Resolution, pixel density, viewing distances, and retina displays

For once, I actually post an entry that’s not bashing Apple. In fact, I’m going to defend Apple somewhat.

So, with the iPhone 4, Apple’s announced something they call a “Retina Display.”

That’s marketing speak for a display with higher resolution than the human eye – the human eye cannot discern pixels on a display of that pixel density, at one foot viewing distance.

One critic has claimed that Apple’s claims are false… which is only true if you ignore 20/20 vision, and go straight to 20/12, according to someone who worked on the optics for the Hubble Space Telescope.

Of course, there are plenty of other critics that go on about how the iPhone 4’s 960×640 resolution was selected purely because of iOS lacking scalability, and it allowing for the highest resolution and pixel density mobile phone display is merely a side effect. While that likely is the case… it’s still the highest resolution and highest pixel density mobile phone display, and it’s even IPS. Apple should be commended for that. (We won’t discuss the “three years ago, I had a Windows Mobile phone that had nearly the same pixel density” comments, other than to say that it’s less than the iPhone 4, still.)

Not that I’m going to buy one, of course. But this entry is about displays, not why I’m not buying an iPhone, if you want those kind of entries, read almost every other entry on this blog.  😉

Anyway, I’ll defer to the experts for discussions of how optics work, but I figured I’d go ahead and make a calculator for all of this. It’s located at

You’ll notice that it has several options.

If you leave it at the default option, you can put a resolution and screen size (in inches) in, and it will give you the aspect ratio, pixel density (in pixels per inch,) and optimal viewing distance (also in inches) for individual pixels to JUST be discernible for someone with 20/20 vision. Change the visual acuity to match yours, and you’ll get the optimal distance for your vision. Useful if you already have a monitor, and want to know how far away to put it.

One thing to note is that the optimal numbers listed are just those that make pixels on the edge of discernibility – not necessarily the optimal for your setup. Certain setups work fine with individual pixels NOT discernible, and certain setups are more comfortable with larger sizes.

Alternately, you can select to get a size calculation. In this case, you’ll need to provide a resolution, your visual acuity, and your desired distance. This will give you the optimal distance for pixel discernibility for a given resolution of display. Useful if you want to buy a monitor of a certain resolution, know how far away you want to put it, and want to know how big it should be.

Finally, you can select to get a resolution calculation. You’ll need to provide an aspect ratio, a size, your visual acuity, and a desired distance. This will calculate a rough approximation of the optimal resolution, at your desired aspect ratio. This one’s useful if you have a certain amount of space, and the monitor’s going to be a certain distance, but you’re flexible on the resolution.

I decided to license this under the WTFPL. Some projects, the GPL works better. Some projects, BSD works better. And some projects, like this one, WTFPL is just the right fit. So, want to use the code? Do what the fuck you want to.

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