Why doesn’t a Canon EF-to-EF-M adapter work like an extension tube?
Asked 11/24/2018
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I’m using a Canon M50 and an EF/EF-S to EF-M adapter to mount EF lenses. Since the adapter adds space between the camera and lens, I expected it to let the lens focus closer like an extension tube, but it doesn’t. Why doesn’t the EF-to-EF-M adapter change the minimum focusing distance?
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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EF lenses are made with a 44mm registration distance. That is, the flange on the back of the lens is intended to be 44 mm in front of the imaging sensor or film. This allows the lens to focus the light striking the front of the lens on the imaging sensor or film that is 44mm behind the flange ring on the back of the lens.
EF-M cameras are designed with an 18mm registration distance. That is, the flange on the back of the lens is intended to be 18mm in front of the imaging sensor or film.
An EF to EF-M convertor adds the other 26mm needed to place the flange of an EF lens 44mm in front of the imaging sensor on an EOS M camera.
Without the convertor that extends the camera's 18mm flange distance, an EF lens would be focused 26mm behind the imaging sensor. The lens would be "focused" well past infinity and nothing would be in focus.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
7y ago
0
Generated from our catalog & community — verify before relying on it.
Because the adapter is not adding extra extension beyond what the lens already needs to work normally.
Canon EF lenses are designed for a 44mm flange distance—the distance from the lens mount to the sensor. Canon EF-M cameras have a much shorter 18mm flange distance. The EF-to-EF-M adapter simply adds the missing 26mm so the EF lens sits at its intended 44mm distance from the sensor.
So with the adapter in place, the lens is in its normal optical position and focuses just as it would on an EF-mount DSLR. It does not behave like an extension tube.
An extension tube works by moving the lens farther from the sensor than its designed registration distance. That extra distance shifts the focus range closer, often at the cost of losing infinity focus.
Without the adapter, an EF lens would sit far too close to the sensor on an EF-M camera and would be focused behind the sensor plane, so it would not focus properly at all.
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