CS178 Assignment 7 - Night and Color

Yosemite at Night. Photo by Jesse Levinson.


Taking photographs at night presents unique challenges. Exposure times must be very long, so hand-holding the camera is out of the question. However, with a steady place to rest the camera (which need not be a tripod), and a long exposure, the light gathering ability of your camera can greatly exceed the human eye, allowing you to capture a shot of something you could not normally see, such as a forest at night, a faraway galaxy, or even a rainbow cast by moonlight.

Additionally, a comment we heard a lot after the midterm is that it was hard to study for, given the scarcity of practice questions. This week we're giving you some exam-style questions to help you cement your understanding of color.


This week there are three photographic requirements, which are similar to previous weeks, and then some questions on color for you to work through.


  • Requirement 1: An Outdoor Night Scene
    Head outside, find somewhere stable to rest your camera, and photograph a picturesque night scene. We want to see absolutely no motion blur due to camera movement in your shot. You may wish to use the self-timer mode on your camera to avoid the problem of moving the camera when you press the shutter button. As this is a night scene, your exposure time should be at least one or two seconds.

  • Requirement 2: Painting with Light
    Take a long-exposure photo in which you use a light source as if it were a painter's brush. You should move the light around to create the scene you want over the course of the exposure. You can do this by making the light source the subject, and moving it around as in our example, or by pointing the light at the parts of the scene you wish to be illuminated over the course of the exposure. One popular way to do this is to walk around in your scene during the exposure with a hand-held camera flash, firing it in places you would like to be illuminated.

  • Requirement 3: Light and Color
    Find a colored object, and a strongly colored light source (such as an LED), such that the color of the object as photographed by your digital camera looks dramatically different under that light source versus under normal lighting. Submit two photographs demonstrating this effect - one under normal lighting and one under colored lighting. Include in each photograph a white (neutral-colored) object, as a comparison.

    To make the two photographs comparable, turn off auto white balance on your camera. In your comments, describe the object and the colored light. Remember that this effect will depend not only on the apparent colors of the objects and light, but on their underlying spectra. Since you don't know these spectra, you may have to experiment a bit to find a good example. The effect will be strongest when using a light source with a narrow spectrum, such as the sodium lamps you find in parking lots or the LEDs on your electronics.

  • Requirement 4: Color Questions
    Head to this page and answer some questions on color. These questions are similar to those that will be in the final exam (except on the final exam we probably won't ask for justifications on true/false questions). You're welcome to discuss the questions in groups of up to three, but everyone needs to submit their own answers in their own words. Your answers to these questions are worth half of the total grade on this assignment.

Upload your photos and add comments.

Upload your photos using the Picasa account you created in the first week to a public Picasa album titled "CS178 Assignment 7 - Night and Color". Add comments below each photo explaining what relevant camera settings you used and why, any image processing done to the photograph afterwards, and most importantly, which requirement the photo covers.

Example Solution

We've put up an example solution to the photographic requirements in this assignment to both let you know what we expect, and hopefully also inspire you to take better photos than the TAs can. You don't have to meet the assignment requirements in the same way as we did - surprise us!

One caveat: In our example solutions we may reuse photos we already had lying around, or borrow photos from other people to illustrate a point. Don't do this - your work for each assignment should be done by you specifically for the course.

If you have questions or comments about the photos we used in the example solution, feel free to add them as comments below the corresponding photograph in Picasa Web, or to email us.


As usual, we will assign grades based on whether you met the requirements of the assignment. For each requirement, you'll be given a check for meeting the requirement, check-minus for not quite meeting the requirement, or check-plus if you do something spectacular that makes your grader's jaw drop.

The questions for requirement 4 will be graded according to their own scheme, with two points available per question.

Due Date

11:59PM, Sunday, May 24, 2009

Assignments are generally due on Sunday at midnight at the end of the week in which they're assigned. This leaves you free to start thinking about the next assignment during your next section. Sometimes lateness is unavoidable. Here's the lateness policy from the course outline:

Since the assignments come in rapid succession, it is important that each be completed on time. Replacing photos after the deadline is not allowed, and will be treated as a violation of the honor code. To allow for unforeseeable circumstances, you will be allowed up to two weekdays of grace for up to two of your assignments. Beyond this, late assignments will be penalized by 10% of the grade for that assignment per weekday that they are late. Exceptions to this late policy will be made only in the case of a necessary (non-pleasure) trip approved in advance by the TA for your section, or severe illness. If you do not submit one of the assignments at all, you will fail the course, even if you are taking it pass/fail. In this case, or if you believe you are in danger of failing, it is your responsibility to come talk to us before the end of the course. Incompletes are given only in exceptional circumstances.


© 2009 Marc Levoy and Andrew Adams