Why doesn’t a Canon EF-to-Sony E adapter act like an extension tube?
Asked 9/12/2018
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I use a Metabones adapter to mount Canon EF lenses on a Sony E-mount body. Since the adapter places the lens farther from the sensor, why doesn’t it behave like an extension tube and let the lens focus closer?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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Because the Canon EF mount lens "expects" to be further from the sensor than a Sony E mount lens; this is known as the flange focal distance or the registration distance - a Canon EF lens focuses the incoming light on a plane 44mm behind the lens, while a Sony E lens focuses it on a plane 18mm behind the lens.
If you somehow bodged it horribly so that a Canon lens was mounted in the same place as the Sony lens, it would be focusing everything on a point 26mm behind the sensor and it would all be a bit of a disaster really. The EF to E lens mount adapter ensures that the Canon lens is mounted 44mm from the sensor so that the incoming light is focused in the correct place.
Originally by user11371. Source · Licensed CC BY-SA 4.0
user11371
7y ago
0
Generated from our catalog & community — verify before relying on it.
Because the adapter is usually restoring the lens to the distance it was designed for, not adding extra extension.
Canon EF lenses are made to sit about 44mm from the sensor, while Sony E bodies place native lenses about 18mm from the sensor. An EF-to-E adapter simply fills that difference so the EF lens ends up at its correct registration/flange distance and keeps its normal focus range, including infinity.
An extension tube, by contrast, adds distance beyond the lens’s intended mount-to-sensor spacing. That extra extension shifts the focus range closer and usually loses infinity focus.
So in your case, the adapter only looks like a tube physically, but optically it is not “extra” extension—it is the required spacing for that lens mount.
If you adapt a lens from a system with a shorter flange distance to a body with a longer one, then a simple adapter would act more like an extension tube unless it includes optics. Some adapters also contain glass, in which case they’re not just spacers anymore; they also act as optical converters.
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