Effective resolution is the actual resolution
of an image after scaling.
Example Scenario:
A 300 ppi (pixels per inch) image has a size of 1″ x 1″. That image is placed into InDesign and scaled to a 2″ x 2″ image. This creates an effective resolution of 150 ppi for that image and causes pixelation due to the lack of information (pixels) in the image file.
Example Images:
300ppi Effective Resolution.
150ppi Effective Resolution.
72ppi Effective Resolution.