For printed pictures on paper what is the industry standard ‘rule of thumb’ for image resolution?
For raster images, 300ppi is the standard for commercial offset printing.
You can get away with a bit less if you’re just printing to a color printer for comping purposes. I’ve done as low as 150ppi.
I seem to recall seeing calculations that proved that 300 dpi was a bit of overkill, but hard drives are pretty cheap these days and there’s more screenless printing now than in the old days.
Black and white line art is usually done at a higher resolution of 600 or even 1200. Thats because there are no intermediate colors to smooth out jagged edges, and those graphics file are still pretty small even at that level of resolution.
The resolutions are slightly overkill, but that’s partly because printers would rather have a little wiggle room, especially when they may need to do some adjusting on the files. I used to know the details about half-toning and conversion to lpi, but it’s been a few years.
Yes, I think you need to distinguish between resolution - the detail in the photogrpah - and print resolution - the detail of the print process.
Old news photos used to be half-toned to dots at incredibly low resolution - from memory, about 100dpi screen, so a dot every hundredth of an inch or even worse. Very large half-toning has become a “artistic” thing nowadays, or even half-toning with lines instead of dots.
Sharpness, on the other hand - the resolution needed so the print process is not visible to the eye - is usually about 600 to 1200 DPI. Part of this depends on the print process and the quality of paper. Too absorbent, then the ink may bleed; then your picture will not be sharp and will not have the right shades of grey as the dots bleed too much. (which is why old newspaper phots used big dots far apart.) Quality photo magazines are on glossy paper which is not as absorbent. Inkjet printers also want you to use the fancier photo paper for hiher quality output.
Photo output is more actually like smooth colour tones that print, which is solid colour dots trying to trick your eye into being a smooth tone.
The photos themselves - really, 100 pixels per inch would probably be acceptable; what matters is detail. We tolerate a lot less detail in a less complex picture; and 8x10 face portrait does not need as much detail as a complex landscape, for example. I have blown up a sharp 2-megapixel picture to 11x14, which is less about 100 pixels-per-inch and it still looks great.