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I have a few very-fast Canon lenses (f/1.2, f/1.4, f/2, etc). They're stunning for portraits and normal terrestrial photography, but I'm not happy with any of them below f/4 for astro.
Some of them are quite good at f/2.8 (e.g. 100L, 135L, 70-200 f/2.8L IS II) but they vignette quite badly on full-frame sensors.
With CA, you'll get red fringing on one side of the focal plane, and green fringing on the other side (or something similar). I find this very helpful for LiveView focusing.
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