advice on extension tubes

I own Triplus tubes which I believe are the same as the Kenko ones. They look fine but in practice they are poorly made. The fit is poor, and the connections are not good. Nikon tubes are insanely expensive but well made.

Given that your lens already goes to 1:1, and to go to 2:1 you would need 100mm of tubes, I would suggest reversing a lens. If you have a 28mm lens, reverse it, and you get 2:1, with a fixed focus distance i.e. no focussing possible! Reverse a zoom, and you get variable magnification.

 

There are several approaches.

The simplest is to buy an adaptor - Nikon BR-2A - which allows you to mount the lens in reverse. You will need to focus at full aperture, then manually stop down before taking the picture as the aperture linkage will be lost. To focus you move the camera and lens, as the lens to sensor distance is fixed. If you reverse a wide angle zoom, you will get a range of magnifications.

An alternative is to use a long focal length lens, such as a 100mm or 200mm , and then mount a reversed lens such as a 50mm in front. The advantage is that you can maintain auto metering and aperture function, but you might get vignetting if you do not choose the lens carefully. This is sometimes called stacking lenses. To mount the lens you can glue together some step up/down rings which screw into the filter mounts of the lenses, but make sure the glue is effective as you do not want a lens falling off. The magnification is given by the ratio of the lens focal lengths

Many people such as John Shaw describe these techniques in various books. And a Google will provide some information too.

Note that focusing is very hard at 1:1 and greater, and vibrations are problematic. A focusing stage can solve the first problem.

 

I have had great success with Kenko tubes. They fit well and have the correct connections for various models of camera and they autofocus quite well in most cases. For the price the Kenko's are great and very useful. I now own a proper macro lens but I still use the Kenko's from time to time because the effect is so much different.

Have a look through the Macro forums as well for posts from Kevin_D50 he is the king of Kenko