Sony FE 100-400mm f/4.5 GM OSS Review: The Best Constant-Aperture Telephoto Zoom Out There?

When Sony announced the FE 100-400mm f/4.5 GM OSS, I was curious whether it would actually be a meaningful upgrade or just another refresh of an already excellent lens. After spending the last few weeks shooting with it, I can confidently say this is much more than a small update. Sony has completely reworked the lens, introducing a constant f/4.5 aperture, an internal zoom design, and autofocus that’s noticeably faster and more reliable than before.

At $4,299, it’s undoubtedly a premium piece of glass, so it won’t be for everyone. But after using it for everything from birds in flight and wildlife to sports and even a few portraits, I can honestly say it fills a gap that has existed in Sony’s lineup for years.

Design, Build Quality, and Handling

The first thing I noticed was the weight, or more accurately, how well Sony has managed it. At 1,840g, this certainly isn’t a lightweight lens, but for a professional constant-aperture telephoto zoom it feels surprisingly well balanced. More importantly, it never feels front-heavy in use.

The biggest improvement is the internal zoom mechanism. Unlike the older 100-400mm GM, nothing extends as you zoom, so the balance stays exactly the same throughout the entire focal range. It sounds like a small change, but in practice it makes a huge difference, especially when you’re tracking fast-moving subjects or shooting handheld for long periods.

Build quality is exactly what I’d expect from a G Master lens. The magnesium-alloy construction feels incredibly solid, weather sealing is comprehensive, and the fluorine-coated front element makes it easy to wipe away rain, dust, and fingerprints.

Sony has also nailed the controls. The zoom ring is smooth with adjustable torque depending on how you like it to feel, the focus ring has just the right amount of resistance, and there are four customizable focus-hold buttons positioned around the barrel so one is always within easy reach regardless of how you’re holding the lens.

You’ll also find the usual AF/MF switch, focus limiter, and Optical SteadyShot controls neatly arranged along the side. The rotating tripod collar feels sturdy and has positive 90-degree click stops, although I’m still disappointed Sony hasn’t made it Arca-Swiss compatible straight from the factory.

The lens accepts standard 95mm front filters, but Sony has also included a 40.5mm drop-in filter system, which is particularly useful if you regularly shoot with ND filters or polarizers on larger telephoto lenses.

Overall, this is exactly how a professional wildlife lens should feel, solid, dependable, and built to survive years of hard use.

Key Specifications

• Focal length: 100-400mm

• Maximum aperture: Constant f/4.5

• Optical construction: 28 elements in 20 groups (including ED XA, XA, Super ED, and ED elements)

• Aperture blades: 11 (rounded)

• Minimum focus distance: 0.64m (100mm) / 1.5m (400mm)

• Maximum magnification: 0.25x

• Autofocus: Four XD Linear Motors

• Optical SteadyShot image stabilization

• 95mm front filter thread

• 40.5mm drop-in filter slot

• Approx. 120 × 328mm

• Weight: 1,840g

• Dust and moisture resistant

• Compatible with Sony 1.4x and 2x teleconverters

Optical Performance and Image Quality

This is where the lens really starts to justify its price.

Sharpness is outstanding throughout almost the entire zoom range, and unlike many telephoto zooms, there’s very little compromise at the long end. At 100mm, the center is incredibly sharp while the corners are already excellent. Zoom all the way to 400mm, and the center remains just as impressive, with only a slight drop in sharpness right at the extreme edges. For wildlife and sports photography, that’s something you’ll rarely even notice.

Stopping down to f/5.6 or f/8 squeezes out a little more edge performance, but I found myself shooting wide open most of the time because the lens is already so sharp at f/4.5.

Contrast is excellent, colors have that rich G Master look, and chromatic aberration is extremely well controlled. Sony’s Nano AR Coating II also does a fantastic job of suppressing flare and maintaining contrast when shooting into difficult light.

The 11-blade aperture produces beautiful background blur. Bokeh is smooth, specular highlights stay nice and round, and distracting onion-ring patterns are almost nonexistent. At 400mm f/4.5, subject separation is excellent, making it easy to isolate wildlife or athletes even against busy backgrounds.

Close focusing is another pleasant surprise. While this isn’t a macro lens, the 0.25x maximum magnification is enough to photograph larger insects, flowers, textures, or product details without feeling overly limited, and the working distance stays comfortable.

Autofocus Performance

Autofocus is probably the biggest improvement over the previous generation.

Sony has equipped the lens with four XD Linear Motors, and the difference is immediately noticeable. Focus acquisition is incredibly quick, virtually silent, and exceptionally confident.

Birds changing direction mid-flight, athletes running directly toward the camera, or wildlife weaving through trees—the lens handles all of it with impressive consistency. Paired with cameras like the Sony a1 II or a9 III, it honestly feels like the autofocus almost disappears because it simply keeps doing its job without getting in the way.

Focus breathing is also extremely well controlled, making this a surprisingly capable lens for video work.

I also found the focus limiter genuinely useful when photographing wildlife, helping reduce unnecessary focus hunting when I knew my subjects would remain beyond a certain distance. Full-time DMF is available as well if you prefer making small manual adjustments without switching out of autofocus.

Image Stabilization

Sony’s Optical SteadyShot continues to perform extremely well.

Working together with in-body image stabilization on compatible cameras, I consistently managed sharp handheld shots at shutter speeds that would’ve been risky with older telephoto lenses.

Mode 2 works particularly well for panning shots, and overall the stabilization feels smooth, predictable, and easy to trust without becoming overly aggressive.

Real-World Performance

This lens quickly became one of those lenses I simply wanted to keep mounted on my camera.

For wildlife, field sports, aviation, motorsports, and outdoor events, the combination of the constant f/4.5 aperture, internal zoom, and incredibly fast autofocus makes shooting feel effortless.

One of the biggest advantages is not losing light as you zoom. Shooting at 400mm with the same aperture you started with means your shutter speed stays consistent, your ISO doesn’t suddenly jump, and exposure remains predictable as subjects move closer or farther away.

The internal zoom also keeps the balance perfectly consistent throughout the zoom range, which makes following moving subjects feel much more natural than with extending zoom designs.

Of course, if most of your work involves tiny birds or distant wildlife, you’ll probably still appreciate the extra reach of a 200-600mm lens or a dedicated super-telephoto prime. Teleconverters help extend the range, although you’ll naturally lose some light and a little sharpness.

For the type of photography I do most often, medium-sized wildlife, sideline sports, outdoor events, and even the occasional environmental portrait, the 100-400mm range feels almost ideal.

Comparisons in My Kit

Compared to Sony’s older 100-400mm f/4.5-5.6 GM, this new version feels like a meaningful upgrade rather than a simple refinement. It’s sharper, autofocus is noticeably faster, exposure stays consistent throughout the zoom range, and the internal zoom dramatically improves handling. Yes, it’s larger and heavier, but I honestly stopped noticing that after a few days because the balance is so much better.

Against the Sony 200-600mm G, you’re giving up reach but gaining a brighter aperture at the long end, noticeably better sharpness, faster autofocus, and smoother background rendering.

And compared to lenses like the 400mm f/2.8 GM, you obviously lose that incredible f/2.8 aperture and the absolute highest level of optical performance. On the other hand, you gain the flexibility of a zoom while spending considerably less money.

Pros and Cons

Pros

• Constant f/4.5 aperture throughout the zoom range

• Outstanding sharpness, even wide open

• Exceptionally fast and accurate autofocus

• Internal zoom keeps handling perfectly balanced

• Excellent weather sealing and professional construction

• Beautiful bokeh and subject separation

• Very useful close-focusing performance

• Compatible with Sony’s 1.4x and 2x teleconverters

Cons

• Expensive

• Larger and heavier than the previous model

• 400mm may still be too short for some wildlife photographers

• Tripod foot isn’t Arca-Swiss compatible

• 95mm filters can be expensive

Who Should Buy It?

The Sony FE 100-400mm f/4.5 GM OSS is clearly meant for photographers who regularly shoot wildlife, sports, aviation, and fast-moving subjects but don’t want to compromise between flexibility and image quality.

If you spend most of your time working within the 100-400mm range and value fast autofocus, excellent optics, and consistent handling, this is one of the best telephoto zooms Sony currently offers.

If your main priority is maximum reach, you’ll still be better served by the 200-600mm G or a super-telephoto prime. And if you’re working with a tighter budget, the previous 100-400mm GM remains an excellent lens that still has plenty to offer.

Final Verdict

After spending several weeks with the Sony FE 100-400mm f/4.5 GM OSS, it’s become one of those lenses I naturally reach for whenever I know I’ll be photographing wildlife, sports, or action.

The constant f/4.5 aperture genuinely changes the shooting experience, the internal zoom makes the lens feel much more balanced than its predecessor, and the autofocus is easily among the fastest I’ve ever used on a telephoto zoom.

It’s certainly not cheap, and it won’t replace a dedicated super-telephoto if you constantly need more reach. But if your photography regularly falls within the 100-400mm range, I honestly think Sony has nailed the balance between versatility, speed, image quality, and handling.

For me, this is the best 100-400mm lens Sony has ever made, and exactly the upgrade I’d been hoping for. What did you guys think of this review, let me know in the comments down below ;)