A 1080p monitor has 2m pixels (1080 * 1920) and on a small screen these pixels are closer together, resulting in a sharper image than on a larger screen of the same resolution. For example, the pixel density of a 19" 1080p monitor is 116pixels per inch (ppi) and on a 24" monitor it drops to 92ppi. A substantial difference, and both fall far short of the 227ppi of the MBPR. We can see that phones and tablets have the highest pixel densities - holding a screen 10 inches from your face is a good way to spot a low ppi count and manufacturers have obviously realised the need to have high end screens on these devices - at the other end of the scale we have some very large monitors from Sony and iiyama, the later of which is a whopping 52" inches, packing a density of just 40ppi. But to be fair, many such displays are used in airports/train stations and are mounted far away from the user, so the apparent resolution isn't too bad... but if you're thinking of buying one to use as your primary monitor, it'll look shocking. In terms of a second monitor to pair with your MBPR, utilising a small 1080p monitor is a cost-effective way to get a high pixel density, but even an 18.5", 1080p will be far off the MBPR screen. 4k monitors are the only thing that will come close: A 24" 4k monitor has a pixel density of 184ppi, a 32" version 138ppi. There is chat that Apple will bring out a new Thunderbolt Display; cross your fingers that it will eclipse 4k with a retina screen of it's own, to match the 227ppi of the MBPR.

This plot of devices simply started as a list of monitors that I was considering buying and then grew bigger as 1)I tried to find devices at each end of the spectrum 2)New devices came out and I wondered how they'd compare to the stack. Where possible I've used the manufacturer's RRP; in scenarios where there are multiple choices (e.g. the various iPads with different capacities / 4G) I've gone for the cheapest one.