Hi @MNshutterbug welcome to the forums. The lens reports the focal length to the camera. The camera records the information the lens provides it. If you’re using lens correction it slightly crops in. So the entire lens image circle isn’t used. But that shouldn’t affect the focal length. Now flash photography can be affected by this. If the lens reports the wrong focal length it will affect the speedlite’s motorized zoom head. Causing over or underexposed shots. E-TTL II uses both focal length and focus distance information (if the lens has a focus distance encoder). To help calculate flash power. Note all Canon brand EF lenses have a focal length encoder. Older EF lenses lack a focus distance encoder for E-TTL II. Most lenses released after about 1990 have a focus distance encoder. Ring Type USM lenses first got these focus distance encoders. Then started to be added to Micro USM & Micro Motor lenses. Note very old Arc Form Drive (AFD) lenses released between 1987-August 1990. Never had focus distance encoders instead Canon discontinued these lenses. With newer Ring Type USM or Micro Motor versions instead. Note some AFD lenses remained in production for years. The EF 50mm F/2.5 Compact Macro was in production from December 1987 until about 2020. This was one of the longest EF lenses in production.
-Demetrius
Bodies: EOS 40D & EOS 5D Mark IV
Lenses: EF Holy Trinity, EF 50mm F/1.8 STM, EF 70-210mm F/4 & EF 85mm F/1.8 USM
Speedlites: 420EX, 470EX-AI, 550EX & 600EX II-RT
