Ladies and gentlemen.
What Apple did with iPhone 4 and 960x640 display was great at the time. They have shaken the industry and made them do better. But is there room for improvements? Let's see.
Relation between human visual acuity, angular resolution and apparent realness of an image
Ability of eye to see detail is called visual acuity. Precise vision is tested by ability to distinguish fine pair of black lines (1 pixel wide) separated by one pixel wide white line.
Like this (first line pair).
You can test it on your screen by moving away from your screen until you see one line (usually gray) instead of two (black). Mark that distance and measure it. Now you have to know your display's height in inches and vertical resolution. By knowing these things, you can calculate your visual acuity, expressed in (cpd) or pixels per degree.
If a person has visual acuity of 60 cycles per degree (cpd), that means that person has visual acuity of 60*2= 120 pixels per degree.
Want to know how to calculate measure and calculate your visual acuity?
OK, so we have:
DH = display height in inches, measure just the display, without bezel!
D = distance from you computer display (not TV! - rectangular pixels! measured in inches when two black lines no longer seem like two lines but one (usually grayish).
VR = vertical resolution. For 1920x1080 display, vertical resolution is 1080 pixels. For 1280x800 display, vertical resolution is 800 and so on...
Here's an calculation example.
DH = 8.16198 inches
D = 72.83464 inches
VR = 800 pixels
Here, in the first field we type or paste DH, in second field we type or paste D. We leave third field empty and press calculate. Voila!
In the third field we got angle (in degrees) which display height occupies. In this case, we got 6.414.
Let's call this angle A.
A = angle (in degrees) which display height occupies.
We're almost there! All you have to do now to calculate visual acuity (VA) is to divide vertical resolution with angle (VR with A)
VA = visual acuity
So for our example: VA = VR/A = 800/6.414 = 124.727
Visual acuity in this example is 124.727 pixels per degree or 62.3635 cycles per degree.
Average visual acuity is about
120 pixels per degree or 60 cycles per degree.
Is iPhone display good enough for our eyes?
Let's see for viewing distance of 12 inches. iPhone display has DH of 2.91217 inches, distance we're calculating for is 12 inches and VR is 960.
Again, we go to
this site. In the first field, we paste DH. As we said, for iPhone this is 2.91217. In second field we type in viewing distance from display to our eyes. In this case, we want to calculate for 12 inches. We press calculate and in the third field we got the A - angle (in degrees) which display height occupies. In this case, A is 13.837 degrees.
We have said average visual acuity is about 120 pixels per degree. To get the needed display's vertical resolution (NVR), we need to multiply angle A with visual acuity VA.
NVR = needed vertical resolution
NVR = A*VA = 13.837*120 = 1660.44
For average viewer, at distance of 12'', iPhone's vertical resolution should be 1660 pixels. To get horizontal resolution, we divide vertical resolution with aspect ratio (1.5). We get 1107 pixels.
So, for average viewer, at distance of 12'', iPhone's resolution should be 1660x1107 pixels.
But is everything beyond this waste of pixels?
Short answer - no.
Even though at some distance, two lines seem like one, we can still perceive quality beyond this number (beyond our visual acuity). How do we know that? Japanese
NHK did tests in which they determined that observers could tell the difference in image quality all the way up to 155 cycles per degree (cpd) - that is astonishing 310 pixels per degree!
Image
Until the day we have it - please don't comment "
Why do we need anything above...".
Have any questions? I'd be glad to answer them. For those who want to know more about the subject - I recommend
this book by Colin Ware.
Thanks.