Exposure is a three-way decision, not a brightness control

Aperture shapes depth

A wide aperture lets in more light and can separate a subject from the background. It also reduces the distance that appears sharp. A smaller aperture increases depth of field but requires a slower shutter speed or higher ISO. Lens aperture matters as much as sensor size when you want low-light flexibility.

Shutter speed shapes movement

A fast shutter freezes a bird, athlete or child. A slow shutter can blur water, traffic or a moving subject intentionally. Stabilization reduces blur caused by your hands, but it cannot freeze a moving subject. For sport and wildlife, autofocus and lens brightness remain crucial even when a body has excellent stabilization.

ISO trades cleanliness for access

Raising ISO amplifies the signal so you can use a faster shutter or smaller aperture. It also reveals more noise and reduces editing latitude. Modern noise reduction is useful, but it cannot fully restore fine texture that was never captured. A slightly noisy sharp image is usually more valuable than a clean blurred one.