What adjustment impacts the amount of light entering the camera lens most directly?

Aperture is the adjustment that most directly impacts the amount of light entering the camera lens. It refers to the opening of the lens diaphragm through which light passes. A larger aperture (indicated by a smaller f-number, like f/2.8) allows more light to hit the sensor, while a smaller aperture (a larger f-number, like f/16) restricts the light entering. This control over light not only affects exposure but also influences the depth of field in the image, helping to determine how much of the scene is in focus.

The other options, while also important for exposure and image quality, do not directly adjust the amount of light entering the lens. ISO affects the sensitivity of the camera's sensor to light, shutter speed controls how long the sensor is exposed to light, and white balance adjusts the color temperature of the light in the image. None of these adjustments directly alter the aperture size, making aperture the key factor in controlling light entry through the lens.