Exposure is the amount of light recorded by the camera. Aperture, shutter speed and ISO are often introduced as three equal controls, but they do different jobs. Aperture and shutter speed change how much light reaches the sensor. ISO changes how strongly the camera amplifies the captured signal. A useful exposure decision therefore begins with the picture—not with a meter reading.
Start with motion. Ask whether the subject should be sharp, intentionally blurred, or somewhere between. A walking person may need roughly 1/250 second for reliable sharpness, while a bird in flight may require 1/1600 or faster. A waterfall can look energetic around 1/15 or silky over several seconds when the camera is supported. These are starting points, not laws: focal length, subject distance, stabilization and your technique all matter.
Next choose depth of field. A wide aperture such as f/1.8 can isolate a portrait but makes focus placement critical. A landscape does not automatically need f/16; f/8 often gives stronger optical quality, and careful focus or focus stacking can handle extra depth. Remember that depth of field changes with subject distance and focal length as well as aperture.
Now set ISO to place the exposure where it needs to be. In changing light, Auto ISO with a deliberate shutter-speed limit is often more reliable than treating Manual mode as a badge of skill. Raise ISO when the alternative is unwanted motion blur or underexposure. Noise can be managed; a missed moment cannot.
Use the histogram to evaluate recorded tones rather than judging the rear-screen brightness. Protect important highlights, especially clouds, skin and reflective surfaces, but do not underexpose everything from fear of clipping. Take a test frame, inspect the histogram and important detail, then adjust one variable for a clear visual reason.
Practice exercise: photograph one static subject in changing light. First keep motion constant and vary aperture. Then keep depth constant and vary shutter speed. Finally use Auto ISO with your chosen aperture and shutter. Compare the files at normal viewing size and write down which compromise actually affected the picture.