Homework
#5 solutions
Light and Color (Physics 1230) Fall 2007
Solutions to Homework #5
Pg. 142-3 Prob P1
(a)
When the lens is closest to the film you are focused on a very
distant object. The farthest
object you can focus on is at infinity (in practice, a distant star). The rays from that distant object
arrive at the camera lens essentially parallel and so converge at the focal
point behind the lens. The focal
length is therefore the smallest distance from lens to film that the camera can
achieve.
Pg. 142-3 Prob P2
(a) The
lens forms a real image of the object on
the light sensitive film, which
records this image.
(b)
The light-tight box
excludes all light that does not come through the lens.
(c)
The shutter allows
light to strike the film for a specified amount of time.
(d)
The diaphragm controls
the effective area of the lens and hence the amount of light that enters the
camera in a given time.
Pg. 142-3 Prob P4
(a) Depth
of field is the range of object distances for which the image of the object
will be "acceptably" in focus.
For a given lens, the depth of field is controlled by the aperture
- a small f-number (large opening)
gives a small depth of field, while a large f-number (small opening) gives a
large depth of field. For a given
f-number, the shorter the focal length of the lens, the larger the depth of
field.
Pg. 142-3 Prob P5
a) 28 mm
á
You want a large angle of view
b) 150 mm
á
You want a telephoto lens so the pitcher appears large
in your photograph.
c) 28 mm
á
You want to get close to the fish with wide angle perspective
and good depth of field.
Pgs 142-143, Prob. P6
a) f/5.6 at 1/100 sec gives the same exposure as f/11
at 1/25 sec. Going from f/5.6 to
f/11 is two stops, or a factor of 4 decrease in effective lens area. Hence the time is 4 x 1/100 sec = 1/25 sec.
b) If you increase the f-number by one stop ((one step
along the list in Table 4.2), you open the hole wider and you must therefore
increase the exposure time by a factor of 2 to keep the same total
exposure. Hence, other equivalent
settings are: f/4 at 1/200 sec,
f/8 at 1/50 sec, and f/2.8 at 1/400 sec.
Pgs 142-143, Prob. P7
(a) f/2
at 1/200 sec in order to stop the action
(b)
f/5.6 at 1/25 sec in order to increase the depth of field
(c)
f/2 at 1/200 sec in order to decrease the depth of field