and the filter factor is 3. The correct exposure is 1/20
second (1/60 x 3 = 1/20 second). However, most
cameras do not have a 1/20 second shutter speed;
therefore, use 1/15 second or the next slowest shutter
camera lens and adjust the exposure in the normal
filters may give faulty readings if used with TTL
metering systems because the meter reads 18 percent
gray. The camera light meter may not be sensitive to the
color of light passed by the filter.
and printing filters. The printing filters include variable
illumination from a sodium-vapor lamp. The color of a
sodium-vapor lamp does not affect (expose)
light-sensitive materials under prescribed darkroom
conditions. The word safe is misleading since light-
sensitive materials are never completely safe from
Compatible safelight filters for use with certain
light-sensitive materials should be selected on the basis
of color sensitivity and emulsion speed of the material.
The best method of selecting a darkroom safelight filter
is to use the filter recommended by the manufacturer of
the light-sensitive material. Safelight filters absorb that
portion of the visible spectrum produced by a tungsten
lamp that would affect the light-sensitive material being
produces a very narrow band of visible light in the
yellow-orange portion of the spectrum. Colorblind
printing papers are not sensitive to this monochromatic
(one color) band of light, whereas the human eye is very
sensitive to it. Therefore, a brighter print room is
possible without the light affecting the printing paper.
By using specially designed filters that further reduce
the narrow band of sodium-vapor light, black-and-white
materials sensitive to green and red light can be handled
under this illumination. Table 3-7 provides some
examples for the application of safelight filters. Always
consult the Photo-Lab-Index to determine the best
safelight for use with various light-sensitive materials.
yellow filters. The magenta filters are used to print
black-and-white negatives that are low in contrast.
Basic Photography Course