Yes—there is a practical limit. Extension tubes move the lens farther from the sensor so the lens focuses much closer, but as extension increases the focus range shifts toward extremely short distances. With enough extension, the plane of focus can end up so close that it is effectively inside or behind the front of the lens, making normal focusing impossible. This is especially easy to run into with very short focal-length lenses, such as an ultra-wide.

The main variables are:

• focal length: shorter lenses reach the limit sooner
• total extension added
• lens design and its normal focusing range
• working distance to the subject
• effective aperture: more extension reduces the amount of light reaching the sensor, so the effective f-number rises, which can make diffraction more noticeable at high magnification

So while extension tubes themselves don’t add optical elements, too much extension can still become impractical because of vanishing working distance, loss of focus usability, and higher effective aperture.