Sony 100-400mm f/4.5 GM is now shipping out at BHphoto , Amazon , Adorama , Fotokoch , FotoErhardt , Calumet , Clifton , WexUK

Alex Phan published the full review of this lens on his Facebook page :

Sony FE 100-400mm f/4.5 GM Field Review: The Ultimate Wildlife Zoom or a $4,299 Luxury?

The new Sony FE 100-400mm f/4.5 GM OSS is a unique beast. At $4,299, it sits right between traditional variable-aperture zooms and those ultra-expensive, five-figure telephoto primes. To see if it actually lives up to that heavy price tag, I threw it straight into worst-case scenarios, hunting for flaws in the absolute harshest field conditions (shooting at high noon, 80+ degree temperature.).
Here is my breakdown after putting it to the test:

Build & Handling:
At 4.1 pounds (1,840g), it is physically heavier than the older variable-aperture models. Sony built this with an internal zoom design, meaning the barrel never expands or contracts. This keeps the center of gravity perfectly stable. In the field, this balanced build actually makes the lens feel significantly lighter and more stable when tracking moving subjects.

Blistering Auto focus:
Under the hood, there are four custom-optimized XD linear motors in a floating focus design. Sony claims tracking speeds up to 3x faster than the predecessor—and it absolutely shows. In real-world tests, it flawlessly locked onto a falcon zipping through a cliff side. It kept perfect focus even as the bird passed through chaotic obstacles and wildly shifting light. Focus acquisition is immediate, predictable, and sticky.
The Ultimate Stress Test: Stacking a 2x Teleconverter:
We all know adding a TC results in image quality loss and less light. But to find the absolute breaking point of this glass, I purposely slapped on a 3rd-party 2x Teleconverter (TC) just to hunt for flaws.
The results completely blew me away. Because you start with a bright, constant f/4.5 aperture, stacking the 2x TC leaves you with an 800mm f/9 setup that holds up incredibly well. Center sharpness remains impressive, and the linear motors still allow for fast, reliable tracking. It’s an exceptional option when you occasionally need massive reach!

What Issues I Noticed in the Field
No lens is perfect. Here are the quirks you need to know about before buying:
* Zoom Creep: The lens has a torque ring with “Smooth” and “Tight” settings. In Smooth mode, gravity takes over. If you point it straight down, it slowly drifts out. A few times I pointed back up and wondered why my framing was off, only to realize the lens had crept all the way to 100mm. Switching to “Tight” fixes it, but it makes the ring a bit stiff, which can cause jerky footage if you are shooting video.
* Slight Vignetting: At the 400mm end shot wide open at f/4.5, there is some noticeable corner vignetting (especially obvious when shooting a bird against a clear blue sky).
* Sharpness Drop at 800mm (Expected): While it handles the 2x TC beautifully for a zoom, there is a drop in sharpness and fine contrast compared to shooting the lens natively. At f/9, you become highly dependent on perfect atmospheric conditions.

The Bottom Line
The Sony FE 100-400mm f/4.5 GM OSS works exactly as designed. It delivers spectacular tracking and brilliant flexibility. If you occasionally need extra reach, it works very well with a teleconverter as a reliable backup—but NOT as a direct replacement for a prime.
The Golden Rule: Don’t buy this lens with the permanent intention of slapping a 2x TC on it to replace a dedicated, long telephoto prime. Stacking TCs should be a temporary tool for versatility, not a permanent shortcut to an 800mm lens. But if you need a versatile, fast-tracking zoom that handles beautifully, this is a masterclass in optics.
Happy Shooting.