Can you increase DPI of an image

[charlesd96530515]

We have a catalogue which has around 3,000 product images - Images vary from 72 PPI - 300 PPI and the images look great for our internal catalogue.

Now i know people get confused about PPI / DPI being the same thing when in fact they're two different things all together and i know PPI doesn't matter that much on an image being used on the web, as typically your screen can only output 72 PPI anyway.

And i know DPI is inportant for printing as the amount of visable pixels, my example is always; billboards use a low DPI (10) because you're not supposed to stand right next to those images, you're suppose to view from 50 yards away or whatever. 

But a catalogue is supposed to be viewed closer, so a higher DPI is fine.

ANYWAY, my question is since our catalogue has 72-300 PPI images and we have a customer asking for these images all as 300 DPI, can i increase the DPI? I was under the impression, though technically i could do that it wouldn't actually be true 300 DPI... you can't 'add' what been taken away so to speak. Is that correct?

And if you can increase the DPI, how would you do this?

[charlesd96530515]

Sorry and would i adjust the DPI in the same way i would adjust an images PPI? i think this is why people get confused between the two because they're sey up the same way  - right?

[John Mensinger · Accepted solution]

"you can't 'add' what been taken away so to speak. Is that correct?"

Your instinct there is essentially correct, with repsect to pixels per inch (ppi). Dots per inch (DPI) doesn't apply to images; it's a measure of printer output; applicable to the hardware. Your images are comprised of pixels, not ink dots.

You can indeed increase an image's resolution from 72 ppi to 300 ppi, but it will only make the print size of the image smaller; it won't actually add pixels unless you resample to the higher density. In neither case will the image quality increase. In fact resampling to a higher density requires that the software used interpolates new pixels to add, invariably degrading the image quality.

[charlesd96530515]

Thanks - I suspected as much. 

I think they're using a program to scan through the images and flag any that are potentially 'low-res' but from a human eye P.O.V it's fine as i've never had a complaint againts my images in our catalogue before.

So, what i'll do is run a script on PS and adjust these so they say '300' just to keep him happy.

Many thanks for clearing that up.