Tech Directions- October 2008

small the print is. It is also independent of lens focal length. All that matters is the radial displacement! For example, assume the camera was 20' from the wall in front or 20' from the base of the length of Photo of the layout shown in Fig. 2 tape attached to the side wall. Having printed your in tape length would be introduced photograph, you measure the disby a 1 mm error in measurement? tance from the center of the photo 2. Why is this method indepento the farthest point of the tape to dent of lens focal length? be 43 mm. The length of the image 3. Why is this method indepenof the tape measures about 13 mm. dent of print magniﬁcation? Therefore, the actual length of tape 4. This method is based on cor(as measured from this image) is 13 responding parts within similar trimm × 20' / 43 mm, or angles. Where are these triangles? 0.302 × 20' or 6.05', 5. Can the subject be tilted and which converts to have you still end up with a good about 72.6". measurement? Why or why not? How long was the piece of tape on the Answers wall? Let’s say 5' 8", 1. It depends on your set-up. or 68". The discrep2. The technique depends on ancy therefore is 72.6 ratios. – 68 or 4.6" out of 3. The technique depends on 68", or about 6.7%. ratios. If you asked class4. One triangle is in front of the mates to estimate camera and one is within the camera. the length of the tape 5. No. The tape must be level. If just by looking at the it’s not level, the lengths of the obscene, how close can jects will not bear the proper ratio to they do this on the their corresponding parts. Making sure the tape is level average? Make this comparison a part cord this distance and call it R. of the project! 12. Using the same units as I would be happy to anabove, measure the length of the swer questions you might image of the black tape on the print. have about this project. Make a record of it and call it D. Contact me by e-mail at 13. To determine the length of the andpph@rit.edu or write to tape from the measurements you Andrew Davidhazy, Imaging have just made you simply multiply and Photographic TechnolD times A and divide by R. ogy, School of Photo Arts The real length of the tape will be and Sciences, 70 Lomb Mein the units you used to measure the morial Dr., RIT, Rochester, distance from the camera to the wall NY 14623. in front of it. D and R must be in the same units but A can be in different units. Note that this process is comQuestions pletely independent of how large or 1. How much of an error Snapping the shot 10. Draw the diagonal lines extending from one corner to the other on the print. The print will start to look like the example in Fig. 2. If you had everything aligned properly before you made the photograph, you will notice that the diagonal lines will cross over or come very close to where the mark was placed on the wall. In vertical aerial photography terms, the nadir, or point on the ground below the airplane and leading to the center of the earth, is located where the optical center of the photograph is determined to be by the crossing diagonal lines. 11. On the print, using any convenient measurement tool, measure the distance in mm or inches (mm are probably more convenient) from the black mark on the wall or the intersection of the diagonal lines (they should be close to the same location), to the farthest point of the black tape attached to the wall. Re- www.techdirections.com TECHNOLOGY EDUCATION 21 http://www.techdirections.com

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