How to check a lens for decentering

There are many things that can be wrong with a photographic lens. Optics require tight manufacturing tolerances in order to produce high resolution images. Copy to copy variation is a serious issue, even (or especially?) nowadays. While most lenses leave the factory in good condition, a non-negligible percentage reaches the market with unacceptable optical flaws. One of the most common issues is decentering.
Since this happend to me a few times already, I decided to write a short article about it. I will first explain what decentering means and then continue with a simple procedure on how to test a lens.

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How IBIS works and why the term “5-axis” is misleading

IBIS is short for in-body image stabilization. It is a way of optical shake reduction to help reduce or eliminate motion blur in photos. In contrast to traditional optical stabilization, instead of moving a glass element inside the lens, the camera sensor is shifted. All major manufacturers of mirrorless cameras nowadays offer at least one model with IBIS, e.g. Panasonic GH5, Fuji X-H1, Olympus OM-D or Sony A7 series. But how does it actually work? And what does 5-axis stabilization mean?

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