A key defining principle of the Canon EOS DSLR camera system was to make it simple and instinctive. Full-time manual focus for lenses with USM focus motors fits that remit. It is useful for subjects that don’t move a great deal; the ability to simply turn the lens’ focus ring if shooting macro or landscapes was so instinctive that you might not even have realized that it was a designed feature. It also means that the lens can be manually pre-focused before you even start the AF.
When EF lenses for DSLRs were introduced with fly-by-wire focus rings, new custom settings were added to the cameras to still enable focus to be adjusted slightly once One-Shot AF was complete.
Moving to mirrorless, Canon's RF lenses have a fly-by-wire focus ring that means you can no longer manually pre-focus in Servo AF, or adjust focus after One-Shot AF locks. The custom setting for activating the focus ring after One-Shot AF finishes is still there, and has extra settings like automatic magnification that help macro shots, but pre-focus before the shot when using Servo AF was lost until recently – and even then is only possible with select RF lenses.
Locate the lens Electronic MF menu item on a DSLR and you’ll see it is active by default, so all EF lenses with fly-by-wire focus rings can be used to adjust focus once One-Shot AF is set. Find the same setting on a Canon EOS R-series camera and it’s disabled by default. R-series cameras add a choice to enable manual focus after One-Shot AF and also magnify; with the latter enabled, the display shows a magnified view where the active AF point is once the focus ring is turned.
Recent cameras have an added electronic full-time MF option, which provides the option to pre-focus a lens when using any AF mode. However, this is limited to a subset of the RF lenses – mostly the longer telephoto zoom and prime lenses. Combining these settings adds back in much of the lost functionality of full-time manual focus – if not yet for all RF lenses.
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