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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-equipped 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, the Retina Display MacBook Pro models -- the MacBook Pro "Core i7" 2.3 15" (Retina), "Core i7" 2.6 15" (Retina), and "Core i7" 2.7 15" (Retina) -- support a maximum resolution up to 2560x1600 on two external displays via Thunderbolt and another display up to 1920x1200 via HDMI. This means it can power as many as three external displays by default. However, if you have enough physical space, you still could theoretically connect additional displays with third-party adapters.
USB Adapter Options
One easy way to attach an additional external display to these Macs is to use a USB 2.0 to VGA or DVI adapter. When this Q&A was first published, there were a variety of adapters available that support a maximum resolution of 1600x1200 or 1680x1050. Since that time, a new generation 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 Mac compatible include the Gefen USB to DVI Graphics Adapter, EVGA UVPlus+19 Adapter and the OWC USB to DVI/HDMI/VGA to Video HD Video Display Adapter.
In basic terms, they 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, each are inexpensive, and all make it possible to support an additional external display on any Intel-based Mac running Mac OS X 10.4 "Tiger" or higher. These are not compatible with PowerPC-based Macs.
For the MacBook Air models, you can connect a second external display with a single adapter (or a second and third external display with two adapters for models with two USB 2.0 ports), on the 13-Inch and 15-Inch MacBook and MacBook Pro models you can connect additional displays with two adapters (using both USB 2.0 ports), and for the 17-Inch MacBook Pro models you can use all three USB 2.0 ports and connect as many as three additional external displays with three adapters.
USB 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 both generations of the technology. The bandwidth provided by USB 2.0 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 is quite possible to use more graphics intensive applications on the "main" external display (connected by the 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.
In a fantastic review of the first generation of the Gefen product that absolutely should be read in its entirety, MetkuMods provides more technical information about how these adapters work and real-world performance details, including this helpful video demonstration of the lag:
Alternate Dual External Display Options
There also are products available that do not result in a "lag time" from Matrox -- the DualHead2Go -- and CinemaView -- the Duo. Although EveryMac.com has mentioned the DualHead2Go previously -- and one advantage of this product is that there is a version compatible with PowerPC-based Macs -- for most Intel-based Mac users a USB 2.0 to VGA or DVI adapter is likely to be sufficient.
The DualHead2Go and CinemaView Duo both require two displays of the same resolution (CinemaView formally requires the displays to be the same model, even) and the displays essentially have to be placed immediately side-by-side (as they behave as one large display).
In addition, the CinemaView Duo only supports displays with a Mini DisplayPort (the DualHead2Go nicely supports Thunderbolt, as well). However, these adapters cost more than a USB 2.0 to VGA or DVI adapter.
Nevertheless, for users who need two or more external displays on a MacBook, MacBook Air, or pre-Retina Display-equipped 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-equipped MacBook Pro (perhaps because they find the relative lack of internal upgrade options and glued battery unacceptable), these alternate options remain well worth consideration, too.
Site sponsor Other World Computing sells an adapter that supports 2048x1152. The adapter is offered as part of a kit that includes a DVI to VGA adapter, a DVI to HDMI adapter, and the necessary drivers.