I use a Metabones adapter for Canon lenses on a Sony body. Why doesn't the lens adapter, which sets the lens further from the sensor, have the same effect as using extension tubes, allowing closer focus?
Answer
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.
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