PPI

Hello, all. Long time, no see.

I am having a debate with somebody about PPI. I maintain it has no relevance to anything, and can be completely ignored except for over-riding it (if necessary) when printing. In other words, you should decide how big the print should be, not the PPI parameter that happens to be attached to the image.

But of course, I might be missing something. Please shed some light on the issue for me.

G

ppi (pixels per inch) is just the definition of how large an image suitable for printing or publication will be. Reduce the ppi and you increase the print dimensions, but then you need to view the print from further away for it to appear sharp.

For normal (up to A3) prints that are generally viewed relatively closely, 300ppi is the norm. The K-5 output actually provides just about an A3 print at 300ppi without doing anything to it. Perfect.

Of course, there can be many reasons why we want to crop the image, in which case we will have to set the ppi and the new image size and let the software interpolate accordingly. All values, ppi, dimensions and number of pixels, are interconnected.

What's the nature of the debate exactly? And does what I have written answer it? With the title by the way, for a horrible moment I thought someone was going to tell me they had several thousand pounds to refund me....but I guess, tongue-in-cheek, you knew that.

Sorry, John, it's not that PPI.

But thank you for confirming my understanding.

Somebody has asked me to produce some work for him (for the web), and amongst other things has specified that each image should be given a PPI of 72. In vain I have tried to tell him that PPI has no effect on screen display, and is merely a way of specifying print size.

He believes that monitors display images at 72ppi, which is nonsense in itself, but he read it on the internet, so knows he is right.

Sadly, I have looked in vain for a simple, straightforward explanation on the internet, and every one I have seen is either wrong, confusing or massively over-complicated.

In a way, that proves that the whole thing can be ignored. When printing, it's just a matter of specifying the output size. There is no need to bring PPI into it.

72ppi was the old Mac screen pitch way, way back which is possibly where he got it from (I believe Windows was always quoted as 96ppi).

Two suggestions. Take the easy way out and change the resolution to 72ppi to keep him happy. Or (try to) prove your point by sending him the same pic with different resolutions so he can see they're the same size on screen.

I suspect that when you and John are referring to printing you mean photo printing. There may be a relevance to ppi when inserting a pic into a document for printing. In the past I found that sizing the pic based on the ppi required to print at the required size made it easier to get the dimensions right in the document with the smallest file size possible.

Thanks, Wasleys. Quite right that he is hung up on early Mac displays.

And I have changed the pics to 72, but I just don't like being told nonsense. So I thought I would double-check.

The point is that you can change the PPI when printing anyway, and indeed have to if you are going to get the size you need.

I often wonder if for the actual best image viewing you should output the jpeg / tiff out from RAW to the PPI of your monitor. i.e my LCD is 24" with a resolution of 1920x1200 which is a ppi of 94.34, or if you're displaying on something like an iPad 4 then with a PPI of 264.

That may be true to an extent robbiwc, but only in terms of our own monitors, and they will all be different. When I make a new wallpaper for my system it looks visibly sharper when I make it exactly the correct pixel dimensions. I'm not sure the ppi matters in this context.

But for the web and for printing it has no relevance at all, as the viwer has their own display or the print has its own requirements.

The 'magic' figure of 72ppi was for the 1984 Mac
It conveniently allowed for a one-to-one match for typography. So, 10pt text was 10 pixels. Microsoft came up with the 96ppi since monitors were viewed about 1/3 further away. So 96ppi 'looked' right at the regular viewing distance, but 72dpi matched size-for-size when holding paper up to the screen. This was only for type

Modern monitors have much higher resolutions anyway! For example, the iPad 3 has a 264ppi and there are higher density displays on the way.

But for web, ppi has ZERO relevance. All browsers work purely on pixels, and all graphic web layout is designed purely pixels.

The only place that I've seen ppi make any difference is when placing an image into Adobe InDesign (desk top publishing). InDesign uses the ppi + pixel dimensions to set the default bounding box for the placed graphic. But even then it's relative - since you can just resize and fit to taste.

Matt

Consider someone selling images or perhaps showing examples of work. As you say, if you create a wallpaper to match your monitor it seems sharper, then if you are presenting a wedding shoot and use something like the aforementioned iPad 4 or perhaps a Nexus 10 (PPI of 300) surely it might make sense in this context to create a set for the device you may be presenting with for that extra pop!

If your wall paper is showing a picture sharper than your image viewer,
it means your image viewer is JUNK!

-

RobbieC,

Quite right. An image that has the same number of pixels as your screen, or fewer, does not need to be scaled in order to fit, and the difference, whilst small, is observable. Which is what you would expect, really.

There was a discussion about this a few years back, I posted some examples: https://www.pentaxuser.com/forum/topic/coastal-sunset-panorama-6484/p-0#comm_54822

Dan