The Question

Date: Thu, 26 Aug 2004 20:45:02 -0700
From: Eleanor Ely <ellieely@earthlink.net>
Subject: [volmonitor] more on DPI, digital photos, etc.

Dear listserv:

I received an e-mail from Ken Cooke (of Kentucky Water Watch) offering a more complete explanation of how to figure out what resolution your digital images are. Basically, you take the image resolution in pixels and divide it by the desired DPI to see what the final size will be at the desired DPI. For example, in the case of the newsletter, the desired DPI is 300 so I would divide by 300. For example, suppose someone sends me an image with dimensions 2260 by 1620 (in pixels). Dividing by 300, I get approximately 7.5 by 5.4 (this represents the dimensions in inches). Since most photos in the newsletter are printed considerably smaller than that, I know that this photo will be fine.

Until I heard from Ken, I didn't know about this formula so I was using my "Windows Fax and Photo Viewer" program to essentially do the math for me. I would click on the "edit" icon, look under "file," click on "properties," and get a dialog box showing DPI, pixels, and image dimensions. When I typed in the desired DPI (300), the image dimensions would automatically change. Now that I know about dividing the number of pixels by 300, I can avoid all these steps.

For those who may be interested in delving further into this issue, here are some excerpts from Ken's e-mail:



"DPI and native resolution are two different settings.

DPI is generally a printer setting only.

You can have a 300 dpi image that's 320 X 240 pixels
You can have a 300 dpi image that's 6000 X 2000 pixels

The first would print about 1 inch x .8 inch. The second would print about 20 inches x 7 inches at 300 DPI.

When going to print, the main calculation you need to do is divide the image resolution in pixels by the print resolution you want in DPI to see if it will be good enough for the size image you want in your publication.

The human eye can't diferentiate much beyond 200-300 dpi. Finer printing resolutions than that help with color definition, but not much more. National Geographic is printed at a whopping 2400 dpi!

One thing about capturing images out of a PDF file you should know:

When you convert a print publication to PDF out of a layout and design software such as Pagemaker, the PDF distiller converts the image to a jpeg at the DPI you set in your pinter preferences based on the size it is in your publication. If you have a 6000 X 4000 pixel image but squeeze it down to a three inch by two inch image in your publication, then convert it to PDF at 300 dpi (the default), the conversion
will downsample the image to 900 X 600 pixels. So when you grab it out of powerpoint, that is the resolution you get."

Hope this is helpful!

Ellie

Eleanor Ely
Editor, The Volunteer Monitor Newsletter
50 Benton Avenue
San Francisco, CA 94112
415-334-2284
ellieely@earthlink.net

================================================================================