Exif history

Honestly, I've been doing this for 10 years and I've never really got my head around ppi/dpi/lpi/spi fully. It was just expected that everything needed to be given to clients at 300dpi and nobody seemed to be able to explain why. I've watched countless tutorials and can see it comes from the older printing press days. I've seen countless people argue for and against the 300dpi myth.

The thing that confused me is when it comes to outputting to, for example, super sizing prints. Let's say you had three files opened. All are 6000x4000px. 24mp images. The first was 72ppi, second 300ppi, third a 3000x2000 72ppi file changed to 300ppi. You need to print at a bus stop 120x80inch print. Enlarging all 3 to print at 120x80 inches.. does that mean the first two images will appear the same in print, and the third suffers in quality as it has been changed from a 3000x2000 to a 6000x4000 image already. Or, will a 120inch... or a 400inch print look the same if you supply the file in 72 or 300 PIXELS per inch. If it's something that can be changed so freely, I'm confused why I'm always asked for files in the magic 300ppi.

Printers have always specified they need the files at 300 or 240 PIXELS per inch. But as I understood it, output resolution should take into consideration desired print size and viewing distance of the print. Billboards can be printed at 15dpi for example.

So my confusion was in having supersized prints - having higher PPI on images would allow the viewing distance to be reduced...

And going back to the original scanning negatives issue. A 3000px scan at 72 or 300 is pretty low res right? But if it was 300 or higher, I could achieve a bigger print size at least?

If this doesn't make sense, don't worry, my brain seems to have always struggled to understand this.. it hurts right now just trying to write all this out and make sense of it!

Thanks for getting back to me. Just to follow up as I might be a little confused here..

So I can see from the data that they used a NORITSU KOKI scanner and then have used Photoshop CS5 on a Windows machine. The current file size I have is 3000x2955px at 300dpi which I can see (at least the dimensions) in the above data. Higher up above that it says :

<tiff:ImageWidth>2079</tiff:ImageWidth>

<tiff:ImageLength>2048</tiff:ImageLength>

Does that not mean it was brought into PS CS5 at 2079x2048 and then enlarged to 3000x2955px?

I only say this because prior to these high res scans, they sent me the low res versions which were at 72dpi and 2079x2048px....

Well actually to be clear and provide a full picture: I was told when processing my 35mm/120mm negs that scanning would be done at 300dpi. In actual fact I received the first scans back at 72dpi but originally at 1039x1024px wide. I'm not too familiar with negative scanners but surely they output jpgs in a more usable way. At the least around 2000px for low res scans..?

I then said I was told in store they'd be at 300dpi but apparently that incurs a further £8 surcharge per roll! So I paid up and went ahead with one roll to check it first as they seemed a bit scatty. 4 days later, I get through the files at 2079x2048px but STILL 72dpi. I told them they're still at 72dpi to which they replied 'Very sorry about that, we will link the correct files to you asap.'

That to me seemed odd because these were seemingly new scans with increased length/width pixel dimensions so they didn't link me to the wrong files. Then a few hours later, new scans come back at 300dpi. That's when I looked at the data and saw the old 72dpi pixel width/length dimensions that now looked to be changed to the larger 3000x2955px. Which to me, looked like they've scanned at 72dpi and resample to 300dpi in photoshop. You can't see at dpi change in the data but it all seems a bit suspect to me...

Does all that add up?