Originally posted by abruzzi The idea is you print the document below, focus on the page, then defocus a little. If it deforms shape when it blurs (i.e. becomes oval instead of circular) the lens is decentered. Is there a drawback to that method?
The target has the be perfectly flat and aligned with the camera parallel to the target and aligned with the center optical axis - otherwise you can get a false positive. I use the star chart with an optics testing bench, testing lenses through a camera is just adding to the number of variables that can make lens testing difficult - a dedicated optics bench can reveal flaws and give you the ability to quantify them which is something that can be difficult to do with just a camera. Having an optics bench also allows me to develop profiles for my own lenses - sometimes due to manufacturing tolerances canned profiles can be inaccurate, which is often the case for some zoom lenses at specific focal lengths.