Pixel Density Explained
Most LCD and LED monitors currently display at standard HD resolution, commonly referred to as 1080p. Without getting too much into the technical aspects, this is the same resolution as a standard, high-definition television (HDTV). What's important to understand is that this pixel resolution -- 1920 pixels wide by 1080 pixels high -- is the same regardless of the size of the screen. So whether you buy a 21", 23" or 27" display, if they are HD displays, you're seeing the same number of pixels. The larger the screen, the lower the pixel density and the more obvious the pixels appear.
Using the calculator with some typical screen sizes, you can see how ppi decreases significantly as display size increases. On a 15" laptop, 1080 resolution displays at a very acceptable 147 ppi, which makes the pixels hardly noticeable. However, on a 27" HD screen, pixel density drops to just 82 ppi, which makes each pixel quite obvious. The end result is an image that looks jagged and more difficult to view, particularly for text-intensive applications.
Enter "Retina" and "4K" Displays
Monitors such as Apple's "Retina" displays and other manufacturers' "UHD" and "4K" displays are cramming more pixels into a smaller space. None of these are a set standard, but simply marketing terms for screens that display at higher pixel densities. At the top of spectrum are the latest iMac and stand-alone 5K Retina displays. Their 210 ppi pixel density is quite striking -- but also a pretty big drain on the pocketbook.
Thankfully, as higher density displays become more common, prices are dropping considerably. Whereas a year ago a high resolution display was out of reach for most people's budgets, there are a number of 4K displays now in the $500-1000 range. Expect prices to continue to drop as the technology becomes more commonplace.
One important caveat: with all of these extra pixels to display, your computer will likely need to have a more powerful video card. In the Mac world, all current models have sufficient video horsepower to drive one or more Apple Retina or aftermarket 4K screens. Other brands of off-the-shelf computers are unlikely to have powerful enough graphics processors to handle these screens.
There are other factors that come into play in terms of usability, like refresh rate of the screen and the video card, but the short version is that you'll need to upgrade the graphics card in a desktop, or buy a laptop or all-in-one computer with upgraded built-in graphics. Your graphics card or computer will also need to include a DisplayPort connector in order to fully take advantage of the screen. Over the next couple of years, expect more computers to come standard with the necessary graphics capabilities as high-ppi screens become commonplace.
If you're ready to take the plunge into the world of 4K right now, here's a great deal: Acer's B286HK 28" display produces a great image at an affordable price of just $499 retail. This screen runs at 3840 x 2160 pixels (157 ppi), which is sufficient to create that sought-after, paper-smooth image. We have a number of these screens in our office connected to our Macs and Windows computers at Digital Media Northwest and are quite happy with them. Or better yet, if don't mind buying manufacturer refurbished, Acer is currently selling these units factory-direct on eBay for just $239 with free shipping (a $50 price drop from a couple weeks ago).
Take it from someone who spends 12-14 hours a day in front of a computer screen: Once you experience 4K or Retina, you'll never want to look at a 1080 screen again.