Auto Focus and Manual Focus TroubleshootingThere 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.
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.)
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.)