Q) I'm working to better understand how megapixels and DPI relate. After looking at the numbers (dimensions and dpi), they don't seem to add up. In the July/August 2004 issue of PCPhoto, you mention the example of a 2500x2100-pixel file (5,250,000 pixels), which becomes a 34x29-inch image at 72 dpi (34 x 29 x 72 = 70,992 pixels), an 8x7-inch image at 300 dpi (8 x 7 x 300 = 16,800 pixels) and a 14x12-inch image at 180 dpi (14 x 12 x 180 = 30,240 pixels). Am I missing something?
A) The concept can be confusing, and I appreciate the effort to understand where the numbers come from. The 72 dpi isn't square dpi; it's dpi in one dimension. You have to multiply each dimension by the dpi (in this case, dpi is also the same as ppi). To make your math work, you need to add a second dpi factor: (34 x 72) x (29 x 72) = 5,111,424. If you allow for some rounding of numbers, you'll find all the sizes add up correctly.