The Pentax Auto Extension Tube Set B contains three tubes of different length---12mm, 19mm, and 26mm---which can be used individually, or in any combination, to obtain the desired magnification. Actual magnification effect changes with each specific lens.
Extension tubes are designed to enable a lens to focus closer than its normal set minimum focusing distance. Getting closer has the effect of magnifying your subject (making it appear larger in the viewfinder and in your pictures). They are exceptionally useful for macro photography, enabling you to convert almost any lens into a macro lens at a fraction of the cost, while maintaining its original optical quality.
The extension tubes have no optics. They are mounted in between the camera body and lens to create more distance between the lens and the film plane. By moving the lens father away from the film or CCD sensor in the camera, the lens is forced to focus much closer than normal. The greater the length of the extension tube, the closer the lens can focus.
When using the camera's TTL metering system, no exposure compensation will be required (Exposure compensation is required for handheld meters).