Update : I just noticed the mag ratio formula had something missing and I also added a few more formulas !

What are extension Tubes ?


Extension tubes are a nice way to achieve great macro pictures and they are available at a low cost for most SLR cameras. The tubes do not contain optical elements and they are mounted between the camera body and the lens with the purpose to move the lens farther away from the sensor / film. The result, the farther away the lens is from the sensor/film, the closer the focus and the greater the magnification. Extension Tubes do come in different length, for example I use a set of Kenko Tubes with 12mm, 20mm and 36 mm. They can be used individually or stacked in any combination. The highest magnification is achieved with all 3 combined.

Extension tubes come either with electrical contacts, which allows you to use autofocus on your camera, or they come without electrical contacts. The tubes without electrical contacts will not allow you to use autofocus. To use autofocus with extension tubes you really need to make sure you have plenty of light available. Why ? Well, the farther away the lens is from the sensor/film, the closer the focus, the greater the magnification, and also the greater the loss of light. And with light loss you might be forced to use manual focus since the camera can loose the ability to auto focus. Usually the loss of light does also require a longer exposure. When using extension tubes the lens will not focus to infinity. The focus range will be greatly limited to a very close focusing distance.
My personal opinion on auto focus vs. manual focus: I do not use auto focus at all, it is giving me too much of a headache since in macro photography you are shooting pictures of tiny subjects and its harder to force the lens to auto focus then it is to manually focus. I highly recommend manual focus , the result is much better.


How to calculate the magnification ratio with tubes with a given lens ( used in normal, unreversed position ) :
[mag ratio] = (amount of added extension+ Master Lens focal length )/(lens's focal length)

Getting the effective aperture :
(effective aperture) = (set aperture)* (1 + [mag ratio])

Exposure Factor = (1+ Image Magnification Ratio )²

Master Lens Advanced Volume = (Master Lens Focal Length)²/ (master Lens Distance Reading - Master Lens Focal Length )

Field Coverage= (Images Size on Film / Sensor)/ Magnification Ratio

The following is very advanced and I have to add a disclaimer that I did not write it.

Extension Tubes and f-stops (PDF)

P.S. Extension Tubes are not the same as Teleconverters !!! A teleconverter is designed to increase the apparent focal length of a lens. An extension tube is designed to increase the magnification ratio on the film or sensor !



Now go have fun with some tubes !

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