Let’s start with some basics.
Flash Synchronisation Speed: DSLR with focal plane shutter, no higher than around 1/250th sec (depending upon camera). Film SLR with focal plane shutter, older models around 1/125th sec (depending upon camera). Camera with BTL/Leaf shutter, all speeds to maximum of 1/500th sec. For the shoot on Friday that means a shutter speed NO higher than 1/250th sec.
Daylight Exposure: Daylight exposure is controlled by BOTH shutter speed and aperture and as you know it is possible to capture the shot with numerous combinations that provide the same overall exposure, eg, 500 @ f8 is the same as 250 @f11 and so on.
Flash Exposure: Flash exposure is NOT affected by your choice of shutter speed (not talking about flash synch here). Flash exposure IS affected by your choice of aperture.
Flash unit: Most flash units have variable output that can be raised or lowered, ie, more output or less.Interestingly this occurs because the duration of the flash can be controlled as opposed to its intensity. The effect is the same however and will not affect our discussion here.
There are two paths we can take when using flash on location, we can use it to fill shadows slightly to increase detail in those areas or we can use it as a main source and let daylight be the fill, this latter choice is effectively what you used on Friday. This choice allows us to create a lighting scheme rather than to simply accept the location lighting as we find it.
Using Flash as Fill: Take an incident reading to determine daylight exposure, let’s assume it to be 60 @ f11, then adjust the monobloc flash unit output and/or position until an incident reading of the flash is 2 stops less than the daylight, f5.6. Your exposure will still be 60 @ f11 but the flash will provide some light to fill shadows only.
Using Flash as Main Source: (what you did on Friday) The task here is to underexpose the daylight by 1-2 stops compared to the flash. Lets assume you wish to underexpose daylight by 2 stops and the daylight incident reading was 60 @ f11, set the camera to 250 @ f11. This does two things, firstly 250 UNDEREXPOSES daylight by two stops, thereby reducing daylight to a fill level, sky is dark etc. Set-up your desired monobloc unit and adjust the position, size and output until an incident reading of the flash on the subject is f11.
Result: Subject is lit with flash which is correctly exposed, daylight exists at a level ¼ that of flash (due to the faster shutter speed) which results in it being a fill light, this has the added advantage of the sky and other background areas to be rendered darker and perhaps richer with the subject cleanly illuminated by the flash, your preferred lighting, size, quality etc.
You’ll remember that Andrew tended to use f11 or at least was aiming for that, depth of field is an issue so f11 would be a good choice. He also recommended you set your DSLR ISO as low as possible. A higher ISO could result in a faster shutter speed than 250 being required to keep daylight exposure down but of course this would be outside your flash synch speed limit.
Summary:
Set ISO as low as possible, ISO 50 perhaps
Calculate daylight exposure and underexpose daylight 1-2 stops with shutter speed ensuring you don’t go higher than the flash synch speed, ie, if daylight is 60 @ f11, use 125 @ f11 or 250 @ f11
Adjust flash size, position and output until your daylight f-number is achieved, ie, f11.
You could also start by setting either 125 or 250 on the camera, calculating a daylight exposure that used a shutter speed 1-2 settings slower and then set-up the flash to match whatever the daylight aperture was, it’s exactly the same just approached from a different angle.
Looking at the metadata on the files from Friday and recalling some discussions about cameras used on the day there were some limitations on flash synch and ISO which affected the exposure decisions.
Remember: daylight is controlled by shutter speed and aperture, flash is controlled by aperture.
