markbarendt
Member
... Adequate exposure of a scene gives faint density in the shadows.
This statement is true but the scene is artificially lit, there's not enough ambient light on the OP's set to fill the shadows on the film, the ambient exposure "falls" somewhere down at the bottom of the film's toe.
With artificial light a few inches to or from the source can make a difference of a full stop.
In the OP's box speed shot the fan is in the right spot, the floor is simply farther away so everything on the floor gets less light per square inch from the strobe and because of that those subjects fall at darker points on the film curve.
Raising flash power does provide more light to the floor and gets those floor subjects up off the toe, the problem is that it also moves the placement of the OP's black card (which is indicative of the portrait sitter's black hair or shirt) to right on the film curve. The most important black point in a portrait is being forced up into a mid-tone grey in order to get detail on the floor.
That strategy can create a usable negative but that negative would require burn and dodge to fix the print.
In the OP's case one fix is probably a large white reflector (bed sheet) hung on the opposite side from the strobe to bounce light back in and fill the dark areas, along with that moving the main light back away from the subject may help too.
More options include adding more ambient light or using a longer shutter time so that ambient light has a bigger effect.