6D vs D600 Astrophotography

Check this warning:

* Note: Though this lens will cover the imaging sensor of a full-frame DSLR, it is better optimized for use with DSLRs containing smaller APS-C format sensors. As such you might notice softer focus and some light fall-off towards the edges of the frame when using a full-frame DSLR. If you do plan on using this lens with a full-frame DSLR we recommend you set the lens no wider (faster) than f/8 for optimum image quality.

Referring the description of BH of Bower 14 mm 2.8 (it´s same lens with another brand).