EF to RF Lens Adapters

[Andrew_H]

I was wondering if any of the canon ef to rf adapters worked for non-canon brands such as sigma?

[Tronhard]

Absoultely. I have shot with Sigma 150-600c and 60-600s lenses and the basic Canon adapter has worked flawlessly

[Andrew_H]

Thank you so much! Do you think it would work well with the sigma ef 18-300mm lens?

[Tronhard]

I have not specifically used that lens but there is no reason why it should not.

The interface standards are very consistent.

[Tronhard]

What camera body do you intend to use the lens and adapter with?

[Andrew_H]

The EOS R5.

[Tronhard]

As I understand it, the Sigma 18-300 is designed for an APS-C camera body and the R5 is a Full-Frame unit - this leads to some complications you may not be aware of. 

In the case of DSLRs, your lens would not attach to a FF body at all.  In the case of the R-series bodies, the adapter will physically allow the camera to be attached to a FF body and will work.  The issue is with the sensor and the area of the projected image it will record.   

An APS-C specific lens will have a narrower projection of the image to the sensor (because APS-C sensors are smaller).  Thus the FF sensor on the R5 will not receive an image across its whole surface area.

So, what happens is that the camera may sense that the lens is not designed for a FF sensor and will automatically go into crop mode, thus cropping the area from which it records your image.  This means that your pixel count will be significantly reduced - by a factor or 2.56 (the square of the crop factor).  Thus your R5, which has a 45MP sensor, will actually only record an image of 17MP.  If the camera does not sense that your lens is a crop EF-S unit,  then you can put the camera into crop mode via the menu system.

To learn more I recommend you download a copy of the R5 manual HERE and refer to P913 for an explanation of this.

If you want a comprehensive explanation of this relationship between mount, sensor size, lens focal length and Field of View (or Capture) see My Article on Equivalence   As part of the paper, I included a series of examples of how the relationship between sensor size and lens format works, and in your case example 6 is exactly your situation, although the camera cited is a R6.

[johnrmoyer]

You are correct as usual.

I use EF-S lenses on my EOS R5. They work better than the same lens worked on my previous cameras. The 1.6x crop mode has a few less pixels than my EOS 80D and more pixels than my EOS 450D. I especially like myEF-S 18-135mm f/3.5-5.6 IS USM on the EOS R5.

I also use a 45 year old Minolta lens from a film camera with an Urth adapter on my EOS R5. The IBIS and manual focus aids are very helpful with this lens that has no electronics.

[Waddizzle]

The camera will not sense that the lens is designed for APS-C sensor bodies because the actual mount is EF, not EF-S.  All third party manufacturers use the EF mount,  no matter if the lens is designed for FF or APS-C bodies.

The camera will not automatically switch to crop mode.  You would have to change it in the menus.

[EngineerKyle]

The camera is still pretty aware of which lenses are designed for crop sensors and which cover the full frame, even from third parties. EF-S and EF are actually identical in every facet except for the physical protrusion of the rear element. Whether or not the camera can detect it is something that the lens or camera is aware of via firmware or database. For example, the sigma 18-35 forces the camera into crop mode and uses a regular EF mount, not EF-S.

Because it's not a fully manual lens, I would be about 99% sure it would crop automatically.