A client recently asked me to provide a file at the “size” of 6000 x 4000 pixels for display on a large array of screens.

I wanted to know the resolution of the screen array for which it was intended. My question was “6000 x 4000 pixels at what PPI?” I was somewhat surprised to find that this information, the screen resolution, was not in fact readily available, and even more surprised to find how difficult I found it to explain exactly why I needed it!

How I would explain it now, after further reflection, is that a specification like 6000 x 4000 pixels by itself does not tell us the size or resolution of the file, because it doesn’t tell us the size of the pixels themselves.

Pixel size is not fixed – it is a variable.

The size of the pixel is determined by how many of them fit in an inch of screen (pixels per inch, or PPI). This may be more easily understood if we think for a moment in terms of print, in which we use dots (of ink) instead of pixels (dots of light).

We are familiar with printed images being made up of dots, and have all seen that that big dots mean low resolution, less detail, and that when we print smaller dots, more dots per inch (DPI), that we can achieve higher resolution, more detail, and finer gradations.

Screen resolution, or pixel density, works the same way. This is why PPI and DPI are interchangeable terms.

The pixel density of screens varies, just as the resolution of printers and prints do.

It used to be that pretty much all screens were made for 72 DPI. But screen resolutions have gotten considerably higher, and will continue to do so.

If I supply that 6000×4000 pixel file at a resolution of 72 DPI, and my client’s screen array resolves at 72 DPI, the image would appear to be approximately 83” wide (found by dividing 6000 by 72).

But if my client has a high-res screen, like a 4K, with a pixel density of 184 PPI, that same file would only appear to be 32” wide. If the screen resolution were to double again, to 368 PPI, the image would be 16” wide instead.

I suspect that most of us have observed this phenomenon when changing our computer resolution (often by mistake), and seen our desktop (alarmingly) either shrink or enlarge. A higher definition monitor, therefore, requires a bigger file.

The larger the screen – the greater the viewing distance.

What works in our favor, in terms of keeping file sizes reasonable for screens, is that generally the larger the screen, the greater the viewing distance. And greater viewing distance is more forgiving of lower resolution.

For a print example, consider a billboard, which uses a big dot pattern, because at a distance the dots blend together. It’s not until you get up close that you can see the dots. Likewise a Jumbotron screen at the ballpark is of a low resolution, because it too is seen only from a great distance.

This low resolution screen is also advantageous in terms of file size, because it can take relatively small files and render them very large.