Description/Explanation of Issue
ETC lens tubes come in two halves. These pieces are cast. For many reasons, castings of different ages may not line up perfectly. Due to the nature of how casting metal works, small variations can arise that will make for imperfections in the castings. The largest of these is wear on the casting tool over time. As a casting tool is used, material is worn away allowing for small changes in tolerances. When a tool reaches the end of its' tolerances, it will be replaced resulting in tolerances at the other extreme. This means that a right half and a left half that were cast at different times have a chance to not match well. ETC also finds that even casts of the same age do not match up. This is addressed in the manufacturing process to verify that the lens tube matches up well and fits properly into a fixture.
For these reasons, ETC strongly encourages people to not mix and match casting halves. This can happen due to a break in a half or if cast pieces get mixed up when cleaning lenses.
