Focus from Auto to Manual

I have seen a few people on here giving tips on certain shots, saying "focus on a subject and then switch focus to manual", my question even though it sounds basic, is this done on the lens or the camera and if so how is it done.

I'm not sure why you would go through the trouble to focus on something and then switch to manual. If you're trying to recompose after focusing on a certain area. Such as a persons eyes. I use single point area mode and single servo auto focus. Once locked on and still having the shutter button pressed half way, you can then point away with out the camera trying to refocus on what the area point is on.

its a question of focusing on a point and then switching to manual so the lens doesnt keep re-focusing thats all, Ive seen people talk about it on here and was just wondering how its done.

Depends on the lens/camera. AF-s lenses have the AF/M switch on the lens itself. Alternatively some cameras also have the AF-C/AF-S/M switch on the camera body. The end result of the "M" (manual) setting is the same.

d3100 has it on the lens the other reason to use it is for instance say you are at the zoo and want to take a pic of animal in cage sometimes the camera focuses on the cage instead so by using auto you get a rough focus then by switching to manual you can guaranty that you focus on what you want rather than what the camera thinks you want.in any case auto/manual switch is on lens barrel just above vibration reduction switch.i also agree w john

Cheers guys