When shooting outdoors in sunlight, it is better to turn people away from the sun, so they do not squint.
So you do it as in this recent shot (with the Hon. Minister Harinder Takhar of Ontario in the centre):
As you see, you now need a flash to light up the subjects. Else, they would become silhouettes.
And the constraining factors are:
- First, the power of the flash. If it is not as bright as the sun, roughly, then you will get an insufficiently exposed image.
- Second, metering. To check if this is an issue, turn the flash to manual, full power, and check if you are now getting an overexposed image.
In this image I used 1/250th second (the fastest I could do with the 1Ds) and f/11 at 200 ISO. Why?
- 1/250th, since higher speeds would have meant high-speed flash would be needed, which would reduce flash power.
- f/11, to get a dark enough background.
- 200 ISO, since higher ISO would not have worked: lighter backgrounds would have resulted, necessitating smaller apertures
That last part is important: raising ISO is not a solution to the need to “nuke the sun”. Only more flash power is.