Originally Posted by reamo
I mean when the images in an original book is converted.. what is the scan dpi used?
It's up to the creator of the ebook, but ideally images are optimized for a particular screen size, as otherwise fine detail is lost when the image is scaled up or down or zoomed to 'full screen'. Unfortunately what works best for DX is not optimal for K2 and vice versa, and mobi format does not allow for inclusion of several sizes for the reader in question to choose from (I think ePub theoretically does allow for this).
A good discussion can be found here:
With the 2.5 update, pan/zoom functionality has been added to viewing PDF pages. I wish they'd add this to image viewing in ebooks as well, then authors could make the images as large as they need to be for readers to view the fine details.