Using Extension Tubes
Extension tubes or extension rings are hollow tubes designed for use in macro photography.
The tubes are usually supplied in a set of three lengths (eg 12mm, 20/25mm and 36mm) and fit between the camera and lens to offer a choice of macro magnifications.
Some manufacturers produce them singly too for those who have a specific requirement, but a set of three offers more versatility because you can use the tubes in any combination to give six magnifications.
As there are no elements inside the tubes, quality is as good as the original lens and the size and versatility is perfect for the nature photographer who may be humping a load of kit around.
Their only real drawback with tubes is that there is a loss of light and this increases the more extension you add. Fortunately modern cameras with through-the-lens metering automatically compensate, even when using flash (providing it is dedicated). If you are using an older manual camera or a non TTL flash system you will have to make adjustments manually.
12mm Extension tube + Pentax 50mm.
25mm Extension tube + Pentax 50mm.
36mm Extension tube + Pentax 50mm.
Once in place adjust the lens focusing as normal. If you cannot fill the frame with the subject add another tube or when the magnification is too high, remove a tube.
When used in conjunction with a 50mm lens, a 25mm tube gives you half-life-size image, and a full set 1:1 reproduction. If you want incredible magnification you could combine more than three tubes. Two sets would give around 140mm of extension which is similar to a standard set of bellows.
If your camera has TTL metering use the suggested exposure, compensating as normal when the subject is predominantly light or dark. If the camera has manual exposure you need to calculate the extension used and increase the exposure to compensate. Check out this exposure table.