In situations with high dynamic range or backlit subjects its useful sometimes to use spot metering and exposure lock. Set the metering on spot or partial, depending on the desired area to be metered, half press the shutter release and lock exposure with the * button. After that, you can recompose and shoot as Canon500D said. Be aware that when you have pressed the shutter button you also activated AF. If you want the focus on the metered spot don’t release the shutter button after you have locked exposure, recompose and take the picture. If you want your focus elsewhere, release shutter button after AE lock recompose activate AF again and take the picture, all within the 4 seconds time.
By using spot or area metering you lose the correct exposure for fill in flash. So if you want to compensate for the back light by using the flash, you have to use evaluative metering.
There is a caveat though with recomposing after locking the focus. If you have a shallow DOF you may lose focus, so the correct procedure is to compose and set the appropriate focusing point so you don’t have to lock the focus and recompose before taking the picture.
Canon PowerShot S100
Canon 50D , SIGMA 10-20 f3.5 ,Canon EF 24-105 L IS USM, Canon EF 100/2.8 macro Canon EF 50/1.4 ,Canon EF 85 f1.8,Canon EF-S17-85 4-5.6 Old Tamron 28-300 inherited from my Canon Rebel G film camera