CS178 Assignment 2 - Sports and Action

Tony the labrador retriever thought the kite needed retrieving. Photo by Natasha Gelfand.

Motivation

One of the first problems photographers hit is that the world doesn't stay still and wait for you to take your photo. Many interesting compositions are fleeting, and the photographer has to be somewhat opportunistic. Sometimes you have to expect to take a hundred photos and only get a handful of good ones. This week we'll be capturing moving things. There are tons of interesting moving things around campus: cyclists, people playing ultimate frisbee, cars driving past, squirrels dashing from tree to tree. Animals in general are delightfully uncooperative, so if you have a dog or cat, this might be a good week to take a photo of them running or jumping or chasing a toy.

The key difficulty in capturing moving objects is getting enough light. The shutter speed needs to be short to prevent motion blur. You usually compensate for this by using a large aperture and increasing ISO. However, this shortens the depth of field and makes precise focus more important. Autofocus takes time, and a slow autofocus system can result in a missed shot. If you're waiting for something to happen so you can take a picture of it, one good trick is to prefocus at the depth you're expecting, so that the photograph can be captured at the instant the interesting event occurs.

Instructions

This assignment is similar in format to the first one, except that there are only four requirements this week. There are a small number of requirements for you to meet by taking photographs, uploading them as a new album to the Picasa account you made last week, and adding comments below them. You must hit each requirement in at least one photo, and every photo should hit at least one requirement. Upload from 5 to 10 photographs. See the example solution below for guidance.

1. Requirement 1: Freeze the action. Take at least one photo with a very short shutter speed to capture something interesting happening. You'll need lots of light for this, so try something outside in the middle of the day. If you happen to have a very strong flash, you could also use that to freeze the action.
2. Requirement 2: Show the action by blurring the foreground. Take at least one photo that gives a sense of how something is moving by the way it is blurred. The main subject of the photograph should be motion blurred, but the background should be sharp.
3. Requirement 3: Show the action by blurring the background. Take at least one photo with a sharp subject and a motion blurred background, in order to demonstrate the motion of the subject. You can do this using a shutter speed of around 1/10s, and trying to keep a moving object centered in your viewfinder as it passes you, by panning the camera in a way that matches its speed. This will take a lot of attempts to get right. You might like to try capturing one of the many cyclists around campus, or a passing car.
4. Requirement 4: Show the action with a burst. Take a burst of photos (at least 3) one after another quickly, so that it shows something happening and tells a small story. Many cameras have a burst mode to make this easier. Once you've taken your action sequence, you should combine the burst of photographs into a single image using Photoshop or similar. Your combination could be as simple as placing all of the photos side-by-side into one larger photo, or it could be a blend of the photos, or it could be something fancier. The choice is yours!
5. Upload your photos and add comments.. Upload your photos using the Picasa account you created last week to a public Picasa album titled "CS178 Assignment 2 - Sports and Action". Add comments below each photo explaining how to you took the photo, what relevant camera settings you used and why, any image processing done to the photograph afterwards, and most importantly, which requirement each photo covers. There's no need to send an email like last week; your TA will look in your Picasa account for the album.

Example Solution

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.

Grading

It's hard to judge artistic quality, so rather than grading the individual photos, the TAs 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.

Practice technical problems

Starting this week, we'll be including some technical questions in each assignment to help you make sure you're keeping up with the material. You don't have to turn in your answers; they're just for your educational benefit. After the assignment's due we'll post the answers here, and we'll also discuss some of them in Section, time permitting. In this case, the first two should be pretty easy and the last two are more in line with the sort of questions we might ask on the midterm.

Note: answers have now been marked in bold.

1. Problem 1. Holding everything else constant, widening the aperture of a camera lens means (check all the apply):
   • a) less depth of field
   • b) more depth of field
   • c) less light hitting the sensor
   • d) more light hitting the sensor
   • e) a wider field of view
   • f) a narrower field of view

2. Problem 2. You have a 50mm f/2 lens. Its maximum aperture diameter is:
   • a) 12.5mm
   • b) 25mm
   • c) 100mm
   • d) 200mm
   • e) Depends on the sensor size

3. Problem 3. In order to compute the field of view of a lens/camera combination, you need to know the lens's focal length and the camera's sensor size. Mary has an old film camera that uses standard 36x24mm film. Joe has a new Nikon D3000 digital SLR with a sensor measuring 24 x 16mm. Mary puts a 60mm lens on her camera and Joe wants to get the same field of view with his camera. What focal length should Joe use?
   • a) 40mm
   • b) 60mm
   • c) 90mm
   • d) Not enough info

4. Problem 4. Joe finds a lens with the correct focal length (from Problem 3) to match Mary's field of view, so now Joe and Mary's cameras have identical fields of view. If both of their lenses are f/2.8, and both point their cameras at the exact same scene, what can we say about the amount of light hitting Mary's film compared to the amount of light hitting Joe's sensor?
   • a) Mary's getting 2.25x as much light as Joe
   • b) Mary's getting 1.5x as much light as Joe
   • c) They're identical
   • d) Joe's getting 1.5x as much light as Mary
   • e) Joe's getting 2.25x as much light as Mary

Due Date

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.

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