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Review: Extension tubes
Like the bellows reviewed above, Macro extension tubes operate on the principle of increasing the distance between the lens and the sensor, thereby increasing the magnification. Extension tubes come in sets of three — 13mm, 21mm and 31mm are typical. These can be used singly or combined to increase the distance. The maximum extension is 55mm with all tubes combined. Take a look at the peppercorn in image 4 to get an idea of the magnification you can expect.
The macro effect won't be as pronounced as with the bellows, as it extends over twice that distance, but the tubes are more robust. The tubes and mounts are aluminium and the locks appear to be steel. They also feature gold coated electronic connections so autofocus and communication with the camera is maintained, unlike the Bellows.
This set is sold under a number of different brands on Amazon. As far as I can make out, they're all coming from the same factory, regardless of the name on the pack. They're not as solidly machined as the Canon version, but do exactly the same job for a fraction of the cost. I've had this set for a few years now with no issues.
Review: Reversing ring and slider rail
A popular method of shooting true macro is by using a reversing ring. These mount to your camera and allow your lenses to be mounted back-to-front. Please note that these rings are not generic — you must use a ring with the correct mount for your camera and the correct diameter for your lens. The ring I use is 77mm which means I can use it with the 24-104mm, 70-200mm and 100-400mm lenses, which all share the same front element diameter.
While this method greatly increases magnification, it also completely disconnects the lens connection to the camera. Unless you are using a lens with a manual aperture, you will need to set the aperture with the lens connected directly to the camera, before using the reversing ring. Depth of field is extremely shallow — I recommend using an aperture of f16.
I made use of the focus slider rail seen in pic 5. This is a useful extra, as it allows you to move the camera incremnentally forwards and backwards, left and right. This is a much easier way to achieve focus.
I've also included a wider shot of the bowl of flowers to give an indication of scale. The macro shots are of the cluster second from the left.