What makes the iPhone camera so great is how fast you can get it out, take a shot, and end up with a great looking photo. Apple's done a lot to make this possible, including adding fast camera access to the lock screen software. What Apple hasn't done, though, is add fast camera access through hardware. And, with iOS 10 on the horizon, I kinda wish they would.
With fast camera access on the lock screen, all you have to do is wake up your iPhone, swipe up the screen, and shoot away. That's still multiple context changes, though. You go from pressing a hardware button to swiping up on a specific region of the screen. That's perceptibly slower and cognitively heavier than a repetitive hardware action.
Buttoning down
Other vendors already provide just that functionality. Click the Home button twice on a Galaxy S7 and the camera launches. Click the volume down button twice on the G5 and same. Click the power button twice on a Nexus and... you get the idea.
On iPhone, double clicking the Home button from a sleep-state is already assigned to Apple Pay. So that's out.
Volume up or down would be convenient, since those buttons will already take a photo when the Camera app is live. So, double clicking to launch, then clicking again to shoot, requires no context change whatsoever. That's the definition of fast.
Some may worry, though, that double clicking a volume button would collide with controlling the volume. Last thing you want is to try and quickly lower a loud tune or show only to have the camera launch unintentionally and kill the sound entirely.
Apple could simply suppress Camera launch when audio is playing, though that would create inconsistency. The company could also use the sleep/wake button instead, since that has no double-click function as of yet. And it's not like anyone is going to try and press it multiple times to sleep/wake the iPhone faster anyway. It's on a different side than the volume buttons used to take photos, though, so it won't be as fast nor as coherent. But it won't collide.
Faster forward
My preference is for a volume button to launch the Camera app. But it's just that: my preference.
I'm not fussy and will gladly go where the usability data points. I'd simply like to pull my phone out and start taking pictures even faster. That's why I very much hope Apple maps a hardware button to the camera launcher. So, come iOS 10 — fingers crossed! — I'll be able to do just that!
I've filed this feature request with Apple: rdar://25671012.
