Why an M42-to-Sony E adapter doesn’t act like a macro extension tube
Asked 5/27/2026
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I’m adapting an M42 55mm lens from a 35mm film SLR to a Sony A7 using a Neewer M42 adapter with no optics. The adapter is about 27mm long. I also have Meike extension tubes totaling about 26mm, also with no optics.
With the M42 adapter, the lens focuses normally, including infinity. But adding 26mm of extension tubes to a native Sony E-mount lens gives obvious macro behavior and loses infinity focus.
If both accessories are just empty spacers of nearly the same length, why doesn’t the M42 adapter have the same macro effect? What is physically different?
Originally by Steve Smith. Source · Licensed CC BY-SA 4.0
Steve Smith
15d ago
2 Answers
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How is this arrangement different to fitting a 55mm lens to my A7 with the 2 macro extension tubes totaling 26mm in length, and why is the effect completely different?
The difference is if you have a 55mm lens in Sony E-mount which can directly attach to your camera without any adapter, it's already designed for the lens' flange to sit exactly 18mm in front of the sensor of your Sony α7. So when you add the extension tube, you're placing an additional 26mm between the lens and the camera with a lens that was designed to sit 18mm in front of the sensor.
With the M42 lens, it was designed to focus light on a film plane or sensor at 45.5mm behind the flange of the lens. The adapter you use adds the needed 27.5mm difference.
Originally by Michael C. Source · Licensed CC BY-SA 4.0
Michael C
15d ago
0
Generated from our catalog & community — verify before relying on it.
The key difference is what distance the lens was designed to sit from the sensor.
An M42 lens is designed for a much longer flange focal distance than Sony E. M42 expects about 45.5mm from lens mount to film/sensor, while Sony E is about 18mm. So the adapter’s job is simply to restore that missing ~27.5mm. It puts the M42 lens at its correct designed distance, so the lens behaves normally and can still focus to infinity.
An extension tube does something different: it moves a lens farther from the sensor than its design distance. That extra spacing lets the lens focus closer and increases magnification, but you lose infinity focus.
So even if the adapter and extension tubes are physically similar hollow spacers, they are not doing the same thing in the system:
- adapter: restores the correct registration distance for that lens mount
- extension tube: adds extra distance beyond the correct registration distance
That’s why a 26–27mm M42 adapter is normal, while 26mm of extension on a native Sony E lens creates macro behavior.
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