film speed - cartridge has no filter notch
light film speed - cartridge has a filter notch
can be seen, the distance from the centre pin to the bottom of
the cartridge notch decreases
as film speed increases.
Thus, the size of the notch itself increases with film speed.
thing that is confusing about the table above is that there are
two collumns for film speeds - daylight and tungsten - and that
the same size notch is used for different film speeds, depending
on whether the film is tungsten or daylight balanced.
For example, both 160 asa tungsten and 100 asa daylight films
have the same notch dimension. How can that work?
The answer is the filter notch!
filter notch, if present, is located bout 2cm below the centering
notch. Tungsten balanced film has this notch, daylight balanced
film does not. Now, all super 8 cameras have an internal colour
correction filter for shooting tungsten balanced film in daylight.
Using this filter, however, takes away a little bit of light that
would otherwise get to the film. Specifically, they reduce
the light by two-thirds of an f-stop. Thus, a 160 asa tungsten
balanced film when used in daylight with the colour correction
filter in place, is effectively becomes only a 100asa film.
That's why the 160asa notch for tungsten film is the same size
as the 100asa notch for daylight film. Really, the 0.5 inch
notch dimension is the 100asa dimension.
what does the filter notch actually do? Cameras that can
read the filter notch do so via a small pin inside the film chamber.
When using tungsten film - ie film with the filter notch - the
pin is not pushed in by the cartridge. In this instance
the internal filter is kept in place. When daylight film
is inserted in such a camera, however, the pin is depressed (because
there is no notch) and the filter is automatically removed.
Thus in our example of 160 tungsten, 100 daylight film, when the
tungsten film is in the camera, the filtern notch pin is not depressed
by the cartridge, so the internal filter is kept in place.
The film speed notch reader indicates a speed of 100 asa to the
camera - which is the right speed for 160asa film when shooting
in daylight. When the 100 daylight film is inserted, the
filter notch pin is depressed and so the filter is automatically
'de-selected'. Again, the film speed notch reader will detect
a speed of 100 asa - correct for this film.
what happens with tungsten balanced film when you want to shoot
under tungsten light? As usual with tungsten film, when
the cartridge is inserted in the camera, the presense of the filter
notch will mean that the internal filter is not
'de-selected'. To shoot under tungsten lights, the internal
filter needs to be deselected another way. This will either
be done by inserting a filter key or a filter screw. In
either case, not only will doing so remove the filter, but
the light meter will also be compensated by two thirds of a stop.
Thus, the filter notch pin when depressed 'de-selects'
the filter but does not adjust the asa, while the filter key or
filter screw when inserted 'deselects' and also
ajusts the asa.
all cameras however have a filter notch detector. Those
that don't are all of the 40/160 asa type. Instead of a
filter notch pin and filter keys or screws, these cameras use
a simple switch to select or de-select the filter. While
easy to operate, the absense of a filter notch pin reduces the
flexibility of the camera. 40/160 cameras that have the
pin can not only read 40 and 160 tungsten film, but can also read
25 and 100 daylight film, while those with just a filter switch
can only read 40 and 160 film, tuntsten or daylight.