EF-S Lens compatible with the EOS-R?

[Jul]

I have a 35mm ef-s lens on my old cam body. Will it fit on the eos-R or do I need an adapter? Does this lessen the quality? 

I am not sure what to make of the notations on the canon website. 

[Waddizzle]

In order to use any EF/EF-S lens on an RF mount body you would need one of the Canon EF to RF mount adapters.  Be aware that EF-S lenses are made for APS-C image sensors, so they project a smaller image circle than a full frame EF lens.  

This means the camera will enter a crop mode if it detects a Canon EF-S lens has been attached to the mount adapter.  Because of the cropping, the full resolution of tne sensor will not be retained in the crop images.

[jrhoffman75]

See page 52 of your manual.  

[Tronhard]

As mentioned, using an EF-S lens is possible on the FF R body via an EF-RF adapter.  However, as explained, the lens projects an image that does not cover the whole sensor, as it is designed for a smaller APS-C sensor.  The R will go into 'Crop Mode' to reduce the area of the sensor to match that of an APS-C unit and avoid a vignette effect.  This has a drastic effect on the resolution of the sensor.

To expand an APS sensor to the same size as a FF sensor would mean increasing its lineal dimensions, measured in both directions by a factor of 1.6.  That is known as its crop factor.  When considering the area covered, rather than the magnification factor, one takes the square of that number: 1.6 x 1.6 = 2.56.  This means that the area of the APS-C sensor is reduced to 39% of that of the FF sensor. 

So, what does this mean?  Well, effectively your 30MP sensor is using only 39%, rendering only 11.5MP which is a significant reduction in quality.

Really, if you want to make the best of the camera and its features, then you need to invest in RF lenses, or even get an EF lens to use with the adapter if you can't afford that.  Canon have made some good and reasonably cheap STM lenses for the RF mount, so you might well afford one of those.

[johnrmoyer]

I have the EOS R5 instead of the R. I often use EF-S lenses with an adapter.

All of my EF-S lenses work better on the newer camera than they did on older cameras. This is an inexpensive way to make use of the lenses one already has.
This photo was made with an EF-S 55-250mm f/4-5.6 IS, Kenko TELEPLUS HD C-AF 2X DGX, and Vivitar SERIES 1 1.4X AF at 700mm focal length and F/16. F/16 is wide open for that combination. It was autofocused.

A Bumble Bee (Bombus pensylvanicus) was on a Zinnia in Norman, Oklahoma, United States on September 2, 2021. I made this photo with a very old lens as an experiment. I purchased the lens in 2011 and nearly wore it out. For this photo, it was attached to a newer camera body with an adapter with two telephoto extenders.

[Tronhard]

So, in brief, to answer your questions:

Will it fit on the eos-R or do I need an adapter?  It will fit on the EOS R with an EF-RF adapter

Does this lessen the quality? The quality of the lens will not be any less, but you will be using only a fraction of the sensor's area, which will result in a major loss of potential recording quality - which is really what it's all about.  Whether that matters to you depends on what you produce.  If you use the camera simply to upload photos to social media, not so much;  for display on digital display or small prints, perhaps not; for printing anything larger quite likely - depending on size.

Like John (in his response) I have tried an EF-S lens with my Canon R5 and the results are ok for digital display.  However, what John didn't mention is that the R5 has a 45MP sensor, so when the reduction of recording area is taken into account you still get 17.5MP - which is a lot better than your R will offer.