PHYSICS AND THE VISUAL ARTS
Notes on Lesson 11
Things to remember from
Lesson 11.
Photography has been around for nearly two centuries in one form or another. By 1839 Daguerre was coating
copper plates with silver and sensitizing them with iodine vapor to form a layer of silver iodide. When light
struck regions of the silver iodide surface the regions changed back to silver. The plates were then developed
by exposure to mercury vapor that formed an amalgam with the silver particles. Then the plate was washed to
remove the unexposed silver iodide. On the developed plate, the regions that had been struck by light
appeared light while the shadow regions appeared dark. The resulting images were very stable and some remain
to this day.
Over the years other photosensitive materials were developed. By the late 1800s, George Eastman had developed
photosensitive material on a flexible cellulose nitrate backing that could be rolled up. Box cameras were
built to use this film and the Eastman Kodak Company was born.
A camera is a light-tight box with a hole that can be opened to
let light enter and strike the photographic film.
Box cameras have a simple fixed focus lens. Typically, they image
objects at distances from 4 ft to infinity from the lens. How can they
image objects over such a wide range?
Simple cameras have view finders to help the photographer aim the camera. Sometimes
the viewfinder is nothing more than a controlled opening. Others are small
Galilean telescopes turned backwards (as in the Kodak Max camera you examine in the
lab that goes with this lesson). In that case the eyepiece is the positive lens and the objective is the
negative
lens and the magnification is less that one. Typically viewfinders are designed to
give a virtual image about one meter in front of your eye.
Many small cameras nowadays have autofocusing, that is, the camera adjust the lens to
get a sharp focus automatically. Most of the time this works OK, but sometimes the
camera adjust to focus on the background or other object that was not intended to be
the subject of the best focus. Several autofocus systems are in use. Polaroid cameras
use an ultrasonic system that relies on the echo time of ultrasonic pulses. Other cameras
use the imaging of light itself. Still others use infrared beams.
Single lens reflex (SLR) cameras have a mirror that allows the
photographer to see the image formed by the camera lens before the
shutter is opened. The image is formed on a ground glass screen
(sometimes with micro prisms and/or a biprism that splits the image
whenever it is not at sharp focus). Usually, the SLR will have a roof pentaprism to allow the user to look in
the direction that the camera is aimed to see the image formed on the ground glass screen. The pentaprism
allows for the image to be reflected in the proper direction. The roof shape acts as a right angle mirror
to correct for the left/right reversal that
would exist if the top of the prism was simply flat.
Some SLRs have manual focus. In that case the photographer adjust the lens until
the image in the viewfinder is sharp. Other cameras are autofocus. Typically the
autofocus SLR cameras have a simpler viewing screens so that they are not as easily
focused manually as those cameras that were designed for manual adjustment.
Shutters may be found behind the lens (in simple box camera), in the
midst of a compound lens (more advanced camera), or just in front of the
focal plane (SLR camera).
A long focal length lens magnifies the image (i.e., acts as a telephoto
lens) and a short focal length lens acts to give a wide-angle image. For
35 mm format the normal lens is a 50-mm focal length. Anything smaller
is considered to be wide-angle and anything larger is considered to be
telephoto.
35-mm film is so designated because the width of the film is 35 mm. The image size in 35-mm cameras is a
rectangle 24 mm high by 36 mm wide. It is know as a 2 x 3 format. A common size for prints from 35-mm film is
4" x 6" which preserves the shape and allows for showing the full frame. Other common sizes for prints
include 5" x 7" and 8" x 10". Neither of these, however, have the correct 2 x 3 shape. Consequently, some of
the image must be cropped out when printing to those sizes.
High quality images require quality lenses. Good camera lenses use several pieces of glass in order to correct
for the abberations. Special care and materials are used to reduce or eliminate spherical and chromatic
abberations as well as coma.
Good cameras allow for controlling the light to the film by changing either or both the shutter speed and the
aperture. The term shutter speed actually refers to the time interval that the shutter is open. Shutter times
can often be varied from as long as several seconds to less that 1/1000 of a second. Typically, shutter speeds
(or times) can be changed in steps of a factor of two. For example, you can go from 1/30 s to 1/60 s to 1/125
s.
The aperture is most often expressed in terms of the f-number to get proper exposure with a camera.
(An alternate term is f-value.)
You need to know the definition of the f-number and how f-numbers are
used with shutter speeds.
f-number = focal length of lens/diameter of the aperture.
Remember that increasing the f-number by a factor of the square root of 2 decreases the light passed by a factor of 2.
Thus, going from f/8 to f/11 decreases the exposure on the film by 1/2.
Similarly, going from f/8 to f/5.6 increases the exposure by a factor of 2 and going from f/8 to f/4 increases exposure by a factor of 4.
The depth of field is the distance over which the subject is rendered
sharply focus in the film plane. In general, the lower the f-number the
smaller the depth of field.
A zoom lens (varifocal lens) has elements that move, thus changing the
effective focal length of the combination. Could you prove from either
ray tracing or using the thin lens equation that changing the separation
between two lenses would change the effective focal length?
Exposure is the amount of light captured by the film, CCD, or CMOS sensor. Recording media vary in their
sensitivity to light, but all need a specific amount of light (exposure) at a given sensitivity (ISO). Too
much light leads to overexposed, washed out images and too little light leads to dark, underexposed
images.
The correct exposure does not require a unique setting of shutter and aperture.
There may be many combinations of f-number and exposure time that give the correct exposure, that is, the
correct amount of light energy delivered to the film (or sensor). The exposure is the
product of the light intensity (energy per second), which depends on the aperture, and the exposure time
(seconds). The intensity is determined by the f-number and the exposure time is determined by the shutter
speed. This behavior of film (or other sensors) is known as reciprocity because of the reciprocal
relationship between the aperture and the shutter time: as the aperture increases the shutter time must
decreases to keep the
exposure constant. Similarly as the aperture decreases the shutter time must increase. This relationship
between shutter and aperture is also known as the reciprocity.
The American Standards Association developed a method for measuring the film sensitivity. From series of
measurements the sensitivity (or film speed or emulsion speed) can be expressed as
S = N2/TB,
where N is the f-number, B the illumination of the scene in foot-candles, and T is the minimum time in seconds
for the proper exposure. S is the ASA number for the film speed.
The ASA rating has been adopted by the International Standards Organization and is now known as the ISO rating
for film speed. Typically the ISO ratings go from as low as about 64 for Kodachrome to 1600 or more for some
Ektachromes and Fujichrome films. The photosensitive device is not limited to film. The CCD sensors in
contemporary DSCs (digital still cameras)are also rated with an equivalent ISO number.
What is a latent image, how do developers work, and what is the
difference between negative film and reversal film?
We discussed the mechanisms of the dominant methods of color photography and saw a
series of transparencies that explained the three layers of sensitive material in the film
and the three layers of dyes in the developed film in both print film and slide film.
Color negative films (such as Kodacolor or Fujicolor) have three photosensitive layers,
one each sensitive to blue, green, and red. Because the green and red sensitive layers are also blue
sensitive a yellow filter is incorporated between the upper blue sensitive layer and the other layers
to filter out any blue that would get through. The yellow layer is removed during development. As the
developer reacts with the silver halide it is oxidized and reacts with color couplers to produce the
dye in proportion to the silver released. The dyes are the complements to the initial light. Thus red
light exposes the red-sensitive layer and the development turns that layer cyan. (The density of the
cyan layer controls how much red light can pass through.) When a print is made from such a color
negative, the resulting color is correct. The cyan
light exposes the blue and green sensitive layers in the print which in turn are developed into
yellow and magenta. Light passing through must go through both layers and emerges as red.
The magenta and cyan dyes are not ideal. To compensate for this a yellow coupler is added to the green
sensitive layer. A similar reddish coupler is added to the red sensitive layer to better control the
cyan dyes. As a result the finished negative appears to have an overall orange cast.
Printing from such a negative requires additional filter to restore the proper color to the finished
print.
Color reversal films (Ektachrome for example) result in a positive image rather than a negative. Thus
for red exposure, upon develoment there is no dye left in the red sensitive layer, but the blue and
green sensitive layers have their complementary dyes
of yellow and magenta so that the result is red.
We also saw an example of false-color infrared-sensitive film that translates IR into red, red into green,
and green into blue. In normal usage the blue light is filtered out with a yellow filter. The
resulting images are in full color but the colors are not
as we would normally observe them. Can you explain why the green leaves on trees appeared magenta in the
false-color IR
image?
Technical details about
Kodak and
Fuji films can be found on
these links.
Nikon has a nice page describing camera lenses. Go to Nikon home page and
follow the link to lenses.
Back to Physics 153 Home.
Back to Physics
Dept web page.
Back to USC.
Maintained by rjones@mail.psc.sc.edu
Last Modified: 03/21/05