When sourcing trial frames, prioritize optical precision and ergonomic adjustability. Key specs include a Pupillary Distance (PD) range of 48mm to 80mm, independent left/right adjustment, and at least 4 lens slots per side (3 front, 1 back). Ensure the axis scale is clearly marked with 5° increments and that the temple length and inclination are adjustable to accommodate different facial structures. High-quality models should use lightweight alloys or medical-grade plastics to ensure patient comfort during long examinations.