(Info on older software is included below for possible benefit of other readers. Apple has changed the built-in camera's name on newer Macs from "iSight" to "FaceTime" and then to "FaceTime HD." Regardless of the name of your built-in camera, the following info applies.)
How you select among cameras connected to and recognized by your Mac depends on which application you are using to operate your iSight. Here is how camera selection works in a few examples:
(1) For iChat, you can choose which iSight you use in the "Camera:" choices bar in iChat > Preferences... > Audio/Video that appears when more than one compatible camera is connected. Although your camera choices will be different, the choices bar will look something similar to the iChat Preferences settings shown here:
If you cannot see the "Camera" choices bar, your Mac is recognizing only one (or none) of your cameras. In that case, consider the suggestions from http://support.apple.com/kb/HT2090
This particular choices bar solution applies ONLY to iChat. Most other applications also have settings that allow you to choose which camera to use. However, they do not all work the same way.
(2) For instance, iMovie HD's camera choice is NOT set via Preferences. When you have more than one compatible camera connected, iMovie uses a drop-down menu choice something like this (depending on which version of iMovie you use):
Note: Because I had no built-in iSight connected when I made this example, and because my external iSight was not connected, the drop-down menu showed only "Time Lapse." Because your Mac has a built-in iSight, your built-in iSight would show in the drop-down menu even when no other camera is connected. Connecting an additional Mac compatible webcam should allow you to choose either camera.
iMove 9 (from iLife '11) uses a different camera choice button shown in this article:
iMove 8 (from iLife '09) is slightly different as explained in this article:
In general, you can use your Mac's help for the application in use to find out how to select among more than one connected camera.
(3) Photo Booth in Snow Leopard 10.6.x uses the Photo Booth > Camera menu command to select which camera to use.
Note for readers with older Mac OS X: The previous version of Photo Booth that came with Tiger (10.4.x) is such a simple, basic app that it offers no menu selectable choice. Unless your built-in iSight is already occupied as the camera being used by some open application before you launch Photo Booth, Photo Booth will use ONLY your built-in iSight. However, 10.4 users can employ the trick from ¶ 4 of http://docs.info.apple.com/article.html?artnum=302781 to let Photo Booth use an external camera.
(If you have an external Firewire camera connected, it may be used in preference to any USB webcam. If that is a problem for you, merely disconnect the problem Firewire device.)
(4) For FaceTime, launch the app and click the Video menu item.
If your Mac recognizes more than one connected compatible camera, a "Camera" section listing the cameras from which you can choose will appear there. Clicking on the desired camera name will let you select the one you want as shown in this image from my Mac Pro and external Apple Display:
If your Mac recognizes only one compatible camera, the "Camera" section will not appear in the Video menu, but FaceTime will automatically use the connected camera that is recognized by OS X.
(5) For other apps, see Help for each app for info on how to select your desired camera.
Bryan Schmiedeler wrote:
My question is about Preview, and there is nothing in Preview's Help to address this issue.
Searching Preview Help for "camera" returns several tpoics including this one:
Import images from a camera
You can import photos from many digital cameras and devices that contain cameras, like your iPhone. Then you can edit the images in Preview.
None of my connected webcams are recognized by Preview. I can IMPORT previously saved pictures from my connected iPad. However, I cannot "take" or "capture" new images using Preview.
Therefore, I do not think you can capture images directly into Preview from any camera. Rather, it seems likely that you can only "import" images that have already been saved. Once you have imported an image that has been saved elsewhere, you can copy or edit it.
Use Photo Booth or some other "capture" application to take pictures using your webcam(s). You can then open those files with Preview (or iPhoto) for editing.