M42 Lens Adapters: Will You Still Get Infinity Focus?

M42 is the friendliest of the old mounts. It is a plain 42mm screw thread, it was used by dozens of makers, and an adapter for it costs very little. The question that decides whether it will work on your camera is not the thread at all — it is one measurement.

Short answer

  • The thread is never the problem. M42 screws onto anything with the right adapter.
  • Infinity focus is decided by flange distance — how far the mount sits from the film or sensor. The M42 figure is 45.46mm.
  • If your camera's figure is smaller than that, a plain adapter works and infinity focus is unaffected. Every mirrorless system is comfortably smaller.
  • If it is larger, a plain adapter cannot reach infinity. This is the situation on some SLR mounts.
  • Check the A/M switch on the lens before you blame the adapter for photographs that came out brighter than expected.

The one number that decides it

Every mount has a fixed distance from the metal flange to the film plane or sensor. A lens is built to focus at infinity at exactly that distance. An adapter is a spacer: it pushes the lens further forward by its own thickness.

So the arithmetic is simple. There has to be room to give away.

  • Camera's flange distance smaller than 45.46mm — the difference is the room the adapter fills. Infinity focus behaves normally.
  • Camera's flange distance larger — there is no room. Even a zero-thickness adapter would sit too far forward, and the lens will focus close up but not quite reach infinity.

Figures published by the makers, for orientation. Worth confirming for your own body before you buy anything:

  • Micro Four Thirds 19.25mm — Sony E 18mm — Fujifilm X 17.7mm — Nikon Z 16mm — Canon RF 20mm — Leica L 20mm
  • Canon EF 44mm
  • Pentax K 45.46mm
  • Nikon F 46.5mm

The mirrorless systems all have tens of millimetres to spare, which is a large part of why old screw-mount glass became popular again. Canon EF has a much smaller margin but is still on the right side of the line. Pentax K shares the same figure as M42, so the adapter for it is a very thin ring rather than a tube — Pentax made an official one, and infinity focus sits right at the limit rather than comfortably inside it.

Nikon F is the mount where the number goes the other way. Adapters exist that add a small corrective lens element to bring infinity back within reach, and they do work, but you are then shooting through an extra piece of glass that the lens was not designed around. If your Nikon body is a Z-mount mirrorless rather than an F-mount SLR, none of this applies and a plain adapter is fine.

The 42mm trap: M42, T2 and M39

Three threads get confused with each other, and two of them are both 42mm across. Measuring the diameter alone will not tell you which one you have.

  • M42 — 42mm diameter, 1mm thread pitch. The one used on Takumars, Praktica, Zenit, Yashica and many others. Sometimes called the universal screw mount or the Pentax screw mount.
  • T2 — also 42mm diameter, but a finer 0.75mm pitch. Used on mirror lenses, telescope adapters and some third-party telephotos. It will start to thread onto an M42 adapter and then bind. Forcing it can damage both.
  • M39 / L39 — 39mm, and a genuinely different size. This covers Leica screw-mount rangefinder lenses and most darkroom enlarging lenses.

If a thread starts smoothly and then stiffens after a turn, stop. That is usually a pitch mismatch rather than dirt. Our guide to identifying your lens mount covers how to tell the families apart.

The A/M switch, and why the first shots look wrong

This catches almost everyone once.

Most M42 lenses have a small pin at the back. On the camera it was designed for, that pin is held in by the body and released at the moment of exposure, so the aperture stays wide open for a bright viewfinder and closes down only when you shoot. On an adapter, nothing presses the pin. The lens therefore has to be told to stop down by itself.

  • If the lens has an A/M switch, set it to M. The aperture ring will then actually close the blades as you turn it. Left on A, the lens may stay wide open regardless of the ring, and pictures come out brighter and with less depth of field than expected.
  • If your lens has no switch, some adapters have a small raised ring that holds the pin in — the opposite of what you want here — and some are flat, letting the pin sit free. A flat one is usually the safer choice.
  • Turn the aperture ring and look through the front before you shoot. If the blades move, you are set up correctly. It takes two seconds and settles the question.

How you meter and focus

There are no electrical contacts on a screw mount, so the camera has no idea what is attached. Everything is manual, and that is normal rather than a fault.

  • Metering. Because the aperture is physically closed while you compose, the camera meters through whatever it can see. Aperture priority, or manual with the meter reading, works well on mirrorless, where the screen brightens automatically.
  • Focusing. Focus peaking and magnified view make manual focus considerably easier than the plain viewfinder of an SLR. On a DSLR, live view magnification does the same job.
  • Recorded data. Aperture will not appear in the file information, and stabilisation systems that ask for a focal length will need it entered by hand.

What to look for in the adapter itself

Adapters are simple objects, which is why the differences between a good one and a poor one are all mechanical.

  • No play. Mounted up, the lens should not rock or rotate. Slack here can show as focus that shifts across the frame.
  • Machined metal, evenly finished. The surface that meets the camera side should be flat and square.
  • Flat on the lens side, for the pin reasons above, unless you specifically want the pin held in.
  • Some means of removal. Screw threads can tighten themselves over time. An adapter with a grip, a small set screw or a release makes it far easier to part the two later.
  • Matt black inside. A shiny inner wall can throw reflections into the corners on bright subjects.

What an adapter cannot do

Worth being clear, because it saves disappointment.

  • It cannot give you autofocus. There is nothing for the body to drive.
  • It cannot make a lens sharper, and it will not fix internal haze or fungus.
  • It cannot change the field of view your sensor sees. A 55mm lens frames differently on a smaller sensor than it did on film, and the lens itself is unchanged.
  • It cannot recover infinity focus on a mount whose flange distance is too large, unless it contains a corrective element — which is a different product with different results.

Before you buy the lens

Two things are worth settling in advance, because both are easier to solve before the lens arrives than after.

The first is the filter size, so a cap or a filter can be ready — see how to find the filter size of any vintage lens. The second is the rear cap: M42 rear caps screw on rather than clip, so a bayonet cap from another system will not fit.

Where to look next

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