Digital image resolution can be a confusing issue, and there is a great deal of misinformation floating around. This is because there are two definitions for the term "Resolution".
Definition One: Pixel Dimensions
The NUMBER of pixels in something. Usually images, cameras, monitors and TVs.
A 4K TV is 3840x2160 pixels. A 6MP camera captures (roughly) 3000x2000 pixels. My laptop screen is 1920x1080 pixels. Those are all resolutions, in the original sense of the word.
This definition is the only "real" one.
Definition Two: Pixel Size
The SIZE of the pixels in something. This is unique to images, and is only relevant when printing. It’s commonly measured in PPI (pixels per inch) but can be measured in any unit at all, if you like – pixels per nautical mile, if you wish.
We often talk about 300ppi for photographic printing, or 150ppi for canvas printing, or 50ppi for billboard printing. Those are just examples.
This definition isn’t "real". It’s just an arbitrary figure assigned to a file. It’s vitally important for us to understand that PPI "resolution" is akin to EXIF data, or ICC profiles – it’s additional data, that’s only recognised by some programs, but not others. Windows Picture Viewer, for instance, has no idea what PPI an image has. Nor do web browsers.
When and why is this a problem?
Usually, these two definitions co-exist peacefully. We can discuss "screen resolution" and "print resolution" (Definitions One and Two respectively) without giving it much thought.
But sometimes the lines get blurred, and confusion sets in. This is common when discussing the provision of digital files to clients.
If somebody tells you that "high resolution" files should be 300ppi, they are probably getting their definitions mixed up. In most cases, PPI doesn’t matter – all that matters is the number of pixels, not their size.
If you have six litres of water, does it make any difference if you store it in two three-litre jugs, or six one-litre bottles? Of course not – either way it’s still six litres of water.
The ambiguity is exaggerated by the concepts of "high" and "low". These aren’t definitive terms, are they? Just like "old" and "young" – my children think my parents are "old", but I don’t think of them that way, and they surely don’t consider themselves as such.
My wife’s Canon 5D was nearly 13 megapixels, which was a lot at the time. Now she has the 5DII, which is about 21MP … does that make the 5D a "low resolution" camera? Of course not – 13MP is still a lot of pixels.
So, how long is a piece of string? Let’s take a look at some situations …
Providing files to members of the public
When providing digital images to members of the public, your only consideration is Definition One.
Definition Two has no relevance whatsoever, because your customers’ computers and programs can’t distinguish the size of pixels – they can’t tell the difference between 72PPI and 1000PPI.
All that matters is the number of pixels you provide. You get to decide, as part of your business plan, what dimensions your "high resolution" files will be.
Some photographers advertise "high res" files, and provide two megapixel files which are just big enough for printing 6x4s. Other photographers provide every single pixel from their DSLRs, suitable for printing much larger, and also call them "high res" files. It’s up to you.
Of course, all digital images need to have a PPI value assigned to them, regardless of its relevance – it’s just a requirement. So if it makes you (and your customers) feel better, by all means assign 300ppi to your files. Just be aware that, ultimately, it makes no difference.
Providing files to graphic designers
If you’re providing files to a graphic designer for use in commercial printing, needless to say you should provide as many pixels as you can. As above, Definition One is most important.
However, graphic designers will be using software (eg Adobe InDesign) which does recognise the PPI value in your files, so it's good manners to make sure the files are 300ppi, unless the designer has specified otherwise.
Providing files to web designers
The PPI value (Definition Two) doesn’t matter at all for web, so you may safely ignore it.
The largest file a web designer is likely to need is 1 megapixel, so they might not appreciate being sent 12 megapixel files! Check with them to see what they require.
Printing your own photographs
Here, finally, we find a situation where Definition Two is genuinely important.
When we’re preparing images to print, we usually use inches as the print dimensions – eg 5×7", 8×10", etc. We also need to specify a PPI value to go along with that.
Most labs require 300PPI, but it can vary from lab to lab, printer to printer, and media to media. Check your lab’s website to be sure.
Even when you’re printing, the PPI only matters if you like to sharpen your images. If you don’t sharpen, then you can ignore the PPI if it suits you.
Sometimes you see or hear people refer to "DPI" (dots per inch), and sometimes even "LPI" (lines per inch).
Never engage a nerd in a conversation about DPI vs PPI. They will bore you to death with complex, historical and irrelevant explanations of printing dots etc. Forget it.
If somebody says "DPI" in reference to digital files, they mean PPI. In modern context, they are the same thing.
If you have a question about this article, please feel free to post it in Ask Damien.