It’s pretty easy. It’s all based on the fact that in almost all standard-def NTSC cameras, the horizontal resolution is 720 pixels across.
For each photo, figure out the smallest 4×3 section you’ll want to see on the TV screen. Then measure it horizontally in inches, divide 720 by that measurement, and you’ll know how many dots per inch you need for your scan of the picture.
Here’s an example: I have an 8×10 picture. I want to zoom into a head & shoulders shot of a guy in the picture. I measure it, and it will be 2 inches across. Now I do the arithmetic: 720/2 = 360. I’m going to need to scan at 360 dpi. Oh, what the heck, I’ll scan at 400 dpi, a little extra resolution can’t hurt.
Here’s another example: I have 3×5 snapshot, and I need to zoom into somebody in the background. I measure it… cripes, 3/4 of an inch! Well, let’s do our division: 720/.75 = 960! Yipes! Well, it looks like a scan at 960-980 dpi.
Here’s a third example: another 8×10 picture. I just need to lose some junk on the edges of the shot, and I’ll be good to go. After measuring, I’m going to need something 7 inches wide. Okay, 720/7 = 102.857142857143… so we’ll call it 120 dpi.
And that’s all there is to it!
Dave LaRonde
Sr. Promotion Producer
KCRG-TV