The Matchup: M42-to-L versus B4-to-NEX
Choosing a lens adapter starts with a two-sided question: what mount is on your lens, and what mount is on your camera? This head-to-head looks at the K&F CONCEPT M42-L for M42 lenses to L-mount cameras, and the B4-to-NEX for B4 lenses to Sony NEX/E-mount cameras. They are built for different lens families and camera systems. The K&F targets L-mount bodies from Leica and Panasonic, while the B4-to-NEX targets the Sony NEX and α E-mount families. Your existing gear decides which one belongs on your camera.
Quick Verdict
Pick the K&F CONCEPT M42-L if you have M42 lenses and an L-mount camera from the Leica or Panasonic L-mount lineup. Pick the B4-to-NEX if you have B4 lenses and a Sony NEX or E-mount body. Let the mount match lead.
Price and Mount Path at a Glance
| Product | Mount Path | Listed Price | Price Bar |
|---|---|---|---|
| K&F CONCEPT M42-L | M42 lens to L-mount camera | USD 29.99 | |
| B4-to-NEX | B4 2/3-inch lens to Sony NEX/E-mount | USD 58.99 |
Where Each Product Wins
The K&F CONCEPT M42-L wins for the M42-to-L mount path. The B4-to-NEX wins for the B4-to-Sony E-mount path. These are different jobs, so the winner is the adapter that matches the lens mount you already own. There is no overall winner across all cameras, only the mount match. There is no right adapter for the wrong mount. That makes the decision about your lens collection first and the adapter second.
Product Notes
K&F CONCEPT M42-L Lens Mount Adapter
If you own M42 screw-mount lenses and an L-mount camera, this adapter is the one that completes the connection. It is built for manual focus, so you stay in control, and the snap-on fastening is part of the listed design. The adapter also allows infinity focus, which helps when you are shooting faraway scenes. It pairs with Leica SL, SL2, T, TL, TL2, CL and Panasonic Lumix S5, S1 and S1R bodies. The clear limitation is the L-mount direction: it will not work with Sony NEX or E-mount bodies, and it is not meant for B4 lenses. For first-time installation, the body menu needs the "Release shutter without lens" option enabled on compatible cameras.
B4-to-NEX Lens Mount Adapter
When your lens is a B4 2/3-inch mount and your camera is a Sony NEX or E-mount body, this adapter is the one to have. The ring is precision-made for B4 2/3-inch lenses, and the material is listed as brass and aluminum. It fits Sony E cameras such as the A6300, A7, A7II, A7S, NEX-7 and NEX-6, along with several NEX camcorders. The limitation is the reverse of the K&F: this adapter is built only for B4 lenses to Sony E-mount bodies, so it cannot be used with L-mount cameras or M42 lenses. If you are not shooting B4 glass, this is not the adapter for you.
Final Choice by Use Case
- M42 lenses + L-mount body: Choose the K&F CONCEPT M42-L.
- B4 lenses + Sony E-mount body: Choose the B4-to-NEX.
The mount path is the deciding factor. If your lens is M42, only the K&F adapter bridges to L-mount. If your lens is B4, only the B4-to-NEX bridges to Sony E-mount. Choose the adapter that lines up with your lens mount, and you will have made the right call.
