Rob & Heelgrasper,
Thanks for your help and advice. I think the DPI values came from the information screen in Irfanview, but clearly their wrong! It shows 180 x 180 and I had to tweak the second up to 204 before ST would enable the OK button. Last night I played about a bit and with some better values got a much better PDF that seems to behave in the Android tablet and is quite quick and responsive loading and navigating. But I'd like to know what the real DPI is...
So how to Calculate the DPI more effectively..... I thought about it and then knocked together a "DPI Calculator", I guess there must be some about? Here's my effort:
You load a typical image. In this example it's just a blank page from a paperback (autofocus is on so obviously it's out of focus). Also, I'm using the images after they've been rotated by the Homer bash script (in the \renamed folder).
You measure the books length with a ruler (it must be a plastic one).
You check the pixels for the length of the image.
Enter the values in the DPICalculator.
Adjust the spin boxes so that the brown bar (which represents the book) is a good match.
Click "OK" and voila!
In this example it turns out to be 400.
Some inner workings...
It just uses the proportion of the 'brown book bar' and the grey areas above and below to the left of the image to work out the pixel length of the book, then divide by it's measured length...
nMeasured = 8.45 // you must use a plastic ruler
nImageHeight = 4000 // from file explorer details view
nTotal = 407.00 // total height in screen pixels of the 'bars' to the left of the image
nProportion = 0.85
Book Pixels = 3380.84
DPI = 400.10
Not sure if it's right though?