Extension tubes work by moving a lens further from the film or sensor.

The closer an object is to the lens, the further back the focussed image will appear, hence extension tubes allow you to focus on objects closer to the lens than would be possible otherwise. They are commonly used a cheap way to improve the macro abilities of a lens.

There are a few ways image quality could be compromised

• Floating elements. Some lenses include a "floating" element (just a lens that moves independently of the focussing group) in order to reduce aberrations when focussing close up. If you set the lens focus to a distant setting and then use extension tubes to obtain close focus then you wont be getting the benefit of the floating element and image quality can suffer.

• Misalignment of the lens. A certain amount of misalignment is inevitable though most cases it will not have a noticeable impact on image quality. In extreme cases this could manifest itself in a tilt in the plane of focus, e.g. the right side of the image might be in focus but the left side out of focus.

• Light leakage or flare (unlikely unless the tube is very badly made, though possible for a DIY effort).