A quick tutorial and remark on display pixel densities (PPI)

Pixel density, measured by pixels per inch (PPI), is an essential feature that should be considered when choosing any display, especially on a laptop. Actual pixel density is solely determined by the display resolution and size. However, there are differences between actual, effective, and perceptible pixel density. There is not one number that is ideal for pixel density. Perceptible PPI is dependent on working distance, which is often determined by the type of device (eg. desktop or laptop). Of course, it’s also prone to personal factors such as eyesight and even personal taste.

 

 At this point I find it important to make the distinction between PPI
and DPI. Generally when talking about a screen, a greater PPI will
mean things are getting smaller, and a greater DPI (some Windows
nomenclature mostly) means things are getting larger.

 

Now for a little bit of history of some common PPIs. For years Windows scaling has been outright dreadful, and it’s been mostly ideal to keep scaling at 1:1 pixel mapping. In these cases, the actual and effective PPI are entirely equal. Let’s look at some common ThinkPads that were praised for having ideal PPI values, items and text should all be comfortable sizes, zooming in should not be necessary under normal working conditions. These PPI are:

 

ThinkPad H V Size Aspect PPI
X220 1366 768 12.5 16:9 125
X300 1440 900 13.3 16:10 128
T420 1600 900 14.0 16:9 131
T60 1600 1200 15.0 4:3 133
T60 1400 1050 14.1 4:3 124
T60 1680 1050 15.4 16:10 128

As you can see, they’ve kept the PPI around 130 for years. Of course, there’s also been plenty of models with lower resolution options, and even a few that are higher. For example, there was a 15.4 inch widescreen version of the T61p which had a resolution of 1920×1200 and PPI of 147.

 

Fast forward to modern day (2016) ThinkPads and here’s the higher-end options:

 

ThinkPad H V Size Aspect PPI
X260 1920 1080 12.5 16:9 176
T460 1920 1080 14.0 16:9 157
T460s /X1C4 2560 1440 14.0 16:9 210
P50 3840 2160 15.6 16:9 282

Keep in mind there are some lower resolution options for these machines, I simply chose to show the high-end options. While the lower resolution options may make more sense from a PPI perspective, sometimes the lower end screens are TN, have worse color reproduction, and viewing angles. Of the following chart, arguably the only usable native resolution would be the 14.0’’ at 1080p. I would be hard pressed to find anyone who uses the other resolutions native. This brings me into my next segment: effective PPI.

Once scaling is employed, you can produce an effective PPI that is lower than the native PPI. This allows you to make the objects and text on the screen bigger. Let’s look at how Apple scales their displays to give us an idea of some usable effective PPI at a really high pixel density:

 

MacBook Pro 13 Retina H V Effective PPI
Native (not selectable) 2560 1600 227
1680 1050 149
1440 900 127
Default (2x scale) 1280 800 113
1024 640 91

By default, the scaling is set to 2x on the MacBook Pro 13 Retina.
This means the effective resolution is actually just 1280×800 and
effective PPI is 113, which is a tad low. Items on the screen are
going to appear larger than on those classic ThinkPads I mentioned earlier, but I can only speculate they did this to appeal to a wider audience who will be able to view content more easily. For those wishing to open more real estate, they have two other scaling options with higher PPI. They also have a crazy low PPI that could probably be used if you’re sitting it up on a desk and sitting far away, or if you’re going blind. But keep in mind, using these fractional scaling factors will take a performance hit, since the CPU or GPU has to do more work. It’s also worth mentioning that the native resolution is not even selectable, Apple assumes that a PPI of 227 is too high for any usability. The MacBook Pro 13 Retina has an upper selectable, effective resolution resulting in a PPI of 149. Also consider that MacBook Pro 15 Retina has a max selectable, effective resolution of 1920×1200, the same same exact resolution and screen size of the 15.4’’ T61p, with a PPI of 147. From these three examples, we can therefore deduce that the upper-end of usable, effective PPI is around 150.

In conclusion, if we look back at the current ThinkPad models with high resolution configurations, we can probably say that all but the
14.0’’ models at 1080p would require some type of scaling, and
even then, a small amount of scaling or occasional zooming of text to bring it back to the classic level of 130 PPI. See what works for
you, but I would aim for PPI from 125-150 in most cases. Although
it’s always easier to keep things at native resolution and PPI,
with improvements of the latest desktop environments for Linux and Windows 10, scaling is now better and easier then ever.

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