The Pentax Helicoid Extension Tube B is an "adjustable" extension tube that provides an infinitely stepless adjustment range from 26.5mm to 46.5mm. Actual magnification effect changes with each specific lens. With a 50mm lens the magnification range is about .5x to 1x. Can also be combined with the Auto or Manual Extension Tubes B for even greater magnification.

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 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.

Auto-exposure operation is not possible with this tube, and manual "stop-down" metering and manual f/stop control is necessary.