What is Extension Tube

What Is An Extension Tube?
This is actually a tube, with no optical elements, having ways for transmitting electronic signals, which can be mounted between the lens and the camera. The extension tube will force the system to focus at closer distances, the result being a recorded image with higher magnification ratio compared to the lens alone (the magnification ratio being the ratio between the size of the image on the focal plane of the lens and the size of the real image). Just beeing able to get closer to the subject you obtain a larger magnifiction ratio. "12", "25", etc. in the name of the device shows the length of the light path through the tube. Extension tubes can be mounted one on another, as many as you want - just be careful to obtain a mechanically ensemble which is stable.

Canon makes tubes with 12mm and 25mm extension, other manufacturers may produce also other dimensions.

Some authors give you mathematical formulas for calculating the magnification ratio using an extension tube, based on length of the tube and focal length of the lens. Modern lenses vary their focal length in the focusing process, so these formulas may become quite complicated, depend on the used lens and are often based on guesses on how certain lenses work. I do not use these formulas. Just know the lens you have, the tube you use and have a look on Bob Atkins tables - see HERE. As a general rule just remember that the magnification ratio is higher when the ratio between the extension length and the focal length is larger.




Advantages Of Using Extension Tubes
Extension tubes are relatively cheap macro accessories. Much cheaper than a specialized macro lens. According to Bob Atkins tables mentioned above, you can get magnification ratios of 0,5x - 1,0x, and even higher than 1x, when using normal or wide focal length lenses. For tele lenses the gain in magnification ratio is modest.

But even so, a medium or tele lens can be made to focus closer, which can be an advantage in some situations. For example EF 70-200 f/4L IS USM has a minimum focusing distance of 1,2m. Using the EF 25 II tube you will be able to focus at 83cm and will get a slightly higher magnification ratio compared to the lens alone.

The extension tubes do not contain optical elements, so they do not interpose "glass" between the subject and the sensor / film. Thus the quality parameters of the recorded image are unaltered.




Disadvantages Of Using Extension Tubes
A minor disadvantage is that you loose some light because of the longer travelling path from the lens to the sensor. Anyhow this is compensated automatically by the TTL measurement system of the camera.

A more important disadvantage is the one that exactly at the focal distances where you get large magnification ratios you need to get very close to the subject. Focucs distance is measured from the subject to the plane of the sensor / film. In this length you have the lens and the tube. So the real working distance may become only a few cm. For example, if the needed distance to subject in order to get a certain magnification ratio is 30cm, if the lens and the tube measure 22cm, and if the distance between the lens mount and the sensor / film plane is another 5cm, you end up with a working distance of only 3cm. Not very funny!

Finally, the ensaemble lens - extension tube will not focus to infinity. So, you will always need to remove the extension tube for taking normal pictures. A specialized macro lens can be used for macro work and for normal photography.




How To Choose An Extension Tube?
Here it is simple. You just need to make sure the selected tube is compatible with your camera. I mean that it has the proper system of transmitting the electronical signals from the camera to the lens. This information can be found in the technical documents of the manufacturer of the tube.

You may check above invoked Bob Atkins tables to see if your ensemble lens - extension tube allow magnification ratios you need.

(info from www.fpix.eu)

Huhu... That's my macro equiptment man~ Muahahhaa,,, Cheap and cute... But mine dun hv any eletronic transmission so need total manual focus.... Thanks max for the information~

thanx for sharing information max...

thanks for the sharing...