My colleague recently alerted me to a strange behavior in InDesign when creating an advertisement for a website.
The client had supplied a brief to create an advertisement with specific measurements: 728×90 px at 96ppi.
So the advertisement was created in an InDesign file with these dimensions and exported to JPG at 96ppi.
But when the file was opened Photoshop to confirm the image size, something was quite odd. Rather than being 728×90 px at 96ppi, it was 971×120 at 96ppi.
The fix for this problem was to go back to InDesign and export again to JPG, but this time at 72ppi.
In Photoshop, the size of the advertisement was now correct, but the ppi was incorrect…
Fortunately, you can change the ppi from 72 to 96 easily in Photoshop in the Image Size panel. Uncheck the Resample checkbox and change the resolution from 72ppi to 96ppi.
How InDesign Handles Resolution and Image Size for Export
So what is going on here? Well, it isn’t a bug, it is a behaviour of InDesign. When using pixels as a measurement, InDesign assumes that the ppi of these documents will always be 72ppi. The easiest way to demonstrate this is to make a new Web or Digital Publishing document in InDesign. Notice there is no option to choose a resolution in the New Document dialog box. In the new document, drag out a guide to 72px, and then change the measurements used in InDesign to inches, and note the measurement of the guideline – 1 inch.
This behaviour extends to the JPG export. Rather than understanding that the image is already at the correct size in pixels, InDesign thinks the image is just over 10 inches wide by 1.25 inches high as a print document. So when asked to export a JPG at 96ppi, InDesign is thinking it is a print document 10 inches wide by 1.25 inches high at 96ppi, and this – when measured back into pixels in Photoshop is 971x120ppi.
Despite all of this, what really matters for on-screen publishing is the finished size of the artwork in pixels, not the pixel density in ppi. Because of the way InDesign behaves, it is important to make sure the artwork is exported at the correct pixel width and height by setting the resolution in the JPG export to 72ppi, and leaving the checkbox Use Document Bleed Settings unchecked.
This article was last modified on July 25, 2019
This article was first published on August 3, 2015
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