Question 33.11: Blown-up photograph. An enlarged photograph looks sharp at n......
Blown-up photograph. An enlarged photograph looks sharp at normal viewing distances if the dots or lines are resolved to about 10 \mathrm{dots} / \mathrm{mm}. Would an 8 \times 10-inch enlargement of a photo taken by the camera in Example 33-10 seem sharp? To what maximum size could you enlarge this 2000 \times 3000-pixel image?
APPROACH We assume the image is 2000 \times 3000 pixels on a 16 \times 24-mm CCD as in Example 33-10, or 125 pixels / \mathrm{mm}. We make an enlarged photo 8 \times 10 in. =20 \mathrm{~cm} \times 25 \mathrm{~cm}.
The short side of the CCD is 16 \mathrm{~mm}=1.6 \mathrm{~cm} long, and that side of the photograph is 8 inches or 20 \mathrm{~cm}. Thus the size is increased by a factor of 20 \mathrm{~cm} / 1.6 \mathrm{~cm}=12.5 \times( or 25 \mathrm{~cm} / 2.4 \mathrm{~cm} \approx 10 \times). To fill the 8 \times 10-in. paper, we assume the enlargement is 12.5 \times. The pixels are thus enlarged 12.5 \times; so the pixel count of 125 / \mathrm{mm} on the CCD becomes 10 per mm on the print. Hence an 8 \times 10-inch print is just about the maximum possible for a sharp photograph with 6 megapixels. If you feel 7 dots per mm is good enough, you can enlarge to maybe 11 \times 14 inches.