taking portraits outdoors typically is associated with dark shadows whether it is in the sun or on a cloudy day, particularly shadows around the eye sockets

flash can be used as a fill-in to lighten these shadows

most of us do not have sufficiently powerful flash units to over-power the sun and make the sun a secondary light source - hence this is not discussed here.

this is the prime issue for most of us

getting a sufficient flash output to provide enough fill-in flash in the sun can be difficult

for most of use doing portraiture, this is a joint prime issue and we need to have as wide open an aperture as possible

when shooting in sunlight, the sunny 16 rule indicates that at ISO 100, at 1/100th sec we need an aperture of f/16

this is a technology invented by Olympus in the 1980's which they call Super FP mode, most other brands now call it HSS

this allows almost any shutter speed to be used and thus wider apertures, BUT, there is a big cost - the flash output decreases substantially as shutter speed gets faster

it is thus NOT useful for overpowering the sun or increasing the ratio of your flash output to sunlight intensity.

nevertheless, it can be handy to use, particularly if you have a relatively powerful flash unit

PocketWizards remote flash controllers have created a new function they call hypersync which allows some flash-camera combinations to effectively gain a faster shutter speed without decreasing flash output as HSS mode does

unfortunately, this requires use of the relatively expensive PocketWizard transmitter and receivers, but it it will only work with some flash-camera combinations (only Canon and Nikon at this stage), and even then the fastest shutter speed possible is different for each flash-camera combination - the Canon 1D Mark III tends to offer one of the better hypersync speeds thanks to its shutter design. Later cameras tend not to perform quite as well.

using this technology, you can generally use shutter speeds of 1/320th-1/400th sec with full flash output and full frame coverage of the flash

some cameras allow you to select a shutter speed faster than the “x-sync” speed while using flash, although most cameras will only do this if you place a flash unit, or flash adapter with only the central PC sync pin and no TTL pins

flashes designed for the camera will generally not allow this to happen as you might do it accidentally and have your photos ruined by a section of your image not being lit by the flash - this section becomes larger as the shutter speed becomes faster.

nevertheless, it is possible to use shutter speeds around 1/400th sec on many cameras in manual flash mode and compose the subject so that the unlit areas of the scene fall on the background only so are not obvious in the image.

on most new digital cameras, this unlit strip will be on the long side corresponding to the top of the camera

this is the manufacturer's fastest shutter speed for flash sync

film SLR cameras generally have sync of 1/60th-/125th sec

entry-level digital cameras and many using remote TTL flash modes will generally have this set to 1/160th sec

pro dSLRs tend to have faster x-sync of 1/200th or 1/250th sec

leaf shutter cameras (some film and the occasional digital camera), allow x-sync at 1/500th sec

hopefully future cameras with global electronic shutters will allow much faster x-sync

make sure you set ISO to LOW (with firmware upgrade) as this gives ISO 100 and allows wider lens apertures in sunlight - add polariser or ND filters to gain wider apertures

use a relatively powerful flash such as the Olympus FL-50R or you might be able to get away with a less powerful flash such as the FL-600R or FL-36

set it to FP TTL (HSS mode)

set camera PASM to A mode

set aperture to your desired aperture for the depth of field you need

set flash exposure compensation level to desired amount (eg -1EV)

use a single pin flash adapter connected to a powerful flash such as a Metz 45CL-4 should allow reasonable fill-in up to around 3-4m

set PASM dial to M for manual so you can control aperture, shutter speed and ISO

set ISO to LOW if you want shallow apertures, otherwise ISO 200 will give slightly better image quality

set shutter speed to 1/320th or if subject suits, use 1/400th or 1/500th sec (remember to compose with unlit area in mind!)

use a ND or polarising filter if you want shallow apertures

set aperture according to your desired ambient light exposure

set manual output on the flash to give the desired amount of flash effect for the flash to subject distance