What is the primary purpose of using a neutral density filter?

The primary purpose of using a neutral density filter is to decrease the light intensity that enters the camera lens without altering the color of the light. This allows photographers and videographers to have greater control over exposure settings, enabling them to use wider apertures or slower shutter speeds in bright conditions. A neutral density filter essentially acts like sunglasses for the lens, reducing the overall brightness while preserving the natural color characteristics of the scene.

This capability is especially beneficial when shooting in bright sunlight or when trying to achieve certain effects, such as motion blur in fast-moving subjects or depth of field control, without overexposing the image. Other options like changing the color of light, increasing contrast, or protecting the lens do not accurately describe the primary function of a neutral density filter, making this answer the best fit for the question.