Sony's 20-70mm f/4 Might Be the Mountain Zoom to Beat
What was the test?
Mark Bergreen of The Bergreens carried two Sony zooms — the 24-50mm f/2.8 G and the 20-70mm f/4 G — on two climbing trips to test them against his Sigma 24-70mm f/2.8 Art. His checklist: 20mm wide, at least 50mm long, usable low light, and light enough to hang from a shoulder strap all day.
Image quality wasn't the decider — every contender produces excellent files. The question was which compromises actually matter on the trail.
Why did the 24-50mm disappoint?
Bergreen expected to love the 24-50mm. You keep constant f/2.8 and lose 20mm of reach, in exchange for a much smaller lens. On the trail the size difference felt smaller than the spec sheet suggests, and the price sits several hundred dollars above the Sigma it was meant to replace.
The 20-70mm won out. Bergreen is wary of f/4 zooms, but 20mm saved him from a 16-35mm swap — he calls it two lenses in one. On full frame, 24mm to 20mm grows the field of view from about 74 to 84 degrees: a whole ridgeline in one frame, without backing up on a cliff edge.
How much does the f/4 compromise actually cost?
The cost shows up at dawn and dusk, when f/4 pushed Bergreen to ISO 12,800. But the penalty has shrunk: since Lightroom added AI-based Denoise in 2023, high-ISO raw files clean up more than when f/4 zooms earned their compromise reputation. One stop is still one stop, and you give up some background blur.
If most of your wide shots are landscapes stopped down for depth of field, the extra width gets used on more frames than the extra light. Bergreen also flags the new Sigma 20-60mm f/2.8-4 Contemporary — though variable apertures complicate manual exposure, and zooming in can leave you a stop underexposed.
Why it matters for you
Buy the lens for the frames you shoot. If your subjects live at the wide end, 4mm of width beats a stop of light you'll rarely use — especially now that AI denoising exists. Bergreen expected to love the 24-50mm and preferred the lens he was skeptical of.
Specs tell you what a lens is. Only using it tells you what it's for.