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Ask TeD: How Many PPI Can The Human Eye See?

It was recently asked of BuB who relayed to TeD:

“Well I know the basics, like the greater the better but there must be some threshold to this whole ppi thing where the human eye at a specific distance wouldn’t be able to differentiate. Could you please shed some light on the subject?”

Thanks for the question, Sahil Tiwari. For a long time, this hasn’t really been an issue in any of our technology because we hadn’t reached a point where it was even a question whether increased resolution was offering a better experience because we could see the difference. We are approaching a point, however, where that might not be the case anymore.

Important Terms

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First, let’s understand some of the terms that we use when talking about PPI. PPI, or pixels per inch refers to the pixel density, or the number of pixels that you would find on one inch of space on a computer, or in our case a phone screen.

The first phone to launch with a 1080p screen was the HTC J Butterfly, which was released only in Japan under that name and released as a Verizon exclusive in the US as the HTC Droid DNA. It used a 4.7” screen, with a 540 x 960 resolution which translated to about 256 PPI.

1080p refers to a screen which is 1080 pixels wide, with lines that run the entire vertical length of the screen. This isn’t to be confused with 1080i, which is 1080-interlaced where the “p” in 1080p stands for progressive scanning. 1080p has much smoother transitions than 1080i and is therefore more common in standard home television sets.

Blinded By Science!

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Now to answer the main question that’s been asked, “How many pixels are enough?” Well, it’s a rather simple question but it bears a rather complicated answer. According to Dr. Optoglass, which I’m simultaneously hoping is and isn’t a real person, it’s a very large number and depends a lot on how close you are from the screen and how perfect your eyes are.

According to his article, Dr. Optoglass tells us that the perfect resolution of the eye can see 2190 PPI at 4 inches from the screen. However, none of us holds our phones that close to our faces, at least I hope you don’t, because that actually is bad for your eyes. In addition, the average visual acuity of a human is about 1 arc minute, which simply refers to a person with 20/20 vision. A person with 20/20 vision at 4 inches (the closest a healthy adult can focus) can see up to 876 PPI, which is considerably lower than the ideal eye, which can see at .4 arc minutes.

The average distance that I prefer to hold my phone from my face is about 1.5 feet, or 18 inches (457.2mm) and we can pretend that thanks to my contact lenses I have perfect vision (20/20). With these assumptions, we can plug that data into the equation below where d represents distance in mm and α represents the viewing angle in degrees, which is dependent on visual acuity.

PPI formula

Despite my best efforts, I was unable to replicate the original calculations, giving me little to no confidence in any numbers that I would have produced myself. However, this excerpt from the article should clear things up to a pretty decent degree:

[td_quote]“If the average reading distance is 1 foot (12 inches = 305 mm), p @0.4 arc minute is 35.5 microns or about 720 ppi/dpi. p @1 arc minute is 89 microns or about 300 dpi/ppi. This is why magazines are printed at 300 dpi – it’s good enough for most people. Fine art printers aim for 720, and that’s the best it need be. Very few people stick their heads closer than 1 foot away from a painting or photograph.”[/td_quote]

What’s The Verdict On Max PPI?

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So, to answer your question – the absolute maximum that we will ever need on a smartphone for a person with the best eyesight is about 720PPI. With 1080p devices such as the HTC One M8, we have already seen approximately 441 PPI which is well above what a person with 20/20 vision can see from a foot away. If you are worried, aim above 300 PPI and it’s hard to go wrong, unless you have 20/2 vision, in which case, you’ll need to wait until 4K makes it to your phone. It would seem that after that, there would literally be no point to increase phone resolution because if you assume the width of an average 5 inch screen to be roughly 2.5 inches, that would mean 720*2.5 = 1800, which is still technically 2K.

In short, we’re there.

Special thanks to Sahil Tiwari for submitting the original question to the Ask TeD page.