Tech Directions- October 2008

To complete the activity, you will need: • A camera (ﬁlm or digital), • A length of black tape about 1'–2' in length, • A tripod that will hold the camera steady, • A level, • A ruler, and • A tape measure. A calculator could come in handy, but the mathematical functions do not involve anything more complex than multiplication and division. Place mark on wall in center of camera’s field of view. Attach length of tape parallel to floor. Camera axis perpendicular to wall Measure this distance so that you can check in-camera measurement (make about 1' to 4'). Procedure You will need to identify a location where there is an “inside” corner on a building or room. Figure 1 should give you a visual image of what the location might look like. To set up for the activity: 1. Attach a piece of 1'- to 2'-long tape to the Measure distance from camera lens to mark Camera angle of view to corner of a room at a height of about 4' to 5' off on wall (make about 20'). encompass mark on wall the ﬂoor. Use a level to make sure the tape is level and parallel to the ﬂoor. Locate one end of the Camera on tripod with back tape exactly in the corner. parallel to wall in front 2. Place the camera on the tripod, adjusting its position so that it is at a distance such that, with the back of the camera parallel to the wall Fig. 1—Layout for testing depth by radial displacement in front of it and perpendicular to the one to your the lens will be at right angles to the wall in front, and the side, the camera’s ﬁeld of view includes the full black tape will appear somewhere to the right of the viewlength of the tape attached to the side wall. Something ﬁnder. You should be able to see the corner of the room like 10' to 20' should work. Again using the level, make as well as the part of the black tape nearest the camera. sure the camera is not tilted up or down. 6. Measure the distance between the camera’s lens and 3. Align the camera so that it aims in a direction paralthe wall in front of the camera in the units you want to lel to the wall located to the side of the camera. The view use in determining the length of the tape attached to the in the camera’s viewﬁnder should be approximately like side wall: feet, inches, or meters. Record this as “distance that illustrated in Fig. 2. A.” In this simulation, it represents the altitude of the 4. Measure the distance from the ﬂoor to the camera’s camera above level ground. lens and from the wall to the side of the camera’s lens. 7. Also measure the tape you attached to the side wall. Now, measure the same distance away from the corner Keep a record of this, as doing so will allow you to see and at the same height that the camera’s lens is from the how close you came to determining the length of the tape ﬂoor. Place a small mark or cross at that point on the wall. “photogrammetrically.” Going back to the camera, aim it so that the mark you just 8. Take a photograph of the scene. If the tape on the placed on the wall is centered in the viewﬁnder. wall was somewhat higher than your camera, the view 5. If the camera is focusable, focus it on the wall directin the viewﬁnder and on a print made from the negative ly in front of it. At this point, notice that the optical axis of or a digital ﬁle would look somewhat like the illustration in Fig. 2 (without the diagonal lines). You can have a print made from a negative or you can download the digital ﬁle from the camera and look at the image on the computer screen. Make R sure the print includes the whole image as photoDiagonals should intersect at center of graphed by the camera, not a cropped version of it. photo where tape was D 9. You can make the following measurements placed on wall. either on a print made from a negative or from a Length of tape = D × A / R print made from the digital ﬁle, or, if you are adept enough, use a computer program that has the where D = length of tape (mm) R = distance from center of photo to end of tape (mm) ability to make measurements of image features A = distance from mark on wall to lens (feet or meters) right off the screen. Photoshop, for example, has a “measure” tool and that works quite well for this purpose. We will assume here that you will measure off a print. Fig. 2—Layout in print made from scene 20 techdirections ◆ OCTOBER 2008

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