The Links panel is a very useful panel indeed, not only does it allow you to manage your links but provides you with lots of information about placed images such as their resolution. A simple way to think of resolution is image quality. The higher the resolution, the more detail there is in the image so the higher the quality of the image. The lower the resolution, the lower the image quality. Detail is capture using pixels, small coloured squares that make up the image. Resolution is measured in Pixels Per Inch, or PPI for short. So the higher the PPI, the better the image quality as there will be more detail.
The typical PPI for print usually varies from 240 to 300ppi. For arguments sake, lets say we want to achieve 300ppi – there are many, many blogs where this is discussed in-detail and passionately, but lets not go there!
Actual and Effective PPI
After placing an image into an InDesign document and looking in the Links panel you can see there is an Actual PPI and Effective PPI and both saying 180, why?
Actual PPI is the resolution of the image at 100% size.
Effective PPI is the resolution of the image when scaled in InDesign.
Because the image was placed into InDesign at 100% and not scaled, the resolution for both is 180 PPI. OK, lets see what happens when the image is scaled 50% smaller.
As you can see above, only the value of the Effective PPI changes. The image size has been reduced by 50%, so the same amount of pixels has to fit into a smaller space, so doubling the Effective PPI. The Actual PPI stays the same as this simply states what the resolution of the image would be if it wasn’t scaled, i.e. at 100%.
OK, one more time, lets scale the image twice its original size – 200%
The image size has been increased to 200% (twice the original size), so the same amount of pixels is now spread across a greater distance, so reducing the Effective PPI to 90 PPI, Again the Actual PPI stays the same as this simply states what the resolution of the image would be if it wasn’t scaled, i.e. at 100%.
So in short, the amount of pixels in a placed image does not change. Scaling the image size affects the Effective PPI. Scaling the image smaller increases the Effective PPI, while scaling the image bigger reduces the Effective PPI.
Effective PPI is king. Always check the resolution of images using the Effective PPI to ensure that it meets your target resolution, as states above I chose 300 PPI. Clicking on the links listed in the Links panel is a great way of checking the Effective PPI of images.
If you are wondering what Effective PPI I aim for when creating documents for on screen viewing, well it’s 150 PPI. Thanks for thinking about that!
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