F II
2x
1
3
Yellow green
F III
2x
1
3
Orange
F IV
3x
1½
5
Red
F V
7x
3 (2¾)
9
Blue
F VI
2½
1½ (1¼)
2
Ultra-violet
--
--
--
--
Polarizing Filter
--
2½
1½ (1¼)
2
Filter factor compensation on Retinette IIA
The filter factor is allowed for on the Retinette IIA by reducing the film speed setting before taking the measurement
with the exposure meter (page 51), by the number of graduations indicated in the table above.
Polarizing Filter
