Tl;dr: Go with the Apple-endorsed LG UltraFine displays if there is no strong reason for not doing so.
A nice external display is more important than ever during this Coronavirus stay-at-home madness. I have had a (previous model, 21.5”) LG UltraFine 4K at home, but we need another display since both of us are now stuck at home starring at the computer for 12+ hours a day. Normally I would have just bought another LG 4K because the display is simply gorgeous with native macOS integration you would not find anywhere else (except the $4,599 Pro Display XDR, of course). However, I got the normally-$699 LG refurbished for less than $300 and it feels weird now to have to pay the full price. So I decided to do some research and seek alternatives, especially that many new 4K displays came into the market during the past two years.
To finish the story: I checked The Wirecutter, which recommends HP Z27 for its functional stand (you can pivot, swivel, and tilt), accurate color reproduction, and USB-C. Since that there were no good deals on the LG UltraFine, I pulled the trigger and got the $540 HP Z27. It is actually a good purchase: The build quality and the design are exceptional for the price. Although the brightness is not comparable to either the built-in display of my MacBook Pro or the LG 4K, it is enough for day-to-day use. To that end, I am satisfied.
However, it was only after the order cancellation window had past did I realized a caveat: The pixel density of Z27 (or any other 27” 4K display) is not optimal. When choosing a display, we usually look at its resolution and size. Although important, they alone do not tell the complete story. This is because the actual “sharpness” of the image is determined by the pixel density instead of the absolute resolution. For example, one could expect blurry image on a 57” HD (1080p) TV. When you want to fit 1920x1080 pixels on a 57” panel, pixels have to be made so big that human eyes can easily tell them apart, resulting in fuzzy image. And the same story goes for computer monitors.
More importantly, macOS establishes that the UI of an application should look good at 110 PPI (pixels per inch, the measure of pixel density) when targeting non-Retina displays, and 220 PPI when targeting Retina displays. Here “Retina” is an Apple term that simply means that the high pixel density makes individual pixels “disappear”. To put it simply, every Mac application (including the system UI) has two “modes”, which are optimal when shown on displays of 110 PPI and 220 PPI respectively. It goes without saying that one should prefer a display with a pixel density of around 220 PPI, given that such pixel density is optimal for the UI and is high enough for sharpness. Three configurations achieve such pixel density: 4K at 21.5”, 5K at 27”, and 6K at 32”. And guess what, The first two are the configurations of the Apple-endorsed LG displays, and the last is the Pro Display XDR.
Now there is a natural question: What if I have a display which is not 110 PPI or 220 PPI? For example, when Apple / LG replaced the 21.5” 4K model with a 24” 4K model last year, many consumers were pissed off: The pixel density drifted off the optimal 220 PPI, to a significantly lower 184 PPI. Or, in my case, the HP Z27 has a pixel density of 163 PPI, right in the middle of 110 PPI and 220 PPI. Does it mean that the display will be totally unusable?
There are two options for owners of displays with non-optimal pixel density. First, one could do nothing but let the OS render the desktop at the “native” resolution. This results in icons and texts that may seem a bit too large, because the same number of pixels are supposed be fit in a smaller display. The benefit of the large display panel (e.g. 27” vs 21.5” at 4K) is larger texts that are potentially easier on eyes.
Second, one could turn on the “scaling” option in System Preferences. The OS will then first render the desktop on an imaginary canvas that is larger than the actual resolution of the display, and then downsamples the canvas to fit the actual resolution of the display. Effectively it is showing more content than it would without scaling, at a cost of reduced pixel density: Recall that applications are designed for 220 PPI. Now there are not enough physical pixels on the actual display to show them all, the OS has to downsample the UI and “approximate” 220 PPI using whatever pixel density you get on the display, e.g. 163 PPI in the case of HP Z27. Besides the most direct consequence of less sharp images, the GPU has to do more work due to the extra downsampling. The good news for me is that my computer seems to be modern enough such that I do not feel any lag when I turn on scaling for my Z27. After all it is supposed to handle 5K with ease, and the particular scaling setting I’m using causes the computer to render at exactly 5K and downsample to 4K. But I have seen others report significant high rendering latency when scaling is turned on.
With most 4K offerings on the market being 27”, resolution scaling is something you can’t avoid unless you really don’t care about screen real estate and is willing to use a 27” display as if it was 21.5”. Or, just go with the Apple-endorsed LG ones. They look boring and only play well with Macs, but they are pretty darn good displays.