Which part of the eye is likely not functioning properly if someone cannot focus on an image?

The lens of the eye plays a crucial role in focusing light onto the retina, allowing for clear vision. When someone has difficulty focusing on an image, it often indicates that the lens may not be functioning properly. This could be due to various conditions such as cataracts, where the lens becomes cloudy and affects its ability to refract light correctly, or presbyopia, which affects the lens’s ability to change shape for focusing on close or distant objects.

The other parts of the eye have different primary functions. The cornea is responsible for a majority of the eye's optical power and helps in focusing light, but it generally does not directly alter focus like the lens does. The iris controls the amount of light entering the eye, while the pupil is the opening that allows light in; neither of these directly affects the ability to focus images. Hence, if someone is unable to focus, the lens is most likely the component that is malfunctioning.