Which mount adapter, EF-EOS R or M, can I use for EOS 1DX Mark III?

[StraightShooter]

I just bought the Canon EOS 1DX Mark III, and have an EF camera (1 red circle on lens mount). According to Canon's site, I should use EF-EOS R or M. https://www.canon-europe.com/lenses/tech-guide/compatibility/

I don't know which mount adapter to use, unless I'm misunderstanding and I can use either.

[Waddizzle]

Neither mount adapter would work.  You can use DSLR lenses [on] MILC bodies, but not the other way around.  MILC lenses cannot be adapted to use on DSLR bodies.

[Tronhard]

Do you mean you just bought the 1DX MkIII and have and EF lens (not camera)?   That lens should fit directly onto the 1DX body without and adapter. 

As Waddizzle said (but you may not have understood his terminology) you cannot fit that EF lens on either an EOS M or EOS R body - there is no adapter because of the physical characteristics of those mounts.

[Lotus7]

"As Waddizzle said (but you may not have understood his terminology) you cannot fit that EF lens on either an EOS M or EOS R body -- there is no adapter because of the physical characteristics of those mounts."    

Whoops!  TREVOR, I'm sure you really didn't mean to say that.

Actually, as Waddizzle said:  A EF lens will directly fit a 1DX MkIII without any adapter, but for use on a mirrorless RF mount [body] would need a EOS-EF R adapter or a EF-EOS M adapter to work on a M camera.

EF and EF S lenses have a back focus distance of 44 mm, so there is plenty of room for adapters to fit the 20 mm RF flange to image sensor space.  However the reverse is not true.  Without including relay optics, it's not possible to place a RF or M mount lens on a EF body and have it work optically.

[Tronhard]

Oops! You are RIGHT. Thanks for catching that.

I meant to say you cannot fit the EF-s, EF-M,  RF or RF-S lenses on the DSLR body you have, which I am guessing is what you want to do since you say you have the 1DX body which is a full-frame DSLR camera..

The only lenses that will fit your camera are EF type lenses.

[StraightShooter]

Ok, so EF lenses. Yes, I have a full-frame hybrid DSLR/ mirrorless.  From my understanding of their page, I can use EF, TS-E, or MP-E lenses.    Scrolling down the page though (on the link I included in my original question), I came across Canon adapters, and that's what confused me and prompted me to ask here. 

Thank you, everyone.

[Tronhard]

To avoid any confusion when engaging with folks - who might be wondering what a hybrid is, the 1Dx is still considered a DSLR - despite the fact that many later such DSLR camera can use the LCD at the back and read straight off the sensor.

[Tronhard]

Since I understand it that the OP has the 1DX body, I am assuming they want to know what lenses will work with that.  To try to sort that out, I enclose the following table:  Now Corrected!

[Waddizzle]

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@Tronhard wrote:

Since I understand it that the OP has the 1DX body, I am assuming they want to know what lenses will work with that.  To try to sort that out, I enclose the following table:

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Wait a second.  Does that chart actually say that EF-M lenses can be used on APS-C DSLR camera bodies with an adapter?  If so, then that would be incorrect.

[Tronhard]

Thank you for the correction Bill, I shall replace that post IMMEDIATELY - I had a prickly feeling at the back of my neck, but could not pin it down.  That's where a second person looking at it is invaluable!