Image Resolution and PPI

It’s annoying when the pros get it wrong.

Scott Bourne tweeted something that included a desire for 640px images at 72 PPI. Apparently he got some flack about the comment and responded with this photofocus blog entry titled Dispelling Common Photo Myths – PPI DOES Matter.

Now Scott has probably forgotten more about photography than I’ve managed to learn – I’m barely out of the taking snapshots stage – but I think he’s missing something here. You can specify image requirements in several ways. You can say “I want an image 8 inches wide at 72 PPI” or you can say “I want an image 576 pixels wide”. It’s the same thing. Saying you want 576 pixels at 72 DPI is meaningless and tends to confuse people.

PPI is a function of the output device. My 24” iMac has a PPI of about 94.5; 1920 pixels / 20 5/16 horizontal inches. If I look at an image I’m going to see it at 94.5 PPI regardless of any PPI/DPI setting in the image metadata. That 576 pixel image is going to be about 6 inches wide on the iMac. My laptop displays 1440 pixels in 11 1/4 horizontal inches for a PPI of 128. That same 576 pixel image is going to be 4.5 inches wide on my laptop, again regardless of any settings in the image metadata.

576 pixels is 576 pixels. Always. [OK… I’m ignoring HiDPI and Retina displays for this discussion.] The size of the displayed image will depend upon the PPI of the display.

In his blog entry he shows file sizes of two images. What he doesn’t show is the X by Y resolution of the images. That’s too bad. I think he’ll find that one does not meet his 640 pixel size requirement. I use Aperture which lets me set the DPI when I export an image.

Aperture Settings

Aperture Settings

I exported the same image twice, once at “72” and a second time at “10,000”. Both times I selected the “Fit Within (Pixels)” and specified a box no wider than 1024 pixels and no higher than 768 pixels. Selecting that option tells Aperture to mostly ignore the DPI setting. The exported images are both exactly 279,645 bytes long.

279645 Mar  9 12:17 p-20110220-0656-2799-10000.jpg
279645 Mar  9 12:17 p-20110220-0656-2799-72.jpg

The difference? In the metadata of the images one contains this:

X Resolution                    : 72
Y Resolution                    : 72

and the other contains this:

X Resolution                    : 10000
Y Resolution                    : 10000

The bits that make up the displayed image are identical.

Image with 72 in the metadata

Image with 72 in the metadata

Image with 72 in the metadata
Image with 10000 in the metadata

Image with 10000 in the metadata

Image with 10000 in the metadata
Click on the thumbs to see the images constrained to fit in the 1024x768 pixel box.