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Unibody MacBook Pro Q&A
Update Published November 11, 2016
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Are there any adapters or "hacks" that make it possible to connect a second external display to a MacBook, MacBook Air, or MacBook Pro model?
By default, all MacBook, MacBook Air, and pre-Retina Display MacBook Pro models are capable of simultaneously supporting the internal display at its native resolution and a single external display either in "mirroring mode" -- where the contents of the internal display are duplicated on the external display -- or in "extended desktop/dual display mode" -- where each display is treated as additional work space.
By default, all Retina Display MacBook Pro models already can support at least two external displays in addition to the internal display at a variety of resolutions depending on the specific model.
However, there are a number of adapters that can be used to "hack" MacBook, MacBook Air, and pre-Retina MacBook Pro models to support a second external display, too.
USB Display Adapter Options
One easy way to attach an additional external display to these Macs is to use a USB to VGA or DVI adapter. When this Q&A first was published, there were a variety of adapters available that supported a maximum resolution of 1600x1200 or 1680x1050. Since that time, new generations of adapters have been released that support larger displays with a maximum resolution of 1920x1200 or 2048x1152.
There are a variety of these adapters available, but three guaranteed to be both (1) Mac compatible and (2) support a maximum resolution up to 2048x1152 include the Kensington Multi Display Adapter USB 3.0 (no longer available), IOGear USB 3.0 to DVI External Video Card (no longer available), and OWC USB to DVI/HDMI/VGA to Video HD Video Display Adapter.
In basic terms, these inexpensive adapters all work in essentially the same way by compressing and decompressing the video signal in order to utilize the limited bandwidth provided by USB 2.0 or USB 3.0. All make it possible to support an additional external display on any Mac notebook running Mac OS X 10.6 "Snow Leopard" or higher, as well. Some earlier adapters also may support Intel-based notebooks running Mac OS X 10.4 "Tiger" or Mac OS X 10.5 "Leopard," but these vintage adapters are difficult to find these days.
Hypothetically, you can support as many as four displays via USB on a Mac, so for compatible MacBook, MacBook Air, and MacBook Pro models you can support between one and three USB displays depending on how many USB ports your particular notebook is packing.
USB Display Adapter Limitations
Being able to connect additional displays to a MacBook, MacBook Pro, or MacBook Air sounds fantastic, but it still is important to be aware of the limitations of the technology. The bandwidth provided by USB is insufficient to "fully support" OpenGL 3D hardware acceleration, and as a result, there is a "lag time".
Consequently, these adapters do not work well with gaming or other applications that place a great deal of emphasis on graphics performance such as video playback or video editing. However, the lag time is unlikely to be bothersome for basic productivity -- word processing, spreadsheets, web browsing, chat, and so on.
It also is quite possible to use more graphics intensive applications on the "main" external display (connected by DVI, mini-DVI, micro-DVI, Mini DisplayPort, or Thunderbolt depending on the model) and less graphics intensive applications on a display connected via the USB adapter.
As part of an excellent review of an earlier adapter that has now been discontinued, but that works in the same way as current products, MetkuMods provides this helpful video of the lag time:
From watching the video you should be able to determine whether or not the lag time is bothersome to you.
Alternate Additional External Display Option
There also are products available that do not result in lag time for an extra external display, like the Matrox TripleHead2Go, which supports three 1920x1080 monitors at 50 Hz.
The TripleHead2Go requires three displays of the same resolution (and that support 50 Hz) and the displays essentially have to be placed immediately side-by-side (as they behave as one large display). This adapter also is expensive. For most users a USB to VGA or DVI adapter is likely to be sufficient.
However, for users who need two or more external displays on a MacBook, MacBook Air, or pre-Retina Display MacBook Pro and find the lag time demonstrated above to be unacceptable (those interested in gaming, video playback, or video editing, for example), but who do not want to consider a Retina Display MacBook Pro (perhaps because they find the relative lack of internal upgrade options a major disadvantage of upgrading), this alternate option remains well worth consideration, too.
USB Display Adapter Conclusion
Based on years of hands-on use of the OWC adapter, EveryMac.com can safely conclude that the lag time is not an issue for general productivity work. If you find yourself using two displays -- one connected via Mini DisplayPort Thunderbolt and another connected via a USB adapter, for example -- it's easy to forget which display is which as usually there effectively is no difference.
If you're not using a USB display adapter, you really should give it a try as an extra display can provide a substantial boost to your productivity.
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