DPI vs. PPI

[rpcrowe]

I realize that DPI is dots per inch which is a printing term, while PPI is pixels per inch which is a digital photo term and they are certainly not interchangeable. I would assume that there are also metric measurements akin to the DPI and PPI.

What I am not sure of is how to interpret a printer/graphic artist's need for a certain DPI (in this case the graphic artist asked for 300 DPI).

My images were 240 PPI. Was I correct in increasing this to 300 PPI in Photoshop as per the request of the graphic artist and does it matter that I did?

[Geoff F]

A correct answer requires to be both very simple and rather complicated. Hopefully someone else will tackle the mathematics and technical details of a full answer.

But basically, yes you are correct.

Dpi and Ppi are frequently, and strictly speaking incorrectly, used to mean the same thing. In this case Ppi was obviously intended.

Some users of digital images require a specific size/density (ppi) for printing, etc, and 300 ppi is often quoted as the ideal size. Although in reality, you may well get an acceptable print as low as 150 ppi; and even lower for a basic print onto soft writing paper, etc.

However, it also depends on the number of available pixels. For example if you have twice the number of required pixels at 150 ppi the image can be resized to 300 ppi with no loss of quality. Provided the end user has suitable software for resizing. But any basic editing software should do this with no problems.

If you had a 'larger' image (more pixels) than was required it doesn't matter if it was at 240 or 300 ppi because it could easily be resized

So, in your case, did the end user also specify dimensions, either in pixels or inches/millimeters? If so it is possible that the photo was required to fit within a predetermined space and it meant less work for them if it arrived correctly sized.

The complicated bit comes when you need to resize an image and have to lose or add pixels. Personally I tend to increase the ppi rather than ditch good pixels although there is little point in going beyond 300 ppi for printing.

For any substantial resizing which involves adding or losing pixels, I always use the best quality option (BiCubic Interpolation - or better if available). I prefer to consider the variation of options with some very basic mental maths to decide how best to resize.

But in reality, I'm not sure that in most cases it actually makes much difference anyway. It's just that I am a bit mean with my pixels and don't like throwing them away!

If you need any more explanation about the full issues regarding resizing, please ask and I'm sure that someone else will explain things better than me.

[Colin Southern]

Hi Richard,

Sorry - I missed your post yesterday somehow.

Just to complicate things even further, in most cases I don't think many printers understand DPI either; many seem to have this perception that if it's "not 300 DPI" then "the world will end", but what also needs to be taken into account is the actual printing device ...

- when it's a "fair dinkum" press then (as I understand it) 300 dpi is indeed the magic number (something to do with 150 dpi 1/2 tones). To be honest, I'm totally ignorant in this area, but there's a good explanation in Real World Color Management.

- when it's an inkjet printer (my area of expertise) then the printer ALWAYS physically prints at a set number of DPI regardless (in the case of my Epson StylusPro 7800 it prints everything (in fine mode) at 1440 dpi. Feed it something that's 3000 ppi, it down-samples it to 1440 dpi; feed it something a 10 ppi, it up-samples it to 1440 dpi. There are great debates over whether it's best to let the printer driver do the up-sampling or just feed it a (massive) file at 1440 dpi in the first place - personally, I normally just chuck a 180 ppi file at the printer and just let the print driver "do it's thing" with it.

So in reality you're faced with 2 possible scenarios ...

1. If you're doing your own inkjet printing or someone else is doing it for you (who understands dpi) then just throw the highest resolution file that you have at them, and 999 times out of 1000 it'll look just fine. If you have a higher resolution version of a file available - and the physical size of the file isn't an issue - then it's silly to throaw away info just for the sake of it.

2. If you're dealing with an offset printer or someone who's "old school" who wants 300 dpi come hell or high water, then just up-sample your file to 300 dpi (saves a lot of time!). The irony is that if a print of set physical dimensions (say 16 x 24") currently works out at 240 dpi - but the printer requires 300 dpi - and you up-sample to 300 dpi, you haven't added any additional information (you can't add detail that wasn't captured), but keeps them happy. We've had shots for calendars submitted at 240 dpi - been rejected - upsampled to 300 dpi in Photoshop - and "that's MUCH better thank you" (doh!). All it means is you end up with a much bigger file on disk.

In terms of actual numbers - in my opinion - people go overboard; way way waaaay overboard. Keep in mind that as the size of the image increases, so does the viewing distance, and of course, at greater viewing distances our eyes can't resolve the same amount of detail. In my canvas prints that I do, 180ppi is "normal" and down to 100 ppi is perfectly acceptable. It's only when it starts betting down to 70 ppi or below that it starts looking pixelated (up close - from a distance even that looks just fine).

Do the maths ... 25mm to an inch (approx) so - if you have 300 ppi then that's 12 pixels per millimeter or 144 pixels per square millimeter. Could your eyes recolve up to 144 tone changes in a single square millimeter (even with reading glasses and a magnifying glass)? Mine sure can't! So it makes my eyes roll when I hear people saying "use at least 360 dpi, but 720 is better". Glossy magazine type prints can benefit from some of this increased resolution, but all the way up to 720? I'm yet to be convinced.

One of the things I keep meaning to do is produce a number of identically sized shots at different resolutions, and see how many people can actually get them in the right order.

Does this help?

[revi]

There's a few additional points (read it somewhere, but can't remember the sources...):

* Inkjet printers need high PPI to get all required half-tones through dithering, so PPI should be (quite a bit) higher than DPI. It's very well possible that the dithering algorithm is optimised for a certain PPI value (say 300 PPI ) so feeding it that PPI should give best results (and don't give Colin's printer a 1440 PPI file, no use...). In Colin's example, he has a printer that does 1440 DPI, and he says that 180 PPI images are perfect. Well, 1440 = 8*180, so that would give the printer an 8x8 matrix to do it's dithering magic, which gives it a nice number of levels/ink.
As for the usefulness of high PPI, I guess that also depends on the support, very smooth surfaces requiring higher PPI values than things like canvas, that have their own structure (though that's getting away from the topic).

* Another possible reason to insist on a given PPI is that you can get moiré effects if you mix grids of different frequencies, (except when the frequency of one grid is an integral multiple of the other). So using 240 PPI instead of 300 might well give some problems, where a 150 PPI could work out better.
And frankly, I'd rather insist on 300 PPI than go explain that either 300 or 150 PPI is acceptable, but other values aren't (most clients would be lost, and do the wrong thing...)

That said, printers tend to be traditionalists, so they might exaggerate the need for high PPI.

Remco

[Geoff F]

My local newspaper once asked me for a photo at 2 column widths wide by 200 ppi. So I had to measure across 2 columns to find the correct size.

I'm sure they only said that to ensure that I bought a copy of the paper!