There are multiple designs on lenses to switch from the auto focus function, to the manual focus function. You may see one of these designs (or similar) on your lens, regardless of brand or format. Some are more self explanatory, while a few are a bit trickier if you’re unfamiliar.
The non-existent switch
Some lenses don’t have a switch at all. In this case, the AF/MF is controlled by the camera focus function only. On film cameras this is usually (but not always) a switch located near the bottom of the lens mount. On digital, it is usually a button or a control in the cameras menu. (Note: The Nikon “G” lenses don’t have a focus switch on the lens, but also does not have an aperture ring)
The manual/ auto focus switch
This is the most common type of AF/MF switch and is found on the side or front of a lens. A simple movement of the switch does the trick.
The manual/auto focus ring
This ring is mainly found on Nikon brand lenses and goes around the barrel of the lens. There is a locking button to the side of the marked M/A box that must be pressed in before the ring is turned to switch your focus function. These rings are more fragile than the “switch”, and should be changed more gently as they tend to crack with use (if cracked, the rings can be replaced by our repair shop).
The locking collar
The focus ring on these lenses also acts as a locking collar for the AF/MF function. The collar needs to be pushed up, or pulled down to change the focus function. On most lenses, it’s an easy push or pull. On some Tokina lenses however, the collar must be switched at one end of the focus scale or the other (infinity or the closest focus). If you have one of these lenses it’s important not to force the collar if it isn’t snapping into place.
Another thing to note about this type of function, is that sometimes the function is missed because the collar, when in MF, covers up the notations on the lens. So for example, in the photos above, when the lens is in AF mode, you see little notations for AF and MF. When it’s in MF, you only see an M for MF and if you don’t know to push the collar up to put it into AF, it can be quite confusing. On the Pentax and Mamiya medium format auto-focus lenses specifically, the notations don’t even exist. (We get a lot of returns from customers that think these lenses aren’t working or auto focusing simply because without a reference manual, it’s tricky to know that this function is there and how it needs to be switched.)
More lens troubleshooting- other possible issues
There are a few other possible issues that your lens may have if it’s not auto-focusing on your camera.
1- It’s not compatible with your camera. (Read about digital lens compatibility here and here)
2- The contacts are dirty. (Read about cleaning contacts here)
3- Something is truly broken within the lens (electronics, loose wire, etc.)