There are a lot of confusing elements when it comes to photography, particularly when you're looking at printing. Perhaps the most misunderstood is PPI — pixels per inch — and when it's useful, because it is often not useful at all.
My first foray into printing was a confusing one. One of my first paying clients as a professional photographer wanted me to capture some elements of his business and what they do, but then get them printed rather large as wall art. Taking the photographs was easy enough, I knew exactly what I wanted to do, but I made a slight error in judgment with the printing side: I presumed it would just be straightforward, despite never having really looked at creating high-end prints.
Fortunately for me, I used the print company's preview service where you pay to have a small version of your print created to check the colors and quality, so I was able to narrow down what I needed from trial and error. However, a rabbit hole it sent me down was what PPI means and how, outside of referencing both digital and physical dimensions at the same time, it is almost useless. DPI (dots per inch) is a printing metric that is easy to understand, but PPI felt more elusive to me and all these years later, I still haven't found a situation within my work where it matters.
Has PPI ever mattered to your photography? If so, how?
A good subject and one that often gets really contentious, really fast. But to me it's simple. DPI and PPI are obsolete concepts that you should just flush out of your mind, today. I wrote a blog post about this which some might find interesting:
https://jimhphoto.com/index.php/2020/12/21/dpi-a-concept-you-should-forget/
I'm a pre-press graphic designer. I care very much about DPI. It's not obsolete at all.
Yes, the main point is that DPI and PPI are not the same thing and are only (very) relevant in terms of physical output i.e. printing. For screen-only or online viewing, PPI is irrelevant.
That is all true, but I was talking about display, not editing - web, iPads etc. - in comparison to ppi for physical prints.
And yes, on a normal res screen like a 120ppi EIZO then viewing at 50% does give you an approximation of print sharpening (it's a cheat but it works), but on a high res screen (264-300ppi) like my 5K iMac, then you are best viewing at 100%. So yes, in that sense knowing the screen ppi is useful.
Thanks for posting the link. Interesting blog in which you make some good points. For a digital image from a camera the dpi or ppi is irrelevant as it of course only has a x value and y value of pixels. Things get rather muddy however when you wish to either display or print the image hence the number of contradictory posts. It’s a topic that confuses many not helped by the software that many of us use that adds to the confusion.
To answer the question posed at the end... Yes, all the time. Most emphatically too. I run a 44" Canon Pro-4100 and you need to be totally across these concepts (and others) to make really good prints. Having said that, if you are not printing at all, then I'd agree that PPI is irrelevant.
Hi if you are given a tiff image 7000 by 5000 pixels and the client wants an A3 print with a 20mm border what resolution values would you give to the image to get the best possible result?
Darn, I was in total agreement until you got to the last sentence. On a webpage, ppi is still important, if not in a precise way. You don't want to use your full megapixel image because it will slow down page loading (and give thieves a better product). Although different screens have different resolutions and the page can be shown in different browser windows, a reasonable estimate of screen ppi and expected viewing dimensions (in inches) allows you to better come up with the total number of pixels to downsize to without losing quality. But if they never want to print or show their photos (maybe just tell everybody how great the pictures are), then don't worry about it.
OK, I'll move to 'pretty much irrelevant". 😁 Pixel dimensions are usually the important numbers, not pixels per inch. Browsers simply display images at a 1:1 ratio on screen. Thus you cannot know exactly how big a viewer will see an image. Ant retina screen complicate things becuase some software takes into account the high screen ppi, and some does not.
You are correct that high res images will load slowly, and can be resized into a web layout 'box' thus slowing things down but ppi for screens has nowhere near the relevance that it does for physical prints - and confuses people no end!
Replying to Eric Robinson: that's a hard one to answer in a short post. Rule of thumb : prints up to A3+ should really be at least 300ppi, over A3+ you can start to drop down towards 240ppi. But it depends on what you are using to print. For Lightroom, there is no resizing for print, you can simply specify the size and use all available pixels. In Photoshop you can do that too (resize in the print dialog) OR resize before printing to, say, 360mm $ 300ppi. It's a bit of a conceptual minefield - if you resize and print in PS you will then have to specifiy precise USM print sharpening parameters too. In Lightroom, you don't.
In reality a pixel does have a physical measurement at exactly 1/96th of an inch and is well defined in CSS. Please see https://www.w3.org/Style/Examples/007/units.en.html.
No. The "px" unit in CSS may have had its name inspired by the word pixel, and its size based on the average size of a pixel on a monitor 30 years ago, but it is not a pixel and it does not define the size of a pixel.
Calling a certain portion of your screen an inch does not make it an inch. Printing one pixel per real inch will not give you the same picture as printing 10,000 pixels per real inch. PPI IS important. But you are correct about people specifying ppi requirements in cases where they are not appropriate (for example, when they say they need, say, 1080 pixels across (when that is important), and then add a 72 ppi requirement (which is meaningless for their application).