John Norman McDonald wrote:
So how do I judge a screen ppi to get good print dpi? At some point I will have to know what ppi will print well in dpi.
If I enlarge a saved 8x10 300ppi tiff to a 16x20 tiff will I lose ppi or res? Should I save at 40x60? Is the same true in raw?
It all explains itself very nicely if you just stop for a second and consider the term ppi: pixels per inch. Think about what that means. It's a unit for pixel density on paper.
It's a simple equation: resolution = number of pixels / physical print size. If you have two of these, any two, you can calculate the third. Just do the math.
Often a minimum required resolution is specified for print. For instance, standard book and magazine print usually requires 300 ppi. Very high quality print may require up to 400 ppi, while newsprint has lower requirements, like 150 ppi. This puts an upper limit on the physical print size, for a file of given pixel dimensions.
As you can see, ppi is not a property of the file - it can be arbitrarily assigned, but so doing determines the maximum print size accordingly. Change ppi and you change the size - but the pixels in your file are unchanged. The file itself is just pixels, it has no size until you assign one by setting a ppi value.
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On screen this is all moot. This is not because it "doesn't matter" - it does; it's just that on screen the pixel grid is fixed. The screen has its own fixed resolution and you can't change that, so a given pixel size will always display at a certain size. This is why retina/high density screens display images considerably smaller than people are used to. The pixels are smaller; the resolution is higher.
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So to answer the question in your last post: A file at 8 x 10, 300ppi, equals 2400 x 3000 pixels. You can print that file at 16 x 20 if you like, but that means the resolution drops to 150 ppi. Again, just do the math. 150 ppi may work for newsprint or draft quality, but probably not for high quality print.