Why the front row is bright and the back row is dark
Softness and even coverage are separate conditions. Use row distances to understand uneven illumination.
Each person has a different source distance
With a 1 m gap between rows, a source 1 m from the front row is 2 m from the rear. Placed 4 m away it is 5 m from the rear. In a point-source model the rear receives about 25% versus 64% of the front-row illumination. Large nearby surfaces, beam angle and room bounce can depart from this model.
Moving the source farther needs a second decision
Greater distance makes row depth smaller relative to the overall source distance, helping uniformity. The same emitting surface also appears smaller and may need more output. Reduce unnecessary row depth and inspect the center, both edges, front and rear instead of trusting a single central reading.
Front-to-back illumination calculator
A single source and both rows lie on the same axis. This assumes a point source and equal incidence angle, excluding near-field area sources and room bounce.
Even light still needs visible faces
No exposure calculation can uncover a face hidden behind another person. Arrange heights and gaps, then inspect each pair of eyes for brightness, blinking and focus separately. The calculator below handles distance only. The photograph keeps its original proportions so the group remains visible.