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Why the 3.5" Form Factor Still Matters - Even in the Age of the iPhone 5S and Later

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It’s good to be regularly reminded about non-obvious testing targets that you might not have been thinking about. Here are some extra reasons you should continue targeting the 3.5” form factor for the foreseeable future.
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In September 2012, Apple unveiled the iPhone 5 at a formal press event. The new iPhone offered five rows of icons in addition to the shortcut bar, aka the dock, situated at the bottom of the screen. Its new 4” form factor would, promised CEO Tim Cook, “fit beautifully into your hand.” That 4” screen rocked a lot of worlds in the development community. Many interfaces that “just worked” on 3.5” screens with their 3:2 aspect ratio no longer fit on the iPhone 5 with its 640x1156 display.

Although Apple had introduced the first generation of Auto Layout tools a few months earlier, many developers had not yet taken advantage of the new technology to build self-adapting interfaces. With the iPhone 5, it became clear that new form factors could and would appear at any moment. Auto Layout wasn’t just a good suggestion – it was now a critical development tool.

Since that announcement, developers have adapted their interfaces to fit not just the original iPhone 3.5” screen and the updated 4” size, but to work with the iPad family as well. Auto Layout, with its rule-based view placement, has become a key player in the development suite. It enables designs to adapt to screens whose aspect ratio and available pixels offer a wide range of targets.

Now, in early 2014, analyst eyes are again looking forward trying to predict if Apple is perhaps ready to introduce yet another form factor this year – one that might take the iPhone to yet another new shape. Rumors range from a modest 4.7” bump to a (relatively) jumbo 5.5” screen. There are admittedly few facts on the ground to back up any speculation. This time, no matter what Apple may announce, more developers are better prepared for the design earthquake that may soon hit.

At each yearly iteration, some hardware falls off the map. These days you won’t find a supported iOS platform that doesn’t, by default, offer a built-in camera. Although end users can still disable cameras and deny access to onboard equipment, you can pretty safely deploy software assuming that a camera exists for nearly all installations.

The same can almost be said for Retina. The only outliers for the current generation are older iPads. The iPad 2 was essentially end-of-lifed on March 18, 2014, and the original iPad mini’s days are numbered. Non-Retina development now has a countdown clock running, and it’s not hard to predict that @2x assets will become the baseline for iOS art in the foreseeable future.

The longer 4” screen has been highly successful, for both the iPhone and iPod touch families. It’s generally accepted that the iPhone 4 and 4S could represent the last hardware models to express smaller 3.5” sizes. What many developers fail to recognize, however, is how the 3.5” form factor – or at least its intrinsic 3:2 aspect ratio – may continue to play a role moving forward. There are two distinct ways this may play out.

First, with new hardware, Apple might introduce a line that returns back to the 3:2 aspect ratio regardless of diagonal inches. It’s not unreasonable to consider a possible 720x1080 device, for example. As diagonal sizes expand, a chunkier screen could support 720p video – not bad for a subtablet that lives in your pocket.

There also is a second reason that the 3.5” form may live on, at least in the immediate future. That’s because it remains the standard for the iPhone-on-iPad presentation.

Developers are familiar with the 3.5” and 4” geometry differential. This is what you see in Figure 1, which shows a barebones application that displays a 3.5” rectangle. The screen shots include both an original and newer generation target. As you’d expect on the 4-inch simulator, the app includes excess space above and below the rectangle.

Figure 1 A 3.5" frame perfectly fits an old-style iPhone. It leaves excess space to the top and bottom on a 4" target

It’s iPad targeting that many developers forget about. When compiled for iPhone-only targets and run on an iPad, applications look like the one you see in Figure 2. As the figure demonstrates, the default iPhone-on-iPad presentation uses 3.5” display, not 4”.

Figure 2 iPhone-only applications currently display on tablets using a 3.5" form factor

In development, it’s important to consider all geometric scenarios an application may encounter. As iPhone-only deployment has grown unpopular, iPhone-on-iPad represents a development target that is often forgotten. An app developed solely for iPhone use should still be thought about and tested for tablet scenarios because there is a customer base that will use it exactly that way.

Of course, there’s no reason Apple could not change the 3.5”-on-tablet rendering in a future iOS update. In fact, there are plenty of good reasons why they might switch the behavior from 3.5” to 4” in iOS 8, especially if Apple commits more strongly to the newer screen ratio. But there are just as many good reasons why they might not, especially if this year’s hardware doesn’t follow a 4-inch geometry.

Speculating on future hardware is always a dangerous and frustrating exercise. So leaving speculation aside, it’s good to be regularly reminded about non-obvious testing targets that you might not have been thinking about today. As a loyal iPhone 4S owner, who’s hoping to delay upgrading her phone for a year or two more, it’s good to know that there are developers out there who continue to keep my phone and me in mind.

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