So I've been playing around with aperture and shutter speeds, most recently this: 15 second exposure, 18mm, f/6.3, flash camera left about 45* set to 1/32 +0.3 power on the rear curtain. so that's an extreme example of using shutter to expose the background and aperture and flash to control light on the subject. here's what i want to do with 'normal' portraits, or at least not ones that are 15 seconds. when part of the goal of the picture is to separate the subject from the background would this work to help aide that? i want to get the shutter speed setting right to expose the background so the histogram is in the middle. then i want to take that same shutter speed and add in the flash and adjust the power and aperture so the flash portion (on the subject) is exposing to the right of the histogram. is that a normal practice? does it end up looking nice, weird, or is it as i suspect a case by case basis?