Will the EOS R3 retain all its functions with an EF lens?

[Brewster]

Greetings to all , I am new here. I am considering purchasing a R3 and am wondering if this camera will retain all of its functions while using EF lenses with an adapter.
Thank you for any thoughts.

[deebatman316]

Yes it will but note that older EF lenses ARE NOT FULLY COMPATIBLE with ALL features found in the EOS R Series. Those include 12 fps, Support for Dual Pixel AF, Slow AF Transitions, Smooth Aperture Transitions also the speed of the AF motor doesn't change. Those are hardware limitations that can't be solved with firmware updates.

[johnrmoyer]

I have EOS R5 with the Canon adapter, not R3, but I hope some of this might be helpful anyway.
I do not know which lenses you have, or which R3 features you might be concerned about.

All of my EF and EF-S lenses work better with my EOS R5 than with my EOS 80D.

I do mostly wildlife photography, so the wide angle advantages of RF lenses are not very important to me. I have not needed faster than 12 frames per second. The image stabilization coordination between IBIS and lens IS using an EF lens is good enough for me.

The EOS R5 has enough resolution that I still get about 17 megapixels in 1.6x crop mode with an EF-S lens.
The filter adapter does not seem to me to work well with my EF100-400mm f/4.5-5.6L IS II USM but does work well using a circular polarizing filter with other lenses.

[deebatman316]

That lens is known to not work well with filters attached to the front element and even the first gen version it doesn't work well either.

[Brewster]

Thank for your responses to my question. I am mostly interested in using the EF 400mm DO ii lens with these bodies. I recognize that this is an older lens but I do believe that it would work well for my interests.

Thanks again.

[johnrmoyer]

I have not used that lens EF 400 DO ii lens, but have tried older lenses on my EOS R5. Your lens seems to me like it would be a very good lens. The IBIS seems to me to really improve older lenses.

I suggest the plain Canon EF to RF adapter for telephoto. The control ring adapter has electronics and the plain adapter works well for me. The filter adapter allows use of one filter with any EF lens and no need to buy various filter sizes.

Some examples of older lenses with newer mirrorless camera body:

As an experiment, I tried EF-S 55-250mm f/4-5.6 IS with stacked extemders on my EOS R5.

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.

I have used a nearly 50 year old Minolta MC ROKKOR-X PG 1:1.4 f=50mm from a film camera.

Rudbeckia hirta (also called Black Eyed Susan) and Castilleja indivisa (also called Indian Paintbrush) blooming in Norman, Oklahoma, June 17, 2022 ; Minolta MC ROKKOR-X PG 1:1.4 f=50mm

EF 17-40 F/4L USM

Asclepias viridis (also called green milkweed, spider milkweed, green antelope horn) blooming in Norman, Oklahoma, United States on May 12, 2023 ; EF17-40mm f/4L USM

[Brewster]

John,

I enjoyed seeing your very nice images. The 400 DO ii does have native IS and I imagine that it would work well in concert with an R series body. I have a long prior history with Canon  but have been using with the Fuji GFX system for a while now. The problem with the Fuji is that it is a bit cumbersome for many of the moving targets that I would like to shoot so nowI am thinking about adding some Canon gear as well...we shall see.

Many thanks for your input and tha great images.

[Waddizzle]

This screenshot is from the R5 manual.  The R6 manual has the same table.  The R3 manual might have a similar table. There have not been any new EF lenses released since the R5 and R6 were released. 

I don’t see the 400mm DO listed. I would take that to mean it’s not fully compatible with the advanced AF features because it doesn’t refocus quite fast enough.

I disable them with my Sigma 150-600mm, and it works just fine. Instead of the dynamic AF tracking squares and rectangles, my AF point display resembles a DSLR, but with a lot more AF points. A whole lot more.  I use the 9-pt AF box most often. 

The IBIS system doesn’t improve lens performance. Image Stabilization improves the camera’s ability to focus by presenting a more stable image for it to analyze. The IS spends over 99.99% of its time stabilizing the image for the AF and the viewfinder.

It spends only a small fraction of second steadying the image for the image sensor. It is essential to remember that IS isn’t a substitute for a mechanical stabilizer or gimbal. 

Here’s my bottom line.  The 400mm DO will yield better results on an R3 than it ever could with a DSLR. 

[Peter]

EF800/5.6 focus preset doesn't work with R6 or M5. 400 DO II is newer, so maybe Canon solved that.

[deebatman316]

Lenses with AF Present don't work with mirrorless cameras so that problem isn't solved either. Also Canon's EF 35-80mm PZ (power zoom lens) doesn't work on mirrorless cameras either.