Physics: Principles with Applications (7th Edition)

Moving closer to the subject decreases the object distance. The focal length of the lens stays constant. Equation 23-8, $\frac{1}{d_{o}} + \frac{1}{d_{i}} = \frac{1}{f}$, tells us that if $d_{o}$ decreases and f stays the same, the image distance $d_{i}$ must increase.