Originally posted by future_retro wait did you mean the flash is compensated automatically?
Perhaps I'm not understanding your question, but my answer would be yes.
There's a flash technique that's called "Dragging the shutter".
You meter the scene for the background, set your aperture and shutter speed to expose this correctly, then you set up the flash to meter for the flash only for a subject in the scene. P-TTL makes this technique very easy, but remember P-TTL requires using either "A" series or AF lenses to work correctly.
Try this -- go to a room that's lit by a single lamp in a far corner, meter the scene so the room is reasonably exposed and take a shot in M mode. Have someone stand in the middle of the room and take a shot with the same settings. The person will be underexposed, somewhat silhouetted by the rear lighting. Now pop up the flash, keeping the same settings, and take another shot. The room (now the background) should still be exposed pretty much the same, but the person should now be reasonably exposed because of the flash. If the person is still underexposed, it should be by less, and you could probably correct this by switching to center weighted or spot metering. Don't worry about blur, IQ, composition, or any other quality but exposure in this test.
What just happened is that you set up the camera (aperture and shutter speed) to expose the background, then P-TTL metered the flash with the preflash to give you a reasonable exposure for the previously underexposed person you added. You've also discovered how to prevent one of the most common objections to flash photography -- a well lit subject but a blacked out background.
If you extrapolate from this, you can use select focus points and the camera's ability to lock AE to AF, so you can select where the flash will be metering and focusing other than just in the center. External flash with flash modifiers gives you even more versatility and control of the light. You can also vary the flash's contribution to the lighting with Flash Ev compensation.
There are, of course limitations -- if the shutter speed is too slow, you might get a blurred background -- but that can be corrected by using higher ISO to keep shutter speeds up (up to the 1/180 max synch speed) -- and with the flash giving you a properly exposed subject, noise is usually much less than you would expect for a given ISO.
You can, of course, use a manual flash, and GN calculations or experimentation, to get to the same place -- but P-TTL makes it pretty brainless, and I personally like that. . .
Scott