Fstopers had one of the best time ever testing three very different telephoto lenses for the Sony system:

• Sony FE 400-800mm f/5.6-9 OSS – Around $3,200
  US: BHphoto , Amazon , Adorama
  EU: Fotokoch , Foto Erhardt , Calumet DE , Wex UK ,
• Sigma 300-600mm f/4 DG DN OS Sports – Around $6,600
  US: BHphoto , Amazon , Adorama
  EU: Fotokoch , Foto Erhardt , Calumet DE , Wex UK ,
• Sony FE 600mm f/4 GM OSS – Around $14,100
  US: BHphoto , Amazon , Adorama
  EU: Fotokoch , Foto Erhardt , Calumet DE , Wex UK ,

Conclusion: All three lenses offer stellar image quality, but they serve different types of photographers. The Sony 600mm f/4 delivers unmatched sharpness even with a 2x teleconverter, but at a painful $14K price point and poor stabilization. The Sigma 300-600mm f/4 is heavier and limited to 15 fps, yet an excellent bargain at $6,600 with fixed aperture and superb optics. But the biggest surprise is the Sony 400-800mm : for a fraction of the cost, it delivers nearly the same image quality, better stabilization, more zoom range, and excellent usability — making it a true winner for enthusiasts and budget-conscious pros alike.