Focal Length and Purpose
These lenses serve entirely different photographic purposes. The Sony 100-400mm is a telephoto zoom designed for wildlife, sports, and distant subjects, offering 4x zoom range with excellent reach. The Sigma 15mm fisheye creates dramatic ultra-wide perspectives with characteristic barrel distortion, perfect for creative architecture, landscapes, and artistic shots. Your choice depends entirely on whether you need to get closer to distant subjects or capture expansive, distorted wide-angle views.
Image Quality and Build
Both lenses deliver exceptional optical performance in their respective categories. The Sony GM lens features advanced coatings, fast autofocus, and optical image stabilization crucial for handheld telephoto shooting. The Sigma Art lens provides sharp corner-to-corner performance with minimal chromatic aberration, impressive for an ultra-wide fisheye. Both feature weather sealing and premium construction, though the Sony's OSS gives it an advantage for telephoto stability.
Versatility and Value
The Sony 100-400mm offers significantly more versatility with its zoom range, making it suitable for various telephoto applications from portraits to wildlife. The Sigma fisheye is highly specialized - stunning for specific creative applications but limited in everyday use. Consider how often you'll use each lens type, as the Sony provides more shooting opportunities despite being slightly more expensive.
