"alex" wrote in news:w_WdnVdMEdy8DbiiXTWc-
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are the terms pixels per inch and dots per inch interchangeable?
While they are often used interchangeably, they mean different things.
PPI is only a *sampling* rate for digital images, back when all digital images were created by scanning either a print or a bit of film. Once the file is generated, the only meaningful measurement is strictly pixel dimensions, as in 800×600. Inches have no meaning in digital files, and even when you specify a PPI value in Photoshop, it is simply an index line. You can change it to whatever you want, it will have no effect on the image if the pixel dimensions are unchanged.
DPI, as others have said, refers to print resolution. It does not have to correspond to PPI in any way, either from the original scan source or from what value someone assigned in Photoshop.
Now comes the confusing part. PPI and DPI are often used interchangeably for two reasons. The first is, people don’t know any better. The second is, having a particular aim for detail in the final printout.
If you scan an 8×12 at 300 PPI, for instance, then print it out as an 8×12 at 300 DPI (or higher), you should get something that looks much like the original. 300 DPI output is considered a good guideline for ‘photo quality’, though I’ve seen a lot of differing opinions on this.
You can print out at a much higher DPI than the pixels account for, and the printer or image software will usually ‘extrapolate’ or ‘upsample’ dots to fill in the gaps not defined specifically by pixels from the digital file. This does not add any useful detail, however. Also, printers that have a high DPI capability (like 1440 and up) may be layering in different color dots slightly overlapped to give much better color control, and it does not mean you should have pixels that match the DPI capabilities of the printer. A printer has three to seven different color inks, while a 24-bit image file has millions of colors, so this has to be faked somehow…
Also, it’s usually bad to print at a lower level that the pixel count, since this dumps detail from the original.
In the end, it’s better to try and have pixel dimensions to match your final output. Want an 8×12 image at 300 DPI for ‘photo qulaity’? Then you make sure your digital image measures at least 2400×3600 pixels. This can mean scanning an 8×12 print at 300 PPI, or a 35mm film frame (about 1×1.5 inches) at 2400 PPI.
– Al.
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