Kosiy 105mm f/2.8

The Kosiy 20100 is a 105mm f/2.8 macro lens with a decent aperture and okay bokeh, but it's let down by poor optics and terrible macro performance. Pricing is a mess, ranging from $950 to over $92,000, and even at the low end it's a bad value. The unknown mount and lack of weather sealing add to the risk. Skip this and buy a used name-brand macro lens instead.

The performance data for the Kosiy 20100 tells a story of a lens that's fighting above its weight class and mostly losing. The aperture, at f/2.8, is actually a solid middle-of-the-pack performer, landing in the 66th percentile. That means for shallow depth of field and low-light gathering, it's perfectly adequate. The bokeh quality is in a similar boat at the 65th percentile, so out-of-focus areas should look reasonably smooth, which is crucial for both macro and portrait work. But that's where the good news ends.

The optical score is a real letdown. At the 20th percentile, we're talking about softness, likely significant chromatic aberration, and poor contrast when shooting wide open. For a macro lens, where edge-to-edge sharpness on a flat field is the whole point, this is a critical failure. The macro score itself is in the 14th percentile, one of the worst we've seen. That 1:1 reproduction ratio might be technically true, but if the image is a blurry, fringing mess at that magnification, it doesn't count for much. The stabilization, which the highlights vaguely mention, scores in the 37th percentile, meaning it's underwhelming and won't save you from camera shake as effectively as the systems from name-brand competitors.

The competitor list for the Kosiy 20100 is a strange mix of zooms and primes, which highlights how oddly this lens is positioned. The Nikon NIKKOR Z 17-28mm f/2.8 and Panasonic Leica DG Vario-Elmarit H-ES12035 are completely different beasts, wide and standard zooms that don't compete on macro or portrait reach. The Sony E 18-135mm f/3.5-5.6 OSS is a superzoom, far more versatile but much slower and not a macro lens. The real comparison is with something like a dedicated macro lens from a known brand, but those aren't in the provided list, which is telling.

The Tamron Di III VC 28-300mm f/4-7.1 and Sigma Contemporary 18-50mm f/2.8 DC DN are the most interesting alternatives here. The Tamron offers insane versatility with its zoom range and has proven vibration compensation, making it a far better all-rounder even if it's slower. The Sigma is a fast standard zoom that's sharp and well-built, a known quantity that will deliver reliable results. Both of these lenses come from manufacturers with a track record. The Kosiy's main trade-off is its f/2.8 aperture and 105mm reach, but you're sacrificing so much in optical quality, build, and support that it's a losing trade for almost everyone.

Talking about value here is almost impossible because the pricing is completely unhinged. Our data shows a spread from $950 all the way up to a baffling $92,069 across different vendors. If you can find this lens at the low end, under a grand, you might be tempted to roll the dice as a budget portrait option. But even at $950, you're competing with used copies of genuinely excellent lenses from the major brands. A used Nikon or Canon 100mm f/2.8 macro with known, excellent optics and reliable autofocus can be found in that range, and they'll run circles around the Kosiy's 20th percentile optical score.

The high-end pricing is frankly a joke. For over ninety thousand dollars, you could buy a car, or a complete set of professional Z-mount S-Line glass with money left over. There's no scenario where this lens's performance justifies anything close to that. The budget score of 38.2 out of 100 reflects this disconnect. It's not a good value even at its lowest price, and at its highest, it's an absurdity. If you see it for a few hundred bucks on a clearance rack, maybe it's worth a curious look. Otherwise, steer clear.

The Kosiy 20100 is a bit of a puzzle box. On paper, it's a 105mm f/2.8 macro lens, which is a classic formula for getting up close with flowers, bugs, and ring shots at a wedding. The spec sheet promises a 1:1 reproduction ratio and some kind of vibration reduction, which sounds great for handheld work. But the more you dig into the details, the more questions pop up. The mount is listed as unknown, the autofocus is a mystery, and there's no weather sealing to speak of. It's like someone described a Nikon Z MC 105mm from memory but forgot half the important bits.

So who is this actually for? The scoring suggests it's best suited for portrait work, where it earned a 47.3 out of 100. That's not a stellar score, but a 105mm f/2.8 can deliver some really nice subject separation if the optics hold up. The budget score of 38.2 hints that it might be affordable, though the wild price range we're seeing across vendors, from $950 to over $92,000, makes that a tough call. This feels like a lens for someone who stumbles upon a deal and is willing to gamble on an unknown quantity for a specific, narrow use case.

But let's be real, the red flags are waving. The optical score sits in the 20th percentile, which is a polite way of saying the image quality is likely disappointing compared to almost any modern macro lens. The build quality is even worse, landing in the 17th percentile. For a lens that weighs a substantial 630g, you'd expect it to feel solid, but the numbers suggest otherwise. This isn't a hidden gem. It's a lens with a very specific set of compromises that you need to go into with your eyes wide open.

You should skip the Kosiy 20100 if image quality matters to you at all. The 20th percentile optical score means your photos will be soft and likely full of color fringing, which defeats the purpose of a prime lens. If you need a real macro lens for product photography, nature, or any kind of detailed close-up work, this is a non-starter. Look for a used Tamron 90mm f/2.8 macro or a first-party option from your camera brand instead. They'll cost about the same as the Kosiy's low-end price and deliver vastly superior results.

Also skip this if you need a lens that can handle a little rain or dust. With no weather sealing, you're one bad outdoor shoot away from a potential failure. And if you value your time and sanity, skip it because the unknown mount and autofocus situation is a support headache waiting to happen. This lens is for tinkerers with very low expectations, not for anyone who needs a reliable tool.

If you're a portrait shooter on an extreme budget and you stumble across the Kosiy 20100 for a price that seems like a typo, say under $400, it might be a fun experiment. The 105mm focal length and f/2.8 aperture can produce a classic look, and the bokeh isn't terrible. You could manually focus it on a static subject and get some pleasing results, as long as you don't pixel-peep. But you'd have to be the type of photographer who enjoys the gamble and doesn't rely on this lens for a paid gig.

For literally everyone else, this lens is a hard pass. The optical quality is a known weak spot, the macro performance is abysmal despite the 1:1 claim, and the build quality inspires no confidence. The complete lack of clarity on mount and autofocus compatibility makes it a risky purchase that could end up being a paperweight. You're far better off saving up for a used first-party macro lens or a proven third-party option from Tamron or Sigma. The Kosiy 20100 is a mystery box where the best-case scenario is mediocrity.