Friday, December 31, 2004

January/February 2005 HelpLine

Image Organizing

    * Bringing Order To Your Images
    * Red-Eye Myth
    * Choose The Right Disc
    * CF Card Differences
    * Resolving Resolution 

DPMag Published in HelpLine
Resolving Resolution

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?

Jeffrey Lim
Via e-mail

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.

If you have any questions, please send them to HelpLine, PCPhoto Magazine, 12121 Wilshire Blvd., Ste. 1200, Los Angeles, CA 90025 or This email address is being protected from spambots. You need JavaScript enabled to view it..


« Prev 5/5 Next

Login to post comments

Popular How-To

  • Always Be Sharpening

    Always Be Sharpening

    Raw image files are great, but they come out of the camera half-baked. If you’re not sharpening your RAW image files, your images won’t look their best.

  • Adding Copyright Information In Camera

    Adding Copyright Information In Camera

    Your ownership information can be embedded directly in EXIF metadata from the moment of capture. Here’s how.

  • Gear For The Soulful Photographer

    Gear For The Soulful Photographer

    Shooting with equipment that complements your individual aesthetic can lead to images with more depth and meaning

Popular Gear

Subscribe & Save!
International residents, click here.