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I'm looking to get a second monitor for my 2018 27" iMac.

The Dell P2715Q seems to be a common choice. It comes in 27" and 24" sizes. The first review on the Amazon site says that

the 24" has the highest pixel density for the resolution -- if you go up to 27 or 28" you will get excellent resolution but not as crisp text

Can anyone verify this? Or does anyone other suggestions for an external display for an iMac?

I will be mounting this second monitor in portrait mode, and using it for coding and reading. I value really clear text, the way my 5k monitor for the iMac provides. I especially like that zooming in with the trackpad preserves the clarity of the text. (Though this may be true for every hi-res monitor.)

My plan is to connect the monitor to the iMac with a display port -> USB-C cable.

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The review reflects a general rule about monitor resolution; given the same resolution, a 27" monitor will be slightly less clear on text and fine lines than a 24" monitor. This is where pixel density matters. Unfortunately, this isn't measured the same way a monitor's screen is measured, as pixel density is about how many pixels there are per a standard unit of measure, but monitor size is measured by the total screen length from the lower left corner to the upper right. Just like that one South Park episode, they measure at an angle to get a higher number.

1920x1200, 24" 3840x2160, 27"

1920*1200 = 2,304,000 pixels 3840*2160 = 8,294,400 pixels

Both of these resolutions have a 16:10 aspect ratio. We now have all of the numbers we need to determine quite a lot about the monitor. We have the hypotenuse of a right triangle (24") but we have a different standard of measure for the sides (1920px L, 1200px W). We can use the Pythagorean Theorem to determine what '24"' means.

1920sq + 1200sq = 3686400 + 1440000 = 5126400

sqrt of 5126400 = 2264.155471693585

3840*2160 = 14745600 * 4665600 = 19411200

sqrt of 19411200 = 4405.814340164597

Now we know how many pixels lie across that hypoteneuse for each monitor at the given resolutions. This lets us determine an estimate of density by ratio.

2264.155471693585 / 24" = 94.33981132056604 approx ppi diagonally.

4405.814340164597 / 27" = 163.1783088949851 approx ppi diagonally.

Clearly, the higher resolution monitor here wins out. That's because the 4k monitor is nearly double the resolution, and is not double the size. The user who left the review likely had trouble seeing some things clearly on the 4k monitor because they were shrunk in size as a result, and lowered their resolution to 1080p; 1080p is not the native resolution on the monitor, and will appear blurry by comparison. This will also happen in fullscreen apps which do not support higher than 1080p.

For text, I'd still use the smaller monitor. Given what I just said about reducing resolution of your display, text will appear smaller in native resolutions at 4k than it does in 1080p at the same font size and kerning.

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