The first time you attach a manual lens, such as the Meike 50mm f/1.7 RF lens in the video below, to your Canon EOS RP you will need to go into your MENU settings and tell the camera to allow a manual lens to be used.
There are certain modes that will not accept manual lenses to work, specifically the AV (Aperture Priority) setting, as even with the Release Shutter w/o Lens enabled, the camera will not read the aperture. It shows up as '00' in the reading. For this to work, it's best to have your camera on set Manual mode.
1) Enable Release Shutter w/o lens (allows manual lens to work on Canon EOS RP):
-- Go to MENU, go to C.fn III - No. 7, select Enable Release Shutter w/o lens
You have to use your eye to gauge the lighting, but this is fairly easy, as you get a WYSWYG (what you see is what you get) view if you're using a manual lens. It's likely that if you're moving up to using manual lenses, you are probably familiar with your manual settings. If not, it's not that difficult to set your camera up in Manual mode to accommodate manual lenses. If not, that should be your primary objective before taking on a manual lens.
It's fine to just leave that set as a default in your camera, so anytime you go from auto-focus to manual lenses, the camera will immediately be ready to go.
2) Manual Focus Peak Settings (makes manual focusing more reliable):
-- Go to MENU, then go to Camera - No 8 - MF peaking settings. Hit OK, select Peaking, hit OK, select On.
You can go old-skool and eyeball your manual focus as you turn the focus ring, or you can use the technology that came with your camera, and get a more precise focus every time, without straining your eyes into a headache.
You can scroll down and change the level from High to Low (I prefer High), and below that you can choose the outline color you prefer. I prefer Red, but you have the choice of Yellow and Blue. If you are in an environment with a lot of red, then you'd want to change the highlight color to yellow or blue, etc. You want a color that stands out and is easy to see when looking at the back of the camera. Red is my default.
3) Now you are all set to use any manual EF or RF lens on your Canon EOS RP (EF lenses require an adapter, and you lose more than half of your pixels, but for social media, online or non-gallery prints, most people would never notice this difference).