I would like to get a little higher magnification out of my Canon 100mm Macro, so I am looking at extension tubes.
I know Canon makes the EF25, but it seems pretty expensive considering there isn't even glass involved. I saw a "Kenko" extension tube set that is only a little more expensive for a 12, 20, and 36mm. Are there any downsides to buying a third party extension tube?
As far as I know there should be no real difference. The extension tube have no lens elements but is merely (as the name suggests) a tube. The Kenko extension tubes feature the same electrical connections as the Canon one (judging from reviews and images on the net). According to this review, there are two versions of the Kenko tubes; an older one that is not compatible with EF-S lenses, and a newer one that is. Worth keeping in mind if you have such lenses.
If there is any downside, it may be in utilizing different materials for the tubes. I don't know if it actually so - and I won't be surprised if it is - but it is possible that 3rd party tubes are made of a more elastic metal or thinner walls (the cheaper you go , the more likely it is). It means that once supporting the weight of the lens, the tube will bend a little, creating some degree of axis offset and asymmetry. The effect should be the same as a tilt-lens at very small tilts.
That said, I did not hear some specifically bad things on the Kenkos, and as an electrical tubes they are considered a good alternative to the Canons.
