Lens adapters solve one very specific problem: making the lens you already own fit the camera you actually use. Between these two adapters, the decision is not about which is objectively better-it's about which mount pair you're trying to connect. The K&F Concept M39-LM carries a listed price of USD 17.99 and is built to bring M39 screw-mount lenses to Leica M bodies. The B4-NEX ring carries a listed price of USD 58.99 and is built to connect B4 2/3" lenses to Sony E-mount cameras. Start with the mount on your lens and the mount on your camera, then use the details below to confirm the fit.
Quick Verdict
Match the lens mount to the camera mount first. If your lens is M39 screw-mount and your body is Leica M-M9, M8, M7, M6, or MP-the K&F Concept adapter is the direct fit. If your lens is B4 2/3" and your body is Sony E-mount-like the NEX series, A6000/A7 lines, or E-mount camcorders-the B4-NEX ring is the one that connects them. The right mount pairing matters more than the price gap.
Price and Fit at a Glance
| Item | Listed Price | Relative Price | Mount Path |
|---|---|---|---|
| K&F Concept M39-LM (50-75) | USD 17.99 | M39 lens to Leica M body | |
| B4-NEX Adapter Ring | USD 58.99 | B4 2/3" lens to Sony E-mount |
Where Each Product Wins
- The K&F Concept M39-LM wins when your lens is M39 screw-mount and your camera is Leica M. It includes a lock pin for infinity focus, and it's the option aimed squarely at Leica shooters.
- The B4-NEX ring wins when your lens is B4 2/3" and your camera is Sony E-mount. It keeps a broadcast-style prime lens in service, and its brass-and-aluminum construction is built for that connection.
Product Notes
K&F CONCEPT M39-LM(50-75) Leica M39 50mm/75mm Lens to Leica M Lens Mount Adapter
The K&F Concept ring is the adapter to reach for when you want to put M39 screw-mount lenses on Leica M bodies such as the M9, M8, M7, M6, or MP. It's made of metal and includes a lock pin for infinity focus, which is helpful for normal shooting. One thing to keep in mind: the ring has no ROM connection, so you'll set diaphragm control manually. Adapting an APS-frame lens to a full-frame camera may create vignetting, so check your specific lens and body combo. If your lens and camera both fit the M39-to-Leica M path, this adapter is the one that gets you there.
Lens Mount Adapter Ring B4-NEX for B4 Lens to Sony NEX E-mount Camera
The B4-NEX adapter solves a different problem: it lets you mount a B4 2/3" lens on Sony E-mount cameras and camcorders, including the NEX series, A6000, A7 family, and E-mount VG models. Its brass-and-aluminum build is designed for a precise fit, and it lets you keep using an irreplaceable prime lens instead of buying new glass. Diaphragm control is manual, and you'll need to confirm your B4 lens works with the specific Sony body you own. The limitation here is scope: this adapter is for B4-to-Sony E, not for Leica M systems. If your setup is B4 glass on a Sony E body, the B4-NEX ring is the one that matches.
Final Choice by Use Case
Choose the K&F Concept M39-LM if you shoot M39 lenses and use a Leica M-mount body. It's built for that pairing and includes a lock pin for infinity focus.
Choose the B4-NEX Adapter Ring if you shoot B4 2/3" lenses and need them on a Sony E-mount body. It's the one built for that broadcast-to-mirrorless connection.
