KINEMATICS - Biomechanics and Motor Control of Human Movement - page 55

Figure 3.9 Differences in the focal length of wide angle, normal, and telephoto lenses
result in an image of the same size.
The larger the opening, the lower the f -stop setting. Each f -stop setting
corresponds to a proportional change in the amount of light allowed in. A
lens may have the following settings: 22, 16, 11, 8, 5.6, 4, 2.8, and 2. f/ 22
is 1/22 of the lens diameter, and f/ 11 is 1/11 of the lens diameter. Thus
f/ 11 lets in four times the light that f/ 22 does. The fractions are arranged so
that each one lets in twice the light of the adjacent higher setting (e.g., f/ 2 . 8
provides twice the light of f/ 4).
To keep the lighting requirements to a minimum, it is obvious that the
lens should be opened as wide as possible with a low f setting. However,
problems occur with the field of focus. This is defined as the maximum and
minimum range of the object that will produce a focused image. The lower
the f setting, the narrower the range over which an object will be in focus.
For example, if we wish to photograph a movement that is to move over a
range from 10 to 30 ft, we cannot reduce the f -stop below 5.6. The range set
on the lens would be about 15 ft, and everything between 10 and 30 ft would
remain in focus. The final decision regarding f -stop depends on the shutter
speed of the movie camera and the film speed.
3.3.3 Cinematography ∗
Many different sizes of movie cameras are available; 8-mm cameras are the
smallest. (They actually use 16-mm film, which is run through the camera
twice, then split into two 8-mm strips after it is developed.) Then there are
16 mm, 35 mm, and 70 mm. The image size of 8 mm is somewhat small
for accurate measurements, while 35-mm and 70-mm movie cameras are
too expensive to buy and operate. Thus, 16-mm cameras have evolved as
a reasonable compromise, and most high-speed movie cameras are 16 mm.
There are several types of 16-mm cameras available. Some are spring driven;
others are motor driven by either batteries or power supplies from alternating
current sources. Battery-driven types have the advantage of being portable to
sites where power is not available.
∗ Representative paper: Eberhart and Inman, 1951.