This question already has an answer here:

By adding an extension between the camera and the lens, in this case a converter so I can use EF lenses on my Canon M50, you would think any lens would focus closer but it doesn't. Why is that? :/

marked as duplicate by scottbb, xiota, mattdm, Hueco, Philip Kendall Nov 25 '18 at 6:49

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.


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.

  • Of course. I was just thinking about the problem. The converter has simply put the unconverted lens where it needs to be on the camera body ie. further away from the sensor. The corresponding f stop would need to adjusted I'd imagine. Allowing for a degree of light loss. Am I right? – Mark Giles Nov 25 '18 at 3:56
  • No adjustment of the f-number would be required (beyond what is already applicable when the lens is used on an EF mount camera), because the lens would be used at the magnification for which it was designed when at the proper registration distance. Only if the lens were extended further forward than 44mm from the sensor would the increased magnification affect image brightness the way an extension tube would. If the lens were placed closer than the designed 44mm registration distance, the (out of focus) image would get brighter due to concentration of the image to a smaller image circle. – Michael C Nov 25 '18 at 16:54

Not the answer you're looking for? Browse other questions tagged or ask your own question.