Increasing the width of the active aperture reduces beam width at the focal point. Which option best describes this effect?

• A. Increases beam width
• B. Decreases beam width
• C. No effect
• D. Increases depth penetration

Answer: (B)

Larger active aperture tightens the focus by increasing the numerical aperture, which reduces beam divergence at the focal point. In a diffraction-limited system, the lateral beam width at focus is roughly proportional to lambda times the focal length divided by the aperture diameter; when the aperture grows, this width gets smaller. So widening the active aperture concentrates energy into a tighter spot, giving a narrower beam width at the focus and improving lateral resolution. This isn’t about more depth of penetration—the depth relation is governed by attenuation and frequency, not the immediate focus width.