How sharp is your monitor really?
What matters is not pixel density alone but how many pixels fall on one degree of visual angle. With normal visual acuity (20/20), about 60 pixels per degree match the resolving limit of the eye.
Why pixels per degree matter
Pixel density in PPI only says how fine the screen is, not how fine it looks to you. What matters is how many pixels fall on one degree of your field of view: a phone with 450 PPI at 30 cm and a monitor with 110 PPI at 80 cm can look equally sharp.
An eye with 20/20 vision separates details of one arcminute, a sixtieth of a degree. From about 60 pixels per degree, single pixels therefore blend into a smooth surface. Young, good eyes reach 75 to 100 PPD. At edges, the eye still spots jaggies well above that, so there is no hard “retina limit”.
Example: a 27″ monitor with 2560 × 1440 has about 52 PPD at 70 cm, with 3840 × 2160 about 78 PPD. Pixels vanish from 80 cm on the first and from 54 cm on the second.
Pixel pitch p = diagonal / √(width_px² + height_px²)Angle per pixel θ = 2 · atan(p / 2d)PPD = 1° / θField of view = 2 · atan(width / 2d)
Frequently asked questions
Is 4K worth it on 27 inches?
At 60 to 70 cm (24 to 28 in) yes, if sharp text matters to you: 4K reaches just under 70 to 80 PPD there, 1440p 45 to 52 PPD. From about 80 cm (31 in), 1440p also reaches 60 PPD and the difference is small.
What does “retina” mean for a monitor?
It means single pixels are no longer visible from the usual distance. Physically that is about 60 pixels per degree for normal vision. How many PPI you need for it depends on the distance.
Why are the values higher at the edge than in the centre?
The screen is flat. Its edges are farther from your eyes and seen at an angle, so more pixels fit into one degree of visual angle there.