How do aperture shutter speed and ISO affect video exposure?
Short Answer:
These three controls form the "exposure triangle". Aperture sets how much light the lens lets in and controls depth of field; shutter speed controls motion blur and, in video, is usually kept at roughly double the frame rate; ISO sets the sensor's sensitivity, with higher values adding noise. Balancing all three gives correct exposure and the look you want.
Overview
Exposure in video is a balance of three settings, each with a creative side-effect. Unlike stills, video constrains shutter speed for natural motion, so aperture, ISO, and lighting do most of the work.
Detailed Explanation
The exposure triangle in video
Aperture (f-stop): controls light and depth of field. Wide (low f-number) = more light and shallow focus; narrow = less light and deeper focus.
Shutter speed: controls motion blur. The video convention is roughly 1/(2 × frame rate) — e.g. ~1/50s at 25fps — for natural-looking motion.
ISO: the sensor''s sensitivity to light. Low ISO is cleanest; high ISO brightens dark scenes but introduces noise.
The balance: change one and you usually compensate with another (or with lighting).
Because shutter speed is best kept near the "180-degree" convention for natural motion, exposure is normally managed with aperture, ISO, lighting, and — outdoors — neutral density (ND) filters to tame bright light.
Example
Filming outdoors in bright sun at 25fps, you keep shutter near 1/50s for natural motion, so instead of speeding up the shutter you add an ND filter and set a wide aperture and low ISO to expose correctly while keeping a cinematic shallow focus.