While an external Mac monitor seems like an obvious use for an iPad, the solutions to-date have been Wi-Fi based. For some applications, that's been OK, but for many others it's just too slow. Duet Display promises to banish lag by using a direct USB connection '" so how does it deliver?
Installing the Mac app is straightforward.
Duet Display uses a pair of apps to make the magic happen. You install a free app on the Mac side, and then a paid app on the iPad. Connect the iPad with a USB cable, launch the Duet app there, and it's like you've connected a separate monitor. Using System Preferences > Displays, you'll see a second screen, and can access different resolutions in the window that appears on your iPad, just like you can when you connect a normal second display.
As usual, Option-click on the Scaled radio button to access additional resolutions.
One issue that I had was that a non-Retina resolution was selected by default, but I was able to choose the correct 'HiDPI' setting from the list of options as expected, and it was a little slower. This may well explain why a non-Retina resolution was chosen by default.
System Preferences > Displays shows the expected 2 screens.
At home, a regular second monitor would be larger, more permanent, and more convenient, but on the road or at a cafe, your iPad can now fill the same role. It's worth noting that in a hotel room, a decent TV can also fill the role of 'second display', but that's not always possible or ergonomically appropriate.
It is possible, but slow and not recommended: MacBook Pro + HDTV + iPad = 3 screens.
It's also possible to use an iPad with Duet as the only screen, but because it can only work after you've logged in, you'd have to set your Mac to log in automatically.
Does it work? Yes, but with some limitations. I found that there was (despite claims) a slight lag in general UI responsiveness, but to be fair, it's better than Wi-Fi, and I don't have the newest gear. My MacBook only has Intel 4000 graphics, and my iPad mini (Retina) that it's paired with isn't the fastest iPad either. A Mac with a real GPU and a newer iPad Air would probably perform better, but there is still a performance hit compared to a regular external screen.
While you can tap on the iPad to click or drag, and use two fingers to right-click or scroll, the clickable targets in many OS X apps are too small to comfortably hit with your finger. That's OK, but you'll sometimes need to use the Mac's input devices instead.
Photoshop behaved quite well, though'"and if you want 'full Photoshop' with a touchscreen, it's certainly a much cheaper option than a Wacom Cintiq. Again, on my setup, there was a small amount of lag, but it would be acceptable for many users.
Using Photoshop to paint in some extra cloud shadows with my finger.
I found that if I put Final Cut Pro X's Viewer on the iPad screen, Scopes showed as a white box, and with or without them, frames would sometimes drop during playback. Certainly, in Retina mode, with an effect applied, frame drops were more likely.
Using the iPad as a Viewer has some issues; the scopes don't work properly yet and it drops frames.
Using the iPad for Events rather than the Viewer worked better. However, since you can't hover on an iPad, you'll have to use your Mac's trackpad or mouse (or keyboard shortcuts) to skim your footage. Alas, when skimming, the current thumbnail frame under the skimmer showed as a white frame, like Scopes did. With any luck these issues may be fixed in a future update, but full-screen video is always going to be the hardest problem to solve.
This white frame isn't normal, but usually you're watching the viewer and not the thumbnail.
Inevitably, there are limits on how well you can use an iPad as a true second screen. While it's not perfect for full-motion video'"at least not on my gear'"it could be good enough, and just fine for other tasks, like viewing full-screen photos while processing in Aperture or Lightroom, displaying a script while recording your main screen, or simply extra screen space for word processing and other general usage.
The verdict? If you're a dual-screen fan who travels, it's faster than Wi-Fi, and should work well as a second screen on the road. For most applications, it's definitely a useful tool, and likely the best solution of its kind.
Pros: An iPad makes a perfect temporary screen on the road. Ties in with system controls when possible.
Cons: Network-free but not truly lag free. Video editing has bugs and limitations. Older iPads or Macs may not perform so well.