Image DPI and Scaling, and Resultant File Sizes

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This article will show the results of some experimentation with modifying the scaling of images, and how this will affect PDF file size. The results are likely to surprise you, since first of all, this is not simply a matter of adjusting the scaling of the image in Properties > Image, since you can also set limits on the DPI and image quality at the moment you export to PDF.

Adjusting the Image File

Here is the simple Scribus document we will work with, a single US letter page with one image frame, and the entire image adjusted to frame size.

Doc with image.png The original unscaled photo was 1.5MB in size, so that the resolution in the image frame was 553.5 DPI. Our first experiment will be to compare PDF file sizes with default settings in the PDF Export dialog, and using Gimp to scale the image. We'll make a PDF with the original image, then make an image copy 50% the size of the original, and another at 25% of the original, each time in Scribus adjusting the image to the frame.
Image Scale DPI in Image tab PDF File Size
100% 553.5 1.6M
50% 276.70 466K
25% 138.4 159K

So we can manage a reduction of PDF size to 1/10 by reducing image scaling to 25% from the original – but a DPI less than 150 generates a warning on export.

What's the Cost?

Now we'll load these 3 PDFs into Adobe Reader, view at 400% magnification, then capture a detail where we can see some sharp contours and big color changes.

553.5 DPI 276.7 DPI 138.4 DPI
Image detail fullsize.png Image detail halfsize.png Image detail quartersize.png

Here we see why you get that warning when image resolution is below 150 DPI, but recall that this is a high magnification in Adobe Reader, so this pixelization could be quite acceptable for a PDF made for the web.