I'm looking for an extension tube for my DSLR. Any suggestions? I see a lot on the internet, but I cannot figure out which is the best.
An extension tube doesn't have any optics in it, it's just a tube that is used to move the lens in relation to the sensor, because there's nothing but air in there all the extension tubes in the world will use the same air will give you the exact same image.
There are 3 ways extension tubes differ:
Tubes with no electrical contacts (the cheap kind) don't have a connection between the lens and camera, you don't have auto focus and don't have aperture control, the camera will only work in M and Av mode.
To control aperture switch camera to Av mode, set aperture, press DOF preview button and hold, while pressing detach lens
Tube with electrical contacts: those are easier to use because you have all the normal camera controls (but I don't think auto-focus will be useful here).
Thicker tubes will increase magnification and move the focus plane closer - but remember if the tube is too thick the focus plane will move behind the lens's front element and nothing will be in focus.
You can buy a set of tubes with different thicknesses that you can combine to have a large number of options.
Just don't put an heavy expensive telephoto on a thin plastic Chinese tube.
Even if macro extension tubes have no glass in them, with cheap ones reflections or play at the mounts can make a huge difference. I recently bought one set branded COMIX, sold by Commlite, from eBay. It is useless because of glare and play. Kenko seems a safe bet. I got so upset with the COMIX tubes that I wrote a review some days ago comparing three equivalent sets of tubes for MFT of different brands and prices. Have a look at the test images, and you will never ever again believe that bad quality or badly designed macro extension tubes will not affect image quality. The review is in instalments: part 1, part 2, part 3.