So, I have a MacBook Pro 15” and I recently bought an LG 27” 4K monitor.

When I connected the monitor, system report said that the original resolution is 4K but the picture looks like 1920x1080 (using 4 pixels for 1 displayed pixel) on the screen. The text was too big for me.

I was then able to access scaling options and opted for the option that looks like 1440p with the original resolution of 5k. This seems to give the best image and size for this external monitor.

Now, I want to understand why the image is clear at what looks like 2560 x 1440 on my 4K monitor since the pixel distribution is not a modulus of 0. In my understanding, there is no way to distribute a 5k picture on a 4K monitor and yet get a clear picture. Since 5k is only 1.33 times bigger than 4k horizontally and vertically, how does it render? Is the picture quality worse than I think?

2 Answers 2


What happens is that the system essentially renders the image offscreen at a much higher resolution where you haven't got "a third of pixel" and similar. Then that image is downsampled to the actual resolution of your monitor and displayed.

This process is ofcourse not "lossless" and will give a slightly worse image than if you had a monitor of a higher resolution where this problem does not occur. However, due to the relatively high resolutions we're working with today, it doesn't look as bad as you might think.

It is always a tradeoff of whether you prefer the perfect quality of having 4k and 1920x1080 match each other perfectly, or you rather have more screen real estate by opting for 4k and 1440p.


When you choose 'looks like 2560x1440' and that is not an integer scale factor of 4K (3840x2160), macOS creates a virtual display twice the size (i.e. 5120x2880). All software (OS and applications) writes to that double size display.

The display driver then downsizes from the 5120x2880 virtual display to the 3840x2160 physical display. This ensures the consistent optimisation which applies across all applications.

As you know, this creates a slight fuzziness but does mean that text looks the intended 'right' size.

When choose 1920x1080 the scaling factor to 3840x2160 is exactly 2 so that the virtual display (twice 1920x1080) is the same size as the physical display. As a consequence no fuzziness, but everything looks far too large.

Macs always look best (text size and crispness) with screens that have about 220 pixels per inch (ppi). Older screens were all about 110 ppi. Anything else there is a compromise between best crispness and best text size. It is not chance that the new 24" (actually 23.5") iMac has a 4.5K screen - 4480x2520 and 218ppi.

Edit (a year later): Marc Edwards at Bjango has an excellent write up illustrating the compromises which must be accepted when a display is not about 220ppi. Thanks to @Wollmich for pointing men to this article.

  • would you have a closeup photograph of text rendered at the same font size on a 5k vs a 4k display, where the 4k has been scaled to have more space so that it fits the same information (space) as the 5k display? (i.e., windows dragged across monitors retain size.) I wonder whether the 5k->4k scaling is "more than good enough visually" or whether apple badly cut corners compared to windows here, making 4k unpleasant enough to justify dropping 3x as much on their 5k displays compared to a 4k display.
    – ivo Welch
    Mar 28 at 6:06
  • @ivoWelch Sorry I don't have a 4K display photo and I am not sure that would help. I really think you need to test 4K displays for yourself to decide if you would find it good enough (or better) at looks like 2560x1440. Do you have a MacBookAir/Pro you could take to a shop for a test?
    – Gilby
    Mar 28 at 9:13
  • Thanks @Wollmich I was not aware of that, though I have a link to a much older Bjango article on the same topic. I have added your link to the answer as it is excellent at illustrating/exaggerating the compromises.
    – Gilby
    May 9 at 23:39

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