I have been looking at the Nikon AF-S 35mm f1.8 lens and it has two focus settings:
- Manual Focus
- Manual-Priority Autofocus
What is "manual-priority autofocus"?
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Manual-priority allows the lens to auto focus but still allows you to adjust the focus manually. Many other lenses lock the focus ring when using auto focus to prevent damage to the internal motor. This lens is designed to allow full time manual override which is very useful to tweak focus without switching out of auto mode.
If it's like Pentax's fulltime manual in autofocus then it's basically autofocus mode with the ability to manually adjust the result without switching out of autofocus. If you have a lens without this, then manual adjustments are bad for the lens when the body is set to autofocus and so you need to switch out of autofocus mode to adjust.
Manual-priority autofocus is a special mode on certain lenses which allows you to stop AF while the lens is still focusing and immediately switch to manual focus.
On most lenses with manual-focus override, while you can adjust the focus manually in AF mode, you can't do this while the lens is in the midst of autofocusing. The manual-priority mode allows you to stop AF right away and manually focus, rather than have to wait until focusing is complete.
On Nikon lenses, this is designated by the M/A position on the autofocus switch. A few professional-grade Pentax lenses have a QFS/M position on the AF switch which does the same.