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I'm trying to programmatically adapt my website's image sizes for differently sized devices. But now I am having trouble telling what sizes I actually need. In Google Chrome emulator, I'm seeing some of my images upsized, e.g. on iPhone 6 from 230x230 natural to 357x357 displayed. The image takes up nearly the entire width of the emulated screen, and looks just slightly degraded, suggesting iPhone 6's width isn't much larger than 357 pixels.

But Apple says the iPhone 6 has a resolution of 750x1334! If that were true, the image should look much worse, I would think.

I've found some contradictory information on iPhone 4 as well.

This site talks about iPhone 4 at 640x960 pixels. Chrome emulator again shows it at half those dimensions, 320x480.

This stackoverflow question says that "the iPhone screen is 320x480 definitely."

What am I missing here? Why do some sources (including Apple) supply dimensions that are twice what Chrome emulator (and my images) say?

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Buttle Butkus
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1 Answers1

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Relax, you're about to understand this mess. Just notice that 2 * 375x667 = 750x1334.

A pixel is not a pixel

The key thing is: one device pixel is different from one CSS pixel.

They are the same in low pixel density devices like your computer screen (96 dpi). However, high pixel density devices like smartphones and printers (upwards of 160 dpi) try to obey the general W3C CSS3 spec understanding that one CSS pixel should always be close to 1/96th of an inch (or 0.26 mm) when viewed from usual distance (arm's length).

They don't obey the spec to the letter, since that would imply 1px being exactly 1/96th of one real inch in high DPI settings, which wasn't ever implemented in any browser AFAIK. However, they try to make their CSS pixels not so minuscule despite very high pixel densities by making one CSS pixel equal to two or more device pixels.

Chrome Device Mode works with CSS pixels, which is what you should use to design text, navbars, headings etc, but not high-resolution images. For these, read the next section.

If you didn't notice, the image above shows that Chrome Device Mode does show you the device scale (how many device pixels equal one CSS pixel).

Fixing image resolution

As you already know, this affects images negatively, since the browser scales the image as well. Your 230x230 CSS pixels picture becomes 460x460 device pixels, using the same quality. To fix that, use the srcset attribute to give the browser links to different resolution files of the same image.

Example (adapted from the link above):

<img src="wolf-400.jpg" srcset="wolf-400.jpg 400w, wolf-800.jpg 800w, wolf-1600.jpg 1600w">

An iPhone 6 will look at that and think "oh, I pretend to be 375px wide but I'm actually 750px, so I'll download wolf-800.jpg."

Just don't forget to use src="" for compatibility. Also, unless you use sizes="", the browser will default to the full width of the device.

André Chalella
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  • `sizes` and `srcset` are new to me but look very promising for mobile. I will definitely be looking into that, since we are always trying to show the best quality image without wasting bandwidth. Also, I had heard about CSS pixels before, but your explanation of the ~1/96th inch basis for it made the logic behind it much more clear. And your blue arrow pointing to the Chrome emulator "Device pixel ratio" indicator is also incredibly useful, because from now on I'll easily be able to tell that ratio for any device I emulate. Great answer! – Buttle Butkus Jun 25 '15 at 18:00
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    Thanks a lot for the feedback! Just a note about Chrome ratio indicator: for iPhone 6 Plus, though it displays 3, the real value is 2.6. I don't know if Chrome just rounds the number or if it uses the actual original scale (which is later downscaled by 1.15, giving ~2.6), but anyway that may be useful to know. It only happens to iPhone 6 Plus AFAIK, which is the only device with decimal ratio. For more info about why it's decimal: http://www.paintcodeapp.com/news/iphone-6-screens-demystified – André Chalella Jun 25 '15 at 18:27
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    That is an interesting link. It seems to me we should think of iPhone 6 plus as 3x, and the phone itself handles downsampling however it pleases. Hmmm, and I wonder if the paintcode app is any good. – Buttle Butkus Jun 26 '15 at 01:18
  • A great answer, indeed. :-) – sjsam Oct 29 '18 at 05:42