The front-ring Ø mark, lens manual and official specification are better evidence than the lens's focal length.
Make the exact-model decision
Match the exact model suffix; Canon RF-S18-45mm and EF 50mm f/1.8 STM both specify 49 mm. Locate the exact lens front-thread size in its manual or spec table, then verify whether the accessory is screw-in, magnetic, square-holder or drop-in.
A manufacturer example
Canon identifies the RF-S18-45mm as a 49 mm-filter APS-C lens and describes automatic APS-C cropping on compatible full-frame EOS R cameras. Use the linked page for its stated model and revision; a different model may need a different support document.
Where to draw the line
Thread arithmetic can identify a ring direction, but it cannot certify hood clearance or an image without corner shading. If a source does not address the exact feature you need, record that feature as unknown rather than treating silence as support or incompatibility.
Your next step
Take the model code from the body or lens label, open the official source below, and compare its scope with the equipment you have. Then use the calculate a filter route to organize what is known. Do not mount an item that resists or appears on a maker’s exclusion list.
Official sources and scope
Checked for this reference on October 30, 2026. Product firmware and support lists can change; inspect the current maker page for your exact revision.