Gosh, almost any basic flash would work...just "a flash unit" that can be synchronized to the camera...I honestly think that some of the old-school Auto-Thyristor flash units that use analogue-style rotating dials or linear, slide-rule-style readouts are the easiest and most-logical types of flash units for portrait type work. Why? Because of their mechanical DIALS or their mechanical slide-rule readouts, the photographer can literally SEE all the distances, and the appropriate f/stops right across from one another, and the entire gamut of settings is visible from one end to the other. This is in contrast to some of the digital readout flash units, where only one bit of information is presented at any one time. Some of the flashes that use the older-style readouts would be made by Metz, or Sunpak, or the older Vivitar 285 design.
Even the camera's built-in pop-up flash could have been utilized here, to help lighten the shadows, and to make the lighting less contrasty. I hate to recommend things like the Canon 580 EX-II as being the required type of flash; although it is a very fine flash,and well-built,and well-designed, in the situation like shot #4, with a family in front of a strongly backlighting window, virtually ANY flash unit could have been used to help brighten up their shadow side--an old used Phoenix, a old, used Crown, or an old, used Vivitar or Nikon flash, hooked up to your Canon via a synch cord and a hotshoe PC outlet adapter...
Please take a look at this Vivitar 285HV rotary control dial photo, and yo can see, literally SEE, the right exposure and distance settings on the dial.
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