Monday, May 1, 2006

May 2006 HelpLine

ƒ-Stops And Bad Crops

    * Digital Camera ƒ-Stops
    * Straight-Line Troubles
    * Cropping Casualty
    * PPI Canon Vs. Nikon
    * Out-Of-Shape Histogram?
DPMag Published in HelpLine

PPI Canon Vs. Nikon

Q)  I have a Canon EOS 5D and a Nikon D70. Nikon JPEG images load at 300 dpi, while those of the Canon load at 72 ppi. I'm printing the majority of my work on my Epson R800 or through at 300 ppi with the Nikon. Since my 5D is new but the total pixels are more, do I need to change the Canon to 300 ppi in Photoshop CS2?

Jim J.
Via e-mail

A)  Many assume that if the Nikon is outputting a file at 300 ppi and the Canon is outputting at 72 ppi, the Nikon is a higher-resolution camera, but this interprets the information incorrectly. (Also, many people interpret dpi and ppi as the same thing, though technically, ppi, or pixels per inch, is more accurate.)

A digital image file by itself doesn't need a ppi; ppi only comes into play when you're outputting that file to something that has a specific size, such as a print or an image in a magazine. In your case, set the ppi to an appropriate value for your use. But you won't be changing pixels at this point. When you first change the image size, be sure interpolation is off. In Photoshop, for example, make sure the Resample Image check box is unchecked. That way, you'll be changing only the metadata of the file (instructions to the computer on how to read the pixels), not the actual image.

Say you start with a 3072 x 2048 image from a camera. When you open it in an image editor, the image size dialog box might show that the resolution is 72 ppi. This would yield a document (if printed) of about 42x28 inches, but the resolution of the printed image wouldn't be very good. If you were to resize the image just by changing the resolution box from 72 to 200 ppi and make sure that Resample Image is off, the new print would be about 15x10 inches. The picture would be much better because inkjet printers generally do best with a resolution of 200 to 300 ppi.

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