Extension Tubes

Extension tubes contain no optical elements. They are simply hollow tubes that allow lenses to be mounted further away from the focal plane of a camera while preserving all the electrical contacts so that the lens functions as if it were attached directly to the camera body.

Thus, they are very different to extenders/teleconverters that do contain optical elements.

What's the point? All lenses have a minimum focusing distance. The addition of an extension tube in between the body and lens reduces the minimum focusing distance. The lens can therefore get nearer to the subject, and the result is a magnifying effect. Using an extension tube is a cheap way of turning a regular lens into a macro lens.

Of course, nothing comes free. The addition of an extension tube also decreases the maximum focusing distance. Most lenses can focus to infinity but with an extension tube they can't, so they lose normal functionality. There is also a loss of light, and vignetting can occur.

Kenko Tubes

Extension tubes have no optical qualities, just electrical contacts. There is no reason why products from different manufacturers shouldn't be exactly the same.

If I had seen a set of Kenko tubes, I would probably have bought them. There are three tubes in the Kenko set (12mm, 20mm, 36mm) and they are cheaper than Canon. All I could find were original Canon items so that is what I bought.

I read one report about lenses attached to Kenko tubes falling off, but I guess that isn't normal.

The original Canon tubes didn't support EF-S. The only difference with the version II tubes is that they support EF-S lenses. In addition to the red EF mount dot, there is also a white EF-S dot.

Magnification and compatibility

How much additional magnification will an extension tube give you?

While Googling this I came across some quite complex formulae. The formula I like (because I can understand it) is:

Additional Magnification = (Extension Tube Length) ÷ (Lens Focal Length)

When using a 25mm extension tube with my Canon EF 100mm f/2.8L IS Macro lens, the extra magnification would be:

25mm ÷ 100mm = 0.25 (or about ¼ more magnification)

What about compatibility?

Extension tubes reduce the minimum (and maximum) focusing distance of a lens. With short lenses that have a fairly short minimum focusing distance in the first place, the addition of an extension tube can actually bring the minimum focusing distance to behind the front element - thus making it impossible to focus.

I haven't tried it myself, but from what I've read it is not advisable to use extension tubes with lenses shorter than about 50mm.

Canon specifications

I've only included the lenses that I use. Information for the 100mm f/2.8L IS Macro lens wasn't available because it was only announced fairly recently. Extension tubes can't be used with the EF-S 10-22mm Ultra Wide Angle lens.

Set up and focusing

Close-up shots, as with long telephoto shots, are subjected to vibration. I used a tripod, mirror lockup, and the two second timer so that I didn't move the camera while operating the shutter.

I focused the lens manually using live view with the 10x magnification option. Autofocus seems to work pretty accurately but Canon advise manually focusing for macro shots.

Post-Processing

Most original images were shot in the RAW format and converted with Adobe Camera Raw. Noise was reduced using Imagenomic Noiseware Professional. Small amounts of image enhancement were applied using Adobe Photoshop CS2 (curves, levels, saturation) before sharpening using the Smart Sharpen filter (Amount: 30% - 120%, Radius: 0.2px - 0.3px, Remove: Lens Blur).

JPG images were saved with a 'High' quality setting of '8' on a scale of 0-12.

Details

Weight: EF 12mm - 2.3oz/66g; EF 25mm - 3.4oz/95g
Soft pouch: Included
Date Purchased: March 2010
Supplier: Parisilk, Singapore
Price: EF 12mm - SGD$75; EF 25mm - SGD$154