Adapting M42 lenses to Sony E

An M42 lens works on any Sony E-mount camera with a plain mechanical adapter and keeps infinity focus. M42 lenses were designed to sit 45.46mm in front of the film; Sony E puts the sensor 18.0mm behind the mount face. The adapter is therefore a 27.46mm tube with a 42mm x 1mm female thread on the front and a Sony E bayonet on the back, and nothing else. This guide covers what the tube needs to get right, what the lens does to complicate it, and how to set up the camera. The M42 to Sony E adapter page lists every M42 lens in the catalog with its effective focal length on full frame and APS-C.

The numbers

Because the adapter only has to be a rigid tube of the right length, a good one costs little. What separates a good adapter from a bad one is machining tolerance. An adapter machined a few hundredths short lets the lens focus slightly past infinity, which is harmless and even useful because it absorbs lens-to-lens variation. An adapter machined long stops the lens short of infinity and no amount of adjustment fixes it. Buy from a maker that states an infinity guarantee.

Flanged versus flangeless adapters

M42 adapters come in two styles. A flanged adapter has an inner lip at the bottom of the thread. The lip stops the lens at a fixed depth and, on lenses with an automatic diaphragm pin, presses the pin in so the aperture ring works. A flangeless adapter is an open thread all the way through; the lens screws in until its own mount face meets the adapter face. Flangeless adapters accept every M42 lens including the ones with protruding tabs, but do nothing for the pin.

Which one you need depends on the lens. The Helios 44-2 is preset and has no pin, so either style works. The Pentacon auto 50mm f/1.8 has an auto pin and no manual switch, so it needs a flanged adapter or it stays wide open. The Super-Takumar 50mm f/1.4 and SMC Takumar 50mm f/1.4 have an Auto/Manual switch: set it to Man and any adapter works. The Fujinon 55mm f/1.8 EBC has the Fujica ST open-aperture coupling tab on its rear, which can hit the lip of a flanged adapter and stop the lens seating; use a flangeless adapter or one with a cut-out.

Aperture pins and the wide-open trap

The most common complaint from a first-time adapter is that the aperture ring changes the number but not the picture. That is the auto pin. On an SLR body a lever pushes the pin at the moment of exposure to close the blades. Off the body, if nothing pushes the pin, the blades stay open at f/1.4 or f/1.8 no matter what the ring says. Three fixes exist: a flanged adapter that holds the pin in permanently, an A/M switch on the lens set to M, or a small pin-depressing ring that some adapter makers sell as an accessory. Check which your lens needs before you order.

Scale alignment

A screw thread has no rotational index, so where the aperture and focus scales end up when the lens is tight depends on the lens and the adapter. Some lenses land with the scale on top, some at the side. It is cosmetic and affects nothing optically. If it bothers you, adapters with a rotating locking collar let you set the position, and a thin brass shim under the lens flange does the same thing permanently.

Camera settings on Sony E

1. Set Release w/o Lens to Enable. Without it the camera refuses to fire because it detects no electronic lens.
2. Set the SteadyShot focal length by hand on bodies with in-body stabilisation. Enter the lens focal length (58 for a Helios 44-2) so the sensor moves the right amount.
3. Turn on Peaking Level (mid or high) and assign Focus Magnifier to a button. Manual focus at f/1.4 is only reliable with magnification.
4. Use Aperture Priority or Manual mode. The camera meters through the lens at the working aperture, so stopping down darkens the finder unless Live View Display is set to Setting Effect Off.
5. Expect no aperture or lens data in EXIF. The file records the focal length you entered for SteadyShot and nothing else about the lens.

Full frame versus APS-C

M42 lenses were made for 35mm film, so on a full-frame Sony (a7, a7R, a7S, a7C, a9, a1) they frame exactly as designed. On APS-C bodies (a6000 to a6700, ZV-E10, FX30) the 1.5x crop trims the edges: a 50mm frames like a 75mm and a 58mm like an 87mm. The aperture, depth of field at a given distance, and exposure do not change. Two effects follow. Swirly bokeh, which lives at the frame edges, is weaker on APS-C. And corner softness, which vintage lenses show wide open, is cropped away, so many M42 lenses look sharper on an a6000 than on an a7 at the same aperture. A focal reducer (speed booster) adapter reverses the crop on APS-C at the cost of optics in the path and a slightly larger effective aperture.

Adapter variants worth knowing

• Helicoid adapters add a built-in extension so any M42 lens focuses closer than its scale allows. A 50mm gains roughly 0.2m of close focus. Infinity is preserved at the collapsed position.
• Tilt adapters exist for M42 to E and give selective focus effects; they are too thin to keep infinity with some lenses, so check the maker's specification.
• Focal reducers for APS-C bodies restore full-frame framing and gain about one stop. They add glass and cost several times a plain adapter.
• Autofocus adapters with a moving helicoid (Techart style) accept an M42 lens through an M42 to Leica M ring and autofocus it on newer a7 bodies with a weight limit around 700g.

What to buy first

A flanged plain adapter from a maker with an infinity guarantee handles the Takumars, the Helios, and most Zeiss Jena and Pentacon lenses. Add a flangeless adapter only if you own a Fujinon EBC or another lens with a coupling tab. Read the M42 mount page for the history and the full lens list, and before buying the lens itself, run the buying checklist. If the lens is a Takumar or other 1960s to 1970s fast fifty, read thoriated glass and yellowing so a warm tint does not surprise you.