Display Fidelity
Work out a display's pixel density, aspect ratio, and the viewing distance at which it crosses into "retina" territory.
About pixels per degree
Pixels per degree (PPD) sets the visual acuity you're calculating for. It's commonly assumed that 60 PPD is enough — that's what Apple used to define "Retina" — but we know that it isn't. Some research [Nat Commun 16, 9086 (2025)] suggests 120 PPD is enough for the 99th percentile of viewers, and around 100 PPD for the 66th. For me personally, 100 PPD is sufficient — an estimate based on using a 4K 27" screen at 25" and 35" viewing distances — so it's the default here.
On field of view
32 inch monitor and 50 inch TV (both in Lawrence) had comfortable distance of about 42 inches and 60 inch (actual 84 inch) respectively, i.e. 37 degrees FOV, which can be used as max. Also, coincidentally that is when a 4k screen almost becomes retina for me. Although, at 130" distance, 65" TV felt pretty big (manageable but not bigger def). So human brain does nonlinear processing.
Actual formulae used here
If horizontal pixels is and vertical pixels is , diagonal length is , pixels per is and field of view in radians is , then
pixels per inch
where is the horizontal screen size,
.