Digital pics. Pixels and dpi when it comes to printing?

Question

I took an educated guess thinking that one of the standard settings of 640 x 480 on a digital camera translated to a 6 x 4 (4 x 6) in. photo. I found out that isn't true. I measured it out after printing one and it came out to be closer to 6.4 x 4.8 in. which makes sense if 1 pixel = .01 in. So, first, why isn't there a setting for standard photo sizes like instead of 640 x 480 using 600 x 400? Also, do I have to consider the dots per inch (dpi) when finally printing the photo? I know 96 dpi is a setting I see over and over. Does this work into a calculation when resizing a photo to print out to an exact size?

Answer 1

You don't seem to be averse to maths, so I'll knock myself out here. DPI is an indication of print quality. 96 dpi means that you're printing 96 Dots Per Inch horizontally, and also 96 dots per inch vertically, for a total of 96*96=9216 dots per square inch.
Computer monitors display just 72 dpi, but fantastic PRINT quality is 300 dpi. This is about the limit you're eyes can resolve if you press your nose to an image. So 96 dpi for prints is pretty low.
Next bit: every pixel in your digital file corresponds to one dot in your print.
If you print a 640*480 file at 96 dots per inch, you'll end up with:
horizontal size 640/96= 6.7 inches
vetical size 480/96= 5 inches
Print the same file at 300 dpi, and the print will only be 2.1 * 1.6 inch!!
A 10 megapixel camera records pictures at 3872 * 2592 pixels. At 300 dpi, this translates to a print size of
horizontal 3872/300= 12.9 inches
vertical 2592/300= 8.6 inches
As you can see, 640*480 pixels is fine for viewing stuff on your computer monitor - it's also the standard VGA format for videos - but it's not really enough for quality prints.