# Lecture 14

```Shutter Speed

Controls the exposure time. It must
be reasonably fast.


1/15 sec, 1/30, 1/60, 1/120, 1/250,
1/500, …
The slower shutter speeds need good
camera support, like a tripod. You can
shoot close-up portraits of people who
can hold still at 1/4 sec, but it’s risky.
Better to use 1/8 sec and faster if your
subject doesn't remain still..
Low light levels require
a slow shutter speed
for proper exposure.
Slow shutter speed reveals motion, shown particularly by the
dancer's blurred hands.
A medium shutter speed
is all that is needed when
the subject is not moving
very fast.
A point and shoot camera will
automatically select a fast shutter
speed in bright sunlight
A fast shutter speed was used
to freeze this bike rider
in mid-air.
Pin-point sharp, stop-action
photography is made possible
by very fast shutter speeds,
but focusing must be accurate
Two types of shutter

Must be able to expose all parts of
the film equally throughout the
exposure


Between-the-lens shutter: near the
lens
Focal-plan-shutter: expose the film
strip by strip, can get deformed images
of the rapidly moving objects.
Stops or Aperture



An opaque barrier with a hole — a
device to limit the width of a light beam.
By varying the diameter of the aperture, you
control the amount of light passing through the
lens into the camera to expose the film.
A large aperture (or wide lens opening) lets in
more light than a small one, and can deliver the
same brightness from a dimly-lit subject as a
small aperture will deliver from a brilliantly-lit
subject.
f-number

f-number = focal length / diameter of
stop’s hole
example:
f = 50 mm, d = 12.5 mm
Then f/d = 4,
which is written as f/4.


For a fixed focal length, the f-number tells
you directly the size of the stop.
For the same f-number, the image has the
same amount of intensity.
f-number

It seems contradictory but is important for
you to remember that a larger ƒ-stop
number designates a smaller lens
opening, and vice-versa.
 For example, an aperture of ƒ/22 is a
very small lens opening,
 whereas an aperture of ƒ/1.4 is a
relatively huge opening, and will let a
great deal more light pass through the
lens.

The bottom row of numbers (16, 11, 8, etc.) on
this lens shows the aperture settings of the lens,
which is currently set at ƒ-5.6. The numbers are
inscribed on an adjustable &quot;aperture control
ring.&quot;
An aperture of f16 is
a small opening!
An aperture of f/1.4 is a
wide opening!
F-number and intensity of light

If you increase the f-number by a
factor, you cut down the light by the
square of that factor.



The area is proportional to diameter
squared.
Double the f-number, get &frac14; of light!
Why f-number involves the focal
length?

For a fixed stop, larger the focal length,
less the intensity of the light.
F-number sequence

a change of f-number to the next in the
sequence corresponds to a factor of 2
change in light intensity,
…0.7, 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32,…

Example:
What is the difference in light intensity
betweenf2 and f22?
f2 lets in more light, f22 lets less,
the difference is
(22/2)2 = 121
Exposure

For a given lighting condition, the exposure
is the product of the light-intensity
times the exposure time, where the lightintensity is related to the f-number.


Light intensity can be measured using exposure
meter.
If film receives the correct amount of light, the
resulting picture reproduces the subject in all of
its proper tones and colors. Such a picture is
said to have been &quot;properly exposed.&quot;
Choosing Exposure

One can get the same exposure by
increase the f-number and the
exposure time by the same factors.
 In a bright sunny day…
f/11 and 1/60 sec
f/8 and 1/120 sec
Gives the same exposure
 If an object is moving, you want to
have shorter exposure time.
High shutter speed froze
this explosion's debris
in mid-air
A small aperture provided
this image with plenty of
depth of field
Film

Chemicals which react when
exposed to light
Silver compound of chlorine, bromine
or iodine
(silver halides), discovered in 1830s


Light strikes the silver halide
(crystal), breaking chemical bond
and yielding metallic silver


Nuclei
Latent image: distribution of the nuclei.

Positive and negative images


Two methods of development



If the film is developed so that the final
image is white where the original
subject is bright, we have a positive
image.
Physical development (Dagurre and
Talbot, used in photocopiers)
Chemical development (what we used
mostly)
Fixing the image

Using hypo to wash away the
unexposed halides.
H &amp; D Curve

How to record the color between
white and black (gray)?


Use crystals of different sizes. Large
crystals are easily exposed by weak
light and small crystals are only active
at very strong light.
The reaction of film, or the density
of the nuclei is a function of the
exposure. The relation can be
shown by a curve, called H &amp; D
curve.
Speed, Contrast and Latitude



Speed.The initial rise (toe) of the H&amp;D
curve represents measure the speed of
the film. Lower the exposure at the toe,
faster the film.
Contrast: the slope of the curve
determines the sensitivity of the film to
the different exposure. Larger the slope,
bigger the contrast.
Latitude: How long is the steep part of
the curve? It represents the range of
exposure.
```