If you want the portrait to pop (i.e. have the background out of focus, and the subject apart from it) then use a large aperture (bigger hole, lower number), make sure the subject is a good distance from the background and the background does not have anything distracting in it (bright spots, things growing out of their head, etc). If you want a photograph that places the subject in their surroundings (e.g. showing someone in the context of their craft/trade) then you need to make sure that you have a greater depth of field (smaller aperture,smaller hole, higher number) to get more into focus, and need to work harder on the lighting.
Remember, when you take a flash photograph, you are actually taking TWO pictures (or two exposures) at the same time:
- The ambient light photograph
- The flash light photograph
The first is controlled by the combination of duration and aperture (as you make the aperture smaller, you increase the duration for the same exposure). For portraiture, your most important setting is usually your aperture as this controls depth of field. This is a key creative decision.
The second exposure is controlled by the power and duration of the flash. A flash typically lasts for around 1/1000th of a second. (By the way, the duration of the exposure on the camera is limited by the fastest speed at which the whole sensor is exposed at the same time – as you increase the speed, shorten the duration, cameras start to move the closing shutter over the sensor BEFORE the leading shutter has completed its journey, thus the sensor is exposed in a moving slot rather than completely, and if a flash of 1/1000th duration went off, it would only be seen on the sensor through the slot).
If you are using a flash in an automated mode and the camera is controlling the flash power, then the camera will increase the power when you bounce the flash off of another surface. Its “judgement” may not be right, and you may have to make some adjustments. Bouncing the light spreads it out and softens it. We are most used to seeing people lit from above.
