On a Fullframe camera like the 6D this lens has lots of vignette at full aperture.
This site:
http://www.the-digital-picture.com/R....aspx?Lens=412
has an excellent preview how lenses perform in sharpness, color errors, etc.
The best value for astrophotography is NOT a FF body but a crop one.
Reasons:
- Use lenses designed for fullframe as the worst part (the edges) falls outside the cropped section.
- clip-in filters such as Astronomik CLS or Hutech LPS V2 can be fitted behind the lens
I have a 7d and 85mm f/1.8 and 70-300 f/4-5.6L excellent value for AP.
The advantage of higher sensitivity due to larger sensor is undone by more squeezing the aperture to prevent vignetting and edge softness.
Get a 70d or 7d or 60d. Cheaper body and invest in glass !
For daylight and landscapes you can use crop lenses such as the EF-S 10-22 or 15-85mm f/3.5-5.6, the latter is called 'the L lens without a red ring'. And the EF(not S) 17-40mm f/4L is of great value too.