I initially posted some info on how corner focus of this lens varies wiht focal legth but after comparing two copies side by side have decided they vary too much to say much about it.
I completed a few more tests today looking at corner-focus behavior of the GM 12-24 when zooming in and out. It appears that when the lens is wide open it is actually reasonably parfocal, i.e corner focus is minimally affected by zooming, at least over the range 14-24mm where my copy is not significantly decentered. This is good news for astro. Here's an illustration crop of the extreme top-right corner at f2.8 and focal lengths of 24,18 and 14mm without altering focus (100% crop from 61MP, unsharpened). Bearing in mind that this copy has a small amount of swing at 14mm and 24mm, focus looks pretty consistent to me.
Also, at 24mm it turns out focus shift is minimal as you stop down. But as you zoom to shorter focal lengths, stopping down increasingly produces more focus shift in the corners. It becomes very large at the wide end of the lens, where stopping down without re-focusing actually makes corner sharpness worse rather than better.
Note also that as a result, the lens become less parfocal the more you stop down, at least up to F8 which is all I tested to.
Oct 12, 2020 at 11:26 PM
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