Evening star., 1952-07-20

Today's column will be q con-
|tinuation of our glossary of photo
graphic terms. The word "posi-
tive" is used photographically to
indicate the appearance Of an i'm
|age that corresponds in tone values
to those of the original subject.
The term can apply to a positive
image on film or on glass as well
as to the paper print with which
we are all familiar. The dark areas
of the positive match the dark
portions of the subject, and the
same is true of the light areas of
both. In a negative, as we know,
the tone values Of the original
subject are reversed, with the
blacks of the subject appearing
white in the negative and the
whites black A positive trans
parency" then is simply a positive
image produced on q transparent
base, such as a lantern slide, film
or glass plate. The positive trans
parency, or print, is sometimes
used as step in the paper mega
tive process, or it may be viewed
as a picture by black-lighting it..
Causes of Fog.
Fog" is one Of the worst buga-
boos Of photography. It can ruin
"a print or film with equal facility.
Fog. Is generally recognized as an
overall graying or blackening of
the fllm or print, although it some
times only affects small portion
OF the sensitized area. There are
several causes Of fog, but light is
the most frequent offender. Paper
or film may be light-fogged in
several ways-an unintentional, of
course.
II the roll or package Of film
H1s opened in an unsafe place, light
Will surely fog it. A roll of film
Is so designed that it can be un-
wrapped and loaded in the camera
almost anywhere except in bril-
liant sunlight with no i'll effects.
But should the strip of film loosen,
light can leak in and fog the
edges. Cut film should never be
opened except in total darkness.
Other causes Of light fog are leaky
f1lm holders, holes in the camera
bellows, lengthy exposure to the
darkroom safelight. or light strik-
ing the film or paper accidentally
before development and fixing are
complete.
Another type Of fog commonly
encountered is chemical fog. This
occurs when the film or paper is
procesSed in a chemical solution
that is too strong or one that con-
tains impurities. 1t will also take
place II a processing solution is
much above the recommended
Temperature and when the devel-
opment process is carried too far
beyond normal chemical fog will
also take place.
Two other types Of Jogging are
known as dichroic fog and aerial
fog. The former is characterized
by a reddish violet color which
may be seen when the negative
is viewed by transmitted light or
a yellow-green hue when reflected
light is used for scanning the
fim. This type of fog is seldom
encountered by the amateur for
it is the result Of chemical reac-
ton in formulas not often used.
Aerial fog, on the other hand, re-
suits from exposure Of the sensi-
tive material to the air when a.
new developer solution is used or
when impurities are to be found
in the solution.
Filters for the Sky.
A "sky filter" is not, as many
seem to think, just a yellow filter.
Only the upper half of the filter
lis yellow, the lower half being
clear glass. The purpose Of this
filter is to enable the photogra-
Ipher to darken the sky without
changing the exposure. With an
ordinary yellow filter the exposure
must be increased to compensate
for the amount Of light held back
by the filter. The clear glass in
the lower half of the sky filter has
no effect on the exposure.
A "high key" picture is one
where the shadows are very lumi-
nous or nonresident. A "low-key"
photograph is one where the
darker tones prevail. High-key
lightings are generally used to
produce a light. cheerful mood,
whereas the low-key lightings are
used to strike a more comber note
lpictorially.
liens turret" is a term desig-
mating rotating plate which is
fitted with several lenses of differ
ent focal lengths. The lens turret
is a feature found on many mo-
tion picture cameras, but can also
be adapted to use with still CAM
eras.
We sometimes discover "pin-
holes" in our best negatives.
Caused by specks Of dust or un-
" A