It’s looking increasingly likely that when Tim Cook takes the stage at the annual WWDC keynote on June 11th, Apple will announce new MacBook Pros and possibly iMacs, and if the rumor mill is to be believed, these new machines won’t just be slimmer and ditch their optical drives… they’ll be the first Macs with Retina displays.

What everyone widely expects from Retina display Macs is an iPhone or iPad-style resolution doubling. So if the current 15-inch MacBook Pro has a 1,440 x 900 display, the Retina 15-inch MBP would have a 2,880 x 1800 display.

What the rumor mill is missing is that there’s no benefit to Apple handling a jump to Retina display Macs this way. The reason the iPad and iPhone going Retina was such a big deal was because they had really pixellated displays. Before the iPhone 4, the iPhone had a display that was only 53% close to being Retina. The iPad was slightly better, at 61%. Roughly, both the iPad and iPhone were only about halfway there, which made the easiest fix to just double the amount of pixels per inch.

But Apple doesn’t need to do this with its line of Macs. In fact, it’s likely that most “Retina Quality” Macs will have fewer pixels than your new iPad. Here’s why.

Macs Are Already Almost Retina Quality

When we talk about an iPad or iPhone having a Retina display, what we’re talking about is that the pixels on that display are so densely packed and so tiny that they are not discernible to the eye of a person with 20/20 vision. Nothing looks pixelated. We measure the sweet spot for how densely packed a display needs to be to qualify as Retina in pixels per inch.

There is no magical number of pixels per inch that automatically equates to Retina quality.

Simple so far, right? Here’s the thing, though: there is no magical number of pixels per inch that automatically equates to Retina quality. The iPhone 4S achieves Retina quality at 329.7 pixels per inch, while the new iPad only needs 214.9 pixels per inch to qualify as Retina. Why? Because you hold your iPad further away from your eyes than you hold your iPhone. That means the iPad doesn’t need as many pixels as the iPhone to seem crisp.

The closer a display is, the smaller and more densely packed the pixels need to be for your eyes not to be able to resolve them. The farther away a display is, the larger and more loosely packed the pixels can be. What all this means if you sit farther back from your Mac than you do your iPad — and everyone does — it doesn’t need as high a resolution to qualify as a Retina display.

There’s a formula for determining how many pixels per inch a display needs to be to achieve “Retina” quality, as provided by Dr. Raymon Soneira of DisplayMate Technologies. Using this formula, it’s pretty easy to tell how many pixels per inch a display needs to qualify as Retina. And helpfully, TUAW’s Richard Gaywood has always done the math.

Here’s how close they are right now:

Model Screen Size (Inches) Resolution Average viewing distance PPI for “Retina” Closeness to Retina 11-inch MacBook Air 11.6 1366 x 768 22 156.3 87% 13-Inch MacBook Air 13.3 1440 x 900 22 156.3 82% 15-Inch MacBook Pro 15.4 1440 x 900 24 143.2 77% 15-Inch MacBook Pro (High Res) 15.4 1680 x 1050 24 143.2 90% 21-Inch iMac 21.5 1920 x 1080 28 122.8 83% 27-Inch iMac 27 2560 x 1440 28 122.8 89%

As you can see, the Mac farthest away from qualifying as Retina is the 15-inch low res MacBook Pro, which is 77%, while the high-res 15-inch MacBook Pro is 90% a Retina display. Almost every other Mac hovers around 80-90%.

See what we’re getting at here? Apple doesn’t need to start doubling the PPI to achieve Retina. Across the board, resolution doubling is overkill. For the most part, every Mac only needs to bump itself up just a little bit. And using the above data and the Pythagorean theorem, it’s pretty easy to figure out exactly what the minimum real world resolution each display would need to be to be Retina.

Model Screen Size (Inches) New Resolution PPI For Retina New PPI Closeness to Retina 11-inch MacBook Air 11.6 1680 x 1050 156.3 170.78 109% 13-Inch MacBook Air 13.3 1920 x 1200 156.3 170.23 110% 15-Inch MacBook Pro 15.4 1920 x 1200 143.2 147.0 102% 21-Inch iMac 21.5 2560 x 1440 122.8 136.61 110% 27-Inch iMac 27 3840 x 2160 122.8 163 133%

See? Not nearly so drastic. And actually, a lot of these minor resolution bumps are actually more “Retina” than the iPhone 4S, which only rates a 105% closeness to Retina, and the new iMac would “out-Retina” the new iPad, which has a 123% closeness to Retina. These resolutions would be just fine.

Besides, there’s there’s another problem with PPI doubling…

Battery Life

The demands a Retina display makes on a Mac’s battery isn’t a problem for the iMac, of course, but it’s a huge issue for the MacBook Pro and MacBook Air.

Let’s look at what happened with the new iPad. When Apple introduced a Retina display to the iPad, they had to increase the battery capacity from 6,944 mAH to 11,666 mAH, an almost 70% bump. The result was a thicker, heavier iPad that took significantly longer than the previous generation to charge.

The display is the most power hungry element of almost any device. More pixels = more battery drain

Why was Apple forced to stuff so much more battery inside the new iPad just for a Retina display? It’s complicated, but the simple answer is that your display is the most power hungry part of almost any device, and if you increase the pixels, you increase the amount of electricity it needs to draw. The more advanced answer is that for LCD displays, the transistors and circuitry that actually connect the pixels together behind the pixels become a far denser web when you increase pixel density, and therefore the device has to output much more light to shine through. Either way, more pixels = more battery drain.

Apple likes to keep their devices as thin and light as possible, and batteries are one of the heaviest and thickest elements of any device. Keeping this all in mind, why would Apple possibly undertake the huge battery hit of a PPI-doubled HiDPI display in the MacBook line when they could simply bump the resolution of each model up by one level each and still be able to accurately describe them as new Retina MacBooks without taking such a hit in power management?

Resolution Independence

So far, we’ve shown that Apple doesn’t have to increase the pixel density of the current MacBook and iMac displays by that much for them to qualify as Retina, and that taking this more modest approach will actually save battery life.

There’s another reason though why Apple doesn’t have to double the pixel density of Mac displays if it wants to go Retina: OS X is much, much more resolution independent than iOS is.

Resolution independence is when elements of a computer screen are rendered at sizes independent from the pixel grid. In other words, instead of rendering an on-screen element — say, the Apple logo — by how many pixels it takes up, with each pixel in the file taking up exactly one pixel on screen, you magnify it according to how big it is meant to appear on the screen, using as many pixels as required.

What resolution independence does is allow you to display UI elements on a wide variety of display types and have them all look roughly the same size, whether that display is a MacBook Air’s 11-inch 1366 x 768 display, or a massive 27-inch Thunderbolt Display’s 2560 x 1440. And it’s a key part of OS X, a desktop operating system built from the ground up to support many, many different sizes of displays, with many, many different pixel densities.

That’s a lot different than iOS, an operating system that was built to only support two different display sizes: 3.5-inches (iPhone) and 9.7-inches (iPad). iOS is not resolution independent at all. In fact, the exact opposite is true: it’s quite resolution dependent indeed.

So when it came time to give the iPhone and iPad Retina displays, the easiest way for Apple to do it and maintain backwards compatibility with apps that didn’t have Retina support was to simply double the pixel density. In other words, if an onscreen element was only one pixel wide and one pixel tall on the iPhone 3Gs or iPad 2, display it as two pixels wide and two pixels tall on the new iPad. That’s effectively four times as many pixels!

But Apple doesn’t have to do this with OS X. The operating system and all of its apps can already handle numerous display sizes, resolutions and pixel densities just fine. There’s no benefit on the desktop side of things for Apple to simply double PPI across the Mac line across the board. Backwards compatibility will be just fine without.

Conclusion

It’s possible that when Apple takes the stage at WWDC, they’ll blow everyone away with a new 15-inch MacBook Pros rocking true resolution doubled displays, but it doesn’t actually make a lot of sense. The higher PPI, the worse battery life becomes, and the Mac line doesn’t need an iPhone or iPad-style evolutionary leap when it comes to the mere resolution of their displays.

Apple already makes some of the best displays on the planet, and even when they go Retina, Apple won’t have to do more than give them a nudge. Besides, resolution’s only important as far as your eye can discern the pixels: once they are invisible to the eye, who cares? There’s more important things ultimately to the quality of a display — brightness, colors, darks — than mere resolution. Maybe once we stop racing to Retina, we can focus on them.