Caution cheaper plastic scrap!

I was looking for güstigen extension tubes to the magnification of my macro lens to increase for some time yet. Electrical contacts are important to me here in order to dim, since I do not use an old manual focus macro lens, but a EF 100L (among other things because I like to shoot close-ups in the forest and me the stabilizer thereby greatly helps).
I wanted to test whether the picture quality at extreme magnifications (ie greater than 1: 1) is better than a close-up lens Raynox DSC250. I also wanted to try how far you get the magnification with achromatic lens and intermediate rings and if that's possible with my tripod equipment. (Is not it, at all, from 2: 1 is my Manfrotto 498RC2 Ball Head with XPROB055 then too unstable.)

Unfortunately, this was a big letdown, who gave me a scare. At the very first test the cross-slide camera extension tube-lens combination on the ball head of my tripod a little (about 5 °) to me is down jerked (yes sure: That was my mistake, I had forgotten to adjust the friction setting right) is nowhere struck, but there are two of the rings broke.
Specifically: The metal rings Bayonettanschlusses are broken from the plastic body of the ring. Such was the cam only a metal ring, also at the lens. And several screws and the rings were loose around Bayonettverrigelungen ...
Much more is happening, thank God, the 100L is then indeed landed lightly unceremoniously on the table, but the drop height was only about 10cm and it went "Geli-advance". I tested with the tripod next to a table on which the object to be photographed was placed, the cam course showed towards table and the lens sticking out accordingly on the table ...
In the further investigation it was found that the really tiny screws have broken out which the metal ring was screwed to the plastic body, several at a time ..

So steer clear if you want to use normal-weight lenses. In the (visually quite good) plastic lenses of the EF-series that may be a good opportunity to hinenzuschnuppern in macro photography, but otherwise unfit. I dare not imagine, if you wanted to use many 70-200 / 2.8 these intermediate rings with the 180L or even to reduce the minimum focus distance of eg a Tamron 70-300 VC or even one.

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