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Old 11-04-2012, 07:15 PM   #1
jswinden
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What does resolution mean for your tablet/phone/ereader?

There is always a big squabble over resolution of various device screens. "Mine has more ppi therefore it is better" type of arguments. Those are normally useless arguments as they seldom compare meaningful measurements. So lets take a short scientific and mathematical look at what our eyes perceive.

There is a principle of 300 ppi equals the limit of the average human retina at 10 inches. In other words, the average human retina with 20/20 vision can distinguish no more than 300 ppi at a distance of 10 inches. To put it another way, the average human retina with 20/20 vision cannot distinguish individual pixels at a distance of 10 inches when the resolution is 300 dpi or higher. We are talking about the average human retina with 20/20 vision, so a person with better or worse vision could experience different results.

If this principle is true, and I'll let you decide that for yourselves, then we can determine the angle at which the average human retina can no longer distinguish between two pixels at 10 inches by this formula:

alpha = 2 * arctan ( h / 2d)

where h = 1/300 inch (which is what 300 ppi equals), d = 10 inches.

Therefore the angle alpha = 0.000333333 radians. Having determined the angle alpha at which the average human retina can no longer distinguish between two pixels, we can use that with various heights (resolutions) to determine at which distance the average human retina can no longer distinguish two pixels at various resolutions.
The formula is:

d = h / tan(alpha)

where d = distance, h = various heights shown below, angle alpha = 0.000333333 radians.
  • For h = 1/132" (132 ppi), distance required = 22.7" (iPad1 and 2)
  • For h = 1/162" (162 ppi), distance required = 18.5" (iPad Mini)
  • For h = 1/170" (170 ppi), distance required = 17.6" (original Fire)
  • For h = 1/216" (216 ppi), distance required = 13.9" (Fire HD, Nexus 7)
  • For h = 1/268" (268 ppi), distance required = 11.2" (iPad3)
  • For h = 1/326" (326 ppi), distance required = 9.2" (iPhone5)

So in theory the above listed devices would have to be held at least as far from the eye as the listed distance before the average human retina would no longer see any individual pixels. Your eyes, of course, will vary.
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Old 11-05-2012, 12:21 AM   #2
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Quote:
Originally Posted by jswinden View Post
There is always a big squabble over resolution of various device screens. "Mine has more ppi therefore it is better" type of arguments. Those are normally useless arguments as they seldom compare meaningful measurements. So lets take a short scientific and mathematical look at what our eyes perceive.

...

So in theory the above listed devices would have to be held at least as far from the eye as the listed distance before the average human retina would no longer see any individual pixels. Your eyes, of course, will vary.
this guy disagrees with you...

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The Apple 4th Generation iPod touch with the retina display makes an excellent reader. I've previously owned (well actually still own) the 1st Gen and 2nd Gen iPod touches. Although they were okay as ebook readers they were plagued by two shortcomings.

One is the small 3.5 inch diagonal screen. That of course is still a problem with the latest 4th Gen touch. However, the small size and portability has made for a decent ebook reader to take along to doctor offices, etc.

The other shortcoming was the lackluster 480x320 pixel resolution. Most eInk screens in comparison have at least 800x600 pixels. But with Apple's new retina display they have increased the resolution to 960x640 pixels. One thing I always noticed on the older touches, and indeed nearly every LCD screen I've owned, is that my eyes would get tired from reading after an hour or so. Extended reading on prior LCDs was out of the question. However, with the new retina display this eye soreness seems to be greatly decreased. I am able to read for hours on my 4th Generation iPod touch.

I wondered why the difference was so great. So I did what i often do and took out my 12 megapixel camera and took some side by side photos for comparison (attached). I loaded the same page from the same book into Stanza on both my 2nd Gen and 4th Generation iPod touches. In the first photo you can see the 2nd Gen on the left and the 4th Gen on the right. Since the photo was a high resolution 12 megapixels, I cropped and shrunk it down to look good in 800x600 pixels. The next photo is a composite of the two touches taken from the first photo. I cropped a full size 800x300 pixel area from each touch screen and placed the 2nd Gen cropping on top and the 4th Gen on bottom. As I said these are full size and because they were from a 12 MP photo they look enlarged, but they are not enlarged. That is the cool thing about high resolution photography.

It is fairly obvious to me why the 2nd Gen iPod touch made my eyes so tired after an hour of reading. I fully expected to see the same pixelized view in the 4th Gen screen but less pronounced. However, the 4th Gen screen is very near to the clarity of an eInk screen. I was quite amazed. Now if Apple would make a 5" iPod touch with a retina display!
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Old 11-05-2012, 03:29 AM   #3
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this guy disagrees with you...
I don't see any disagreement.
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Old 11-05-2012, 03:57 AM   #4
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Thanks for the scientific approach

I don't know if it is the fact that I take out my contacts before going to bed, but I was able to read on a shoddy iPhone 3's screen just fine. Before that I read on a Sony-Ericsson P1i which had a really tiny screen and I was fine with that, too.
Or maybe it's my job as a graphic designer that puts my mind at ease when it sees pixels so that my eyes don't try to "sharpen" somethign that cannot be sharpened ?
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Old 11-05-2012, 09:00 AM   #5
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It really depends on your eyes.

I can read easily without glasses at ~6 inches and under and with glasses at ~18 inches, so that's where I hold things to read. Which means all those devices with 200-300 ppi don't look any different to me from lower ppi devices at normal reading distance.

Specifically, I can't see any difference between the iPad mini's screen and the iPad retina screen at my normal reading distance and I can't see any pixelation on my 127 ppi laptop screen which is 24 inches away from me.

The size of the font affects me more than the ppi. I can't read for any length of time on my retina screen iPod Touch, the tiny print fatigues my eyes, but I can read for 10 hours at a time on my 150 ppi Kindle Keyboard. I increase the font on every device I use, at least to 14 pt, but larger if I can.
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Old 11-05-2012, 06:53 PM   #6
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Interesting.

So why did laser printers go from 300 dpi a few decades ago to 600 or 1200 dpi?
The print certainly looks nicer/cleaner on a higher resolution device. Of course, for laserprinters it is important to be able to produce grays, which is not an issue on an lcd screen (but possibly on an eink device)

Nevertheless, I personally still consider a lower ppi eink device (a.k.a. kindle Keyboard) more pleasing for reading than a higher ppi lcd device (e.g. Kindle Fire), unless it is for nighttime reading, thus requiring light.
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Old 11-06-2012, 05:10 AM   #7
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They seem to have really optimised the rendering on the kindle, somehow. I was looking quite closely at the curve of a C the other day, trying to make out the pixels, and I failed. On my kobo, which I believe is the same resolution, I can see the pixelation fairly easily. And on this iPad 2, the text gets a bit fuzzy at small sizes.
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Old 11-06-2012, 08:22 AM   #8
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Quote:
Originally Posted by jswinden View Post
So in theory the above listed devices would have to be held at least as far from the eye as the listed distance before the average human retina would no longer see any individual pixels. Your eyes, of course, will vary.
What about monkey eyes? My digital world is fraught with pixelated curved yellow fruit

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Old 11-06-2012, 09:37 AM   #9
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Interesting.

So why did laser printers go from 300 dpi a few decades ago to 600 or 1200 dpi?
The print certainly looks nicer/cleaner on a higher resolution device. Of course, for laserprinters it is important to be able to produce grays, which is not an issue on an lcd screen (but possibly on an eink device)
You are confusing dpi (dots per inch) with ppi (pixels per inch).

Laserprinters (and film imagesetters, for that matter) produce on/off dots, not pixels with shades of grey. When reproducing continuous tone images (like photographs) to get the same effect as 16-greys 166ppi eink screen you'd need a 600+dpi laser printer.

Of course, if you're not producing continuous tone images, the higher resolution is more important. Personally, I'd rather have a black/white 600dpi eInk screen than a 16-grey 166dpi eInk screen, but that's not an option yet.
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Old 11-06-2012, 10:04 AM   #10
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Thanks for the post, jswinden.

The only time pixelation has really jumped out at me is when I switch from any retina display iPhone to our 2nd generation iPod touch. The difference is pretty night and day.

When I had a chance to play with the mini, there wasn't any pixelation visible at a normal viewing distance, but the difference could be seen when placed next to an iPad 4. If you read in bed with the mini propped up on your chest, for example, I can see some people noticing the pixels.
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Old 11-06-2012, 11:49 AM   #11
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Originally Posted by pdurrant View Post
You are confusing dpi (dots per inch) with ppi (pixels per inch).

Laserprinters (and film imagesetters, for that matter) produce on/off dots, not pixels with shades of grey. When reproducing continuous tone images (like photographs) to get the same effect as 16-greys 166ppi eink screen you'd need a 600+dpi laser printer.

Of course, if you're not producing continuous tone images, the higher resolution is more important. Personally, I'd rather have a black/white 600dpi eInk screen than a 16-grey 166dpi eInk screen, but that's not an option yet.
I see it the way you do.

Apart from illustrations, I personally do not need many more colours than white and black for reading ;-)

Best regards,
Andy
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Old 11-06-2012, 12:52 PM   #12
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Thanks for the scientific approach

I don't know if it is the fact that I take out my contacts before going to bed, but I was able to read on a shoddy iPhone 3's screen just fine. Before that I read on a Sony-Ericsson P1i which had a really tiny screen and I was fine with that, too.
Or maybe it's my job as a graphic designer that puts my mind at ease when it sees pixels so that my eyes don't try to "sharpen" somethign that cannot be sharpened ?
Same here.
I'm reading eBooks since 2000.
Back then on Pocket PCs and Palm Pilots.
No problems at all.
Then read the 7 Harry Potter books on the first iPhone in 2007 - again, no problems whatsoever.
And I'm reading extensively. Often 5 hours in a single instance.
Often in bad lighting conditions: On the train, in bed, in hotels with low lights...

Maybe I've gotten used to low resolution and the worst display qualities in my old Commodore VC20 and C64 days, hooked to a television and playing for endless hours...
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Old 11-06-2012, 01:08 PM   #13
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Originally Posted by DrNefario View Post
They seem to have really optimised the rendering on the kindle, somehow. I was looking quite closely at the curve of a C the other day, trying to make out the pixels, and I failed. On my kobo, which I believe is the same resolution, I can see the pixelation fairly easily. And on this iPad 2, the text gets a bit fuzzy at small sizes.
The kindle probably has better anti-aliasing. See: http://en.wikipedia.org/wiki/Font_rasterization and http://en.wikipedia.org/wiki/Spatial_anti-aliasing for an explanation and example images
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Old 11-06-2012, 08:18 PM   #14
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For reading EPUB files and the like, where the viewing program does the kerning, and fonts with thick strokes can be chosen, a lower ppi display doesn't do much harm. You really see a problem though when trying to read PDF files on low ppi displays. The fonts are usually thin and intended for ink on paper, and the characters are kerned for the same, so that the text becomes blurry when anti-aliased for the much lower resolution e-ink screens. This makes reading technical PDF documents much harder on the eyes than, say, novels in some HTML-based form.
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