CS 39J: Session 6
http://inst.eecs.berkeley.edu/~cs39j/session06.html
28 February 2002
Student work
We critique each student's work.
The Case of the Leaning Tower
Note that the tower appears to be leaning. This is due to perspective,
and is exacerbated by the use of a wide-angle lens. There are several ways to
address this. Consider taking a photograph of a small building that has
just a couple of stories. From a geometry/mathematics point of view, what
constraints would avoid having the building appear to be leaning? The
answer is to keep the camera parallel to the building, in which case all the
verticals remain vertical.
Professor Barsky holds up a model cube made of Tinkertoys and uses the light
from the overhead projector to form a projection of the cube onto the screen.
When you look up at an object, vertical lines appear to converge in perspective.
However, when you arrange to look perpendicular to the lines, then the
vertical lines will remain parallel. Ensure that the plane of the camera
is parallel to the lines. Lines that are parallel to the plane of the camera
will not get distorted; all other lines will. (This is a basic precept.)
Parallel lines appear to converge in perspective because the distance between
them, which is constant in 3D, shortens in depth in perspective projection.
This is just projective geometry!
We seem to be more concerned about such convergence or "leaning"
for vertical lines than for horizontal lines (for example, railroad tracks converging
into the horizon).
In some cases, a "quick fix" for photographs with "leaning"
towers is merely to crop at a rotated angle to "square" the structure.
Overexposure and Light
Photographers refer to an area of a photograph that is too bright (overexposed)
as "hot".
There is a tradeoff between harsh light and soft light. Harsh light can
cause exposure problems due to the disparity between the amount of light in
the brighter and darker areas whereas soft light can cause a photograph to look
rather dull and flat.
Clouds in the sky diffuse the sunlight, making the light in photographs not
as harsh and glaring.
Reciprocity failure — Exposing film for a very long exposure time (e.g.
a second or more), the film's response characteristics change and become less
responsive. Furthermore, each of the primary color layers behaves differently
for long exposures, which can engender color shifts.
Standard film is daylight film, tuned to the color characteristics of daylight.
When photographs are taken indoors, color shifts can occur due to the color
of the artificial lighting. For example, photographs taken under fluorescent
lights tend to appear green. Here is an experiment: take photographs
with different types of light: halogen, fluorescent, incandescent, and
natural sunlight.
When photographing a CRT monitor, having a refresh rate slower than the shutter
speed produces artifacts in the photograph.
The Campanile on a sunny day has very different light values on its adjacent
faces.
Inverse square law: If you take a point light source — imagine I have
a little candle here — the intensity of light further and further away decreases
by the square of the distance. When shining a point light source upon
an object, the light spreads across the object more as the point light
source is further and further away from the object. However, shining an
area light source upon an object provides a constant light intensity, independent
of the distance between the light source and the object. When there are
many point light sources that are close together, then together they can be
approximated as a single area light source.
Miscellany
Consumer-grade digital cameras take lower-quality photographs, compared to
a traditional manual-exposure camera for the same hardware price. (Steven Chan)
The "Foveon company has just announced that they developed the first CCD
that has a higher resolution than a 35mm traditional camera.
Michael's photograph turned out yellowish with his halogen lamp. (Michael Fu)
Zoom: Image looks compressed in depth.
Polarizing filters: They provide for a deeper sky and brighter clouds.
Polarizing filters darken a bright sky to distinguish it from the white clouds.
Take a notepad and write down your settings, such as aperture, shutter speed,
and so forth. Record such settings so you can contemplate them when viewing
your developed photographs.
The sky is not uniformly blue; it tends to have a gradation that is lightest
at the horizon and gets darker the higher one looks.
Return to the Schedule Page
  |
Current webmaster: Steven Chan ().
CS 39J: The Art and Science of Photography is a freshman seminar taught
by .
Site design by .
|