What Is an M42 Lens Mount? A Complete Beginner’s Guide

An M42 lens mount is a 42 mm screw-thread standard, formally M42 × 1, that screws lenses onto 35 mm SLR and modern mirrorless cameras through simple mechanical adapters.

A lens that refuses to seat on a modern body usually isn’t broken — it just belongs to a 1949 screw standard that predates every autofocus system on the market. M42 was introduced on the East German Contax S built by VEB Zeiss Ikon, and KW used it on Praktica cameras the same year. Because Pentax and Praktica both built around it, the mount picked up the names Praktica mount, Pentax screw mount, and Pentax/Praktica screw mount. All three describe the same M42 × 1 mechanical thread, even though lens coatings and aperture mechanisms varied from brand to brand.

Production of M42 SLR bodies faded through the late 1970s and early 1980s, though Russian Zenit cameras kept the standard alive for years afterward. Today the mount survives mainly because adapters let these old lenses work on new cameras.

M42 Specs: What The Numbers Actually Mean

The core number is 42 mm — the thread diameter — paired with a 1 mm thread pitch. That pitch is the detail that matters most, and it’s the one beginners get wrong.

• Formal designation: M42 × 1, where the second figure is the thread pitch in millimeters.
• Flange focal distance: 45.5 mm, the span from the mounting surface to the film or sensor plane. Pentax-related specifications often list 45.46 mm instead.
• Mount type: threaded screw mount, with no rotational locking tabs like a bayonet.
• Original format: 35 mm film, roughly 36 × 24 mm.
• Electrical contacts: none on the original mechanical standard.

An important distinction: “M42” describes a physical thread, not a manufacturer, lens formula, coating, or image-quality tier. Lenses were made by many companies and sold under dozens of brand names, so two lenses with identical mounts can perform very differently.

How Do You Use M42 Lenses On A Modern Camera?

You use an M42 lens on a modern camera by buying a camera-specific adapter whose thickness compensates for the difference between your camera’s flange distance and M42’s 45.5 mm. Get that spacing right and everything else follows.

Adapters exist across the major systems. For Fujifilm X bodies, M42-to-FX adapters cover models including the X-E1, X-E2, X-E3, X-Pro1, X-Pro2, X-T1, X-T10, X-T20, and X-T2. For Canon EF and EF-S DSLRs, M42-to-EF adapters fit cameras such as the EOS 5D, 6D, 7D, 90D, 80D, and Rebel-series bodies. Mirrorless mounts are generally the easiest case, because many sit closer to the sensor than M42’s 45.5 mm. Nikon F DSLRs are the exception — Nikon F’s flange distance is longer than M42’s, so some adapters need corrective optics to reach infinity focus, and those optics can cost you image quality. If you’re shopping for glass to pair with an adapter, this roundup of tested M42 mount lenses for modern cameras covers the options worth mounting.

A basic mechanical adapter carries no electronics. Autofocus, automatic aperture control, lens-based image stabilization, and EXIF data transfer all stop working. Focus and aperture move to your hands. Many cameras also need a menu change — usually a “release without lens” or “shoot without lens” setting — before they’ll fire at all. Metering varies by body, and stopped-down metering or full manual exposure is often the practical route.

M42 Mistakes That Cost Beginners Money

Most M42 problems trace back to one of five mix-ups, and each one is cheap to avoid once you know it exists.

1. Confusing M42 with T-mount. T-mount shares the 42 mm diameter but uses a 0.75 mm pitch instead of 1.0 mm. Forcing the wrong thread damages the lens or the adapter — check the pitch before you twist.
2. Assuming every Pentax lens is M42. Later Pentax K lenses use a bayonet mount and need a completely different adapter.
3. Expecting autofocus or automatic exposure. Original M42 lenses are mechanical, and an unchipped adapter cannot add electronic functions.
4. Ignoring aperture variations. Older preset, automatic, and direct-aperture lenses behave differently. Confirm the aperture pin or actuator works with your specific adapter and body.
5. Assuming every adapter reaches infinity. Incorrect thickness, rough machining, or a camera with a longer flange distance can all block infinity focus.

On the Wikipedia M42 lens mount reference, the thread figures are stated as 42 mm diameter and 1 mm pitch, which is the check worth running against any lens you’re about to buy. M42 lens mount specifications confirm the same 45.5 mm flange distance used throughout this guide.

Is An M42 Lens Worth Buying Today?

An M42 lens is worth buying if you want manual control and don’t mind giving up autofocus, electronic aperture, and stabilization. It is not worth buying if you need any of those features to work automatically.

What you get is real optical glass for very little money, plus a threading ritual that forces you to slow down and think about focus and exposure. What you give up is speed and convenience. The mount itself is simple, well documented, and unlikely to surprise you — the surprises come from individual lens conditions and from adapters of varying quality.

Before buying, confirm three things: the lens is genuinely M42 × 1 and not T-mount, the adapter exists for your exact camera mount, and the aperture mechanism suits your shooting style. Get those right and the rest is practice.