CS 39J > Schedule & Notes > Session 6 Detailed Notes

# 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.

 Current webmaster: Steven Chan (mr_chan@uclink.berkeley.edu). CS 39J: The Art and Science of Photography is a freshman seminar taught by Professor Brian A. Barsky. Site design by Steven Chan.