See wiki's description of the iSight camera, specifically the section titled Built-in iSight.
...the built-in iSight has a plastic lens, is fixed-focus, and uses a CMOS Active pixel sensor, rather than the CCD used in the external iSight. With the LED-Backlit MacBook Pros, Apple has moved to using a sensor capable of 1280x1024 pixels.
[EDIT] This question was "solved" while I was writing.
I leave my answer here for possible benefit of other readers.
(Thanks to curly1 for the ★ "helped" feedback!)
curly1 wrote: ... should I conclude that there is no longer a useful mega pixel number attactched to these built in FT cameras?
(1) I think you have come to a good conclusion, curly1.
Although the megapixel "number" doesn't seem particularly useful to me, either, it is possible for you to determine it with certainty for your particular iMac. Here is how to do it.
• Make a short test recording using your Mac's QuickTime Player (v.10.x) with the "Quality" (QTX's label for resolution) setting at "Maximum" (your max hardware resolution.) If you don't already know how, this screen shot shows how to access QTX's Quality settings:
• After you stop recording, use the QuickTime Player > Window > Show Movie Inspector menu command to open the Inspector. You will find the resolution pixel numbers in the "Format" section of the Inspector window as shown in this screen shot example (I have x'd out my computer's specific information):
• Finally, multiply your iMac's 1280 x 1024 pixel count and dividing by 1,000,000. Megapixel size is customarily rounded to no more than one decimal place, so yours is properly computed to be 1.3 MP.
(2) Now that you know that your iMac has 1.3 MP, it is important that you understand that hardware resolution is the maximum your iMac can use. In practice, software controls all Mac cameras. Different software applications have different maximum software resolutions. Therefore, the fraction of the maximum 1.3 MP hardware resolution your iMac is actually using is dependent on which of your camera-enabled software applications you use.
Here are a few examples of other software resolutions that common apps provide:
• As shown in the iChat System Requirements article, depending on your system,
• FaceTime can record 720 x 480 on my Mac.
• iMovie 9 (sometimes called iMovie '11) on my Mac offers a choice between
standard definition and 720p Hi-Def by using the "Video Size" choices bar
shown in this screen shot image:
• To test other apps, make a short test recording at maximum quality/size/resolution
settings and then use Finder > File > Get Info to see the resolution in the
"More Info" section of the Get Info window.
Note for other readers: Other inbuilt cameras may give lower maximums.