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Improve the quality of printed iPhoto books
Authored by: mflider on Jul 20, '05 10:57:07AM

Actually, if the pictures were smaller, a lower DPI would be more noticeable. Consider that if you use 300 dpi for an image that's 4 inches across, and 150 dpi for an image that's 2 inches across, the smaller image would have 1/4 the area but only 1/16 of the number of pixels!

In short, you'd be looking more closely at a smaller picture (possibly squinting), but could lean back a bit to see the larger picture. So it makes sense that one would want a larger DPI on smaller pictures -- for visual acuity in the book, and bandwidth constaints for Apple. I agree with the hint, though: 300 dpi is the minimum at which anyone would want to print their pictures.



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Improve the quality of printed iPhoto books
Authored by: jen729w on Jul 26, '05 08:43:18AM

The mathematical me won't let that lie... :-)

Think about it. An image 4 inches across (let's call it a 4x4 square) has 4 times the area of an image 2x2. Halving the width doesn't half the area; it's an inverse square relationship.

DPI, of course, stands for Dots Per Inch. Per -inch-. One inch, 300 dots. Two inches, 600 dots. Each inch has its own number of dots. It doesn't matter a damn, therefore, how many inches you have.

The confusion here may arise because a 1024x728 image printed at A4 size will have a certain number of dots per inch, and the same image printed at A5 (that's half A4, Americans) will have double the dots per inch. In this case, however, I'm specifying a fixed image size and a fixed paper size; not a DPI value.

Hope that makes sense. Maths is so beautiful. :-)

j.



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