Quick answer
The SquareHood Summicron-C 40mm Adapter Ring lets you use standard E39 filters on your Leica 40mm lens by converting its rare 39mm thread (0.70mm pitch) to the common E39 filter thread.
Why you need this adapter
Leica used an uncommon 39mm thread with a 0.70mm pitch on the Summicron-C 40mm lens. Standard 39mm filters use either E39 (0.50mm pitch) or the more modern 0.75mm pitch found on most Japanese cameras. This mismatch means you cannot directly screw on most filters.
The SquareHood adapter ring is machined from solid brass with a black paint finish. It screws into the lens and provides a standard E39 female thread, allowing you to attach your existing filters without buying rare or expensive Leica-specific ones.
What you can and cannot use
The adapter accepts any filter with a standard E39 (39mm x 0.50mm) thread. This includes UV, circular polarizer, ND, and protective filters from most major brands.
It does not convert the lens to accept 40mm filters or step-up rings. The adapter is designed solely for E39 filters. If you need larger filters, you will need an additional step-up ring that fits the E39 thread.
How to mount the adapter and attach a filter
- Clean the lens thread: Use a blower or soft brush to remove dust from the 39mm thread on the front of your Summicron-C 40mm lens. Any debris can cause cross-threading or scratches. Check for grit before threading.
- Align and hand-tighten the adapter: Hold the SquareHood adapter ring with the female E39 thread facing outward. Gently place it on the lens thread and rotate clockwise until you feel resistance. Stop immediately if it does not turn smoothly—do not force it. If it binds, remove and re-align; cross-threading can damage both parts.
- Attach your E39 filter: Screw your standard E39 filter (UV, polarizer, ND, etc.) onto the adapter's female thread. Tighten only until snug—overtightening can make removal difficult. Use a filter wrench if the ring is stuck, never pliers.
- Check for vignetting: Look through the viewfinder or take a test shot at your widest aperture. Thick filter rims or stacked filters may cause corner darkening. If you see vignetting, use a slim-profile filter or remove one layer. Slim filters reduce the risk of vignetting on wide apertures.
Product details
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SquareHood - Summicron-C 40mm Adapter Ring: Leica used an extremely rare 39mm thread with 0.70mm pitch on the Summicron-C 40mm lens; standard E39 filters use 0.50mm pitch. Without this adapter, you cannot mount most standard filters directly on the lens, limiting your creative options.
SquareHood - Summicron-C 40mm Adapter Ring
