How to Identify Your Lens Mount
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Before you can answer "will this lens fit my camera," you have to answer a smaller question first: what mount is on the back of it? Here's how to work that out from the lens in your hands, rather than from guesswork.
Short answer
Turn the lens over and look at the back.
- A spiral thread — it's a screw mount. Measure the thread diameter to narrow it down.
- Tabs, notches or a flanged ring — it's a bayonet mount. These are maker-specific, and the reliable identifier is the marking on the lens, not the shape.
Then check it against the camera from the other direction: identify your camera model, and you'll know exactly which mount it takes. That's usually the faster half of the problem.
Screw mounts
A screw mount is a plain spiral thread. You rotate the whole lens several turns to fit it.
Measure straight across the thread with a ruler or calipers. Two diameters come up constantly on vintage gear:
- About 42mm — the M42 screw mount, used by a lot of makers, which is why it's often called universal.
- About 39mm — a 39mm thread, found on some rangefinder lenses and also on enlarging lenses.
Two warnings about measuring, because a number alone isn't proof:
Thread pitch matters as well as diameter. Two threads can share a diameter and still not screw together, because the spacing of the thread differs. If a lens starts to thread and then binds after half a turn, stop — forcing it damages both parts.
Not every lens with a thread is a camera lens. Enlarging lenses, for example, are built for a completely different job — projecting a negative onto paper in a darkroom, at close range. They were never designed to focus at infinity on a camera body. Some of them use thread sizes that also appear on camera lenses, so a matching thread on its own doesn't tell you what a lens is for.
Check what the lens was made to do, not just whether it screws on. A listing will normally say — enlarging lens, projection lens, bellows or macro gear all behave differently from an ordinary taking lens.
Bayonet mounts
A bayonet has tabs or notches around the rim. You line up a dot or mark, push, and twist a fraction of a turn until it clicks.
Each maker designed their own, and most makers used more than one over the decades. They can look extremely similar across brands. Identifying a bayonet by eye from a photo is unreliable, and confidently wrong answers are common.
What to look at instead:
- Markings on the lens. The name ring, the barrel, or the rear flange often carries the mount name or a series code.
- Coupling parts. Prongs, levers, pins and small tabs around the mount are there to communicate with a specific camera system. They're a strong clue that a lens belongs to a particular generation of a particular system — worth photographing and describing when you research it.
- Electrical contacts. Gold contacts mean the lens expects to talk to an electronic body. Their presence, number and position are system-specific.
- The manual or the box. If they survived — and with Japanese-market gear they often did — they'll name the mount directly.
Work from the camera's side too
This is the half people skip, and it's usually easier.
Find your camera's model name — normally on the top plate or the front. Once you know the exact model, the mount it takes is a matter of record, documented in its manual and in every review ever written about it. You then only have to match one known thing to one unknown thing, instead of two unknowns.
Look at the camera's mount opening as well: screw thread or bayonet, and if bayonet, how many tabs and where the coupling parts sit.
What about adapters?
Adapters let a lens from one system fit a camera from another, but there's a rule underneath that decides whether a given adapter can exist at all.
Every system has a fixed distance between the mount and the film or sensor. For an adapter to work properly, the camera's distance has to be shorter than the lens's — the adapter fills the gap. That's why mirrorless cameras, which have very short distances, can adapt an enormous range of vintage lenses, and why adapting between two older SLR systems is often impossible.
When the distance doesn't allow it, adapters sometimes include a corrective lens element to reach infinity focus. That works, but you're now shooting through extra glass, which affects image quality.
Also worth knowing before you buy an adapter: most simple adapters pass no information. Automatic aperture, metering coupling and autofocus generally stop working, and you operate the lens manually. That's normal, not a fault.
Why we don't publish a compatibility chart
You'll find plenty of them online, and they contradict each other more often than you'd hope. Mounts changed within systems, transitional versions exist, and some combinations physically attach while damaging one part or failing to focus.
We'd rather give you the method than a table we can't stand behind. Identify the mount on the lens, identify the mount on your camera, and confirm against the documentation for both.
Before you order
- Do you know your camera's exact model?
- Do you know what's on the back of the lens — thread or bayonet, and any markings?
- If it's a thread, have you measured rather than assumed?
- If you'll need an adapter, does one exist for that direction — and are you happy focusing and setting the aperture by hand?
Answer those four and the guesswork disappears.