Pencil erasers can remove and clean, but rubber is an insulator. Don't go crazy with this, but just a bit of 91% rubbing alcohol on the end of a Q-tip applied gently to contacts will remove oxidation and improve electrical contact. Do the lenses first (I assume more than one). If this still doesn't help, then the problem is with the camera. This is a service issue (repairman). If you only have one lens, try it on another camera. Since you say "sometimes" I think cleaning will do the trick.
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You are sure on both, the camera and the lens, AF is selected?
If you did and the lens does not work, please try anther lens, to find out, if it is a malfunction of the lens or of the camera.
You also could check if all contacts on the lens and in the camera are clean and not corroded. If the are not clean and shining, please use a dry cloth to carefully rub the contacts clean. Be careful not to bend any contact in that process. please be careful not to leave any dust n the camera or lens. Use the lens and camera caps when you are not cleaning one of them.
First, make sure that both the camera and lens are set to auto-focus. The problem is possibly as simple as dirty contacts between the lens and the camera's lens mount. Depress and hold the lens release button while you fully rotate the lens back and forth about ten times. The friction between the contacts on the lens and the contacts on the lens mount will clean the contacts.
This to me is a connection problem and may only require moving the AF-MF button back and forth a few times or removing the lens and cleaning the contacts between the lens and camera body. Generally the "L" series lenses are pretty resilient in construction of offer the user few problems. The lens contacts can be cleaned with a soft red rubber pencil eraser where you lightly polish the contact area of the gold contacts. Clean away any rubber dust with a lens cloth not a tissue, be extra careful not to get dust into the camera body cavity. reassemble the lens and try the camera lens operation Diagram
Hi tina549, do any of your other lens auto focus? If so then it may be this lens. If they do not then look on the right side of the lens mount (with camera facing you) there is the AF and M switch. Something to try is clean the electrical contacts on both the D70 and the lens. When I had a D70s the auto focus was giving me fits, worked sometimes then it did not so I traded it for the D80. The 18-55 lens is a nice lens however, it is not the most robust of lens.
Did it ever work on this camera?
I have seen some Sigma lenses that are auto focus on canon film cameras but then when put on the digital they dont work right. If you have used it on this camera then make sure the lens is clean.sometimes dirt on the front wil make it try focusing on the dirt. Next try focusing in manual focus to make sure the focus is working. Then take the lens off and then put it back on and make sure it clicks into place.
Let me know if none of those things work. also make sure to try focus on something close and somthing far away.
If it's any consolation, I have the same setup and the same problem. I'm now at the point where I'm probably going to have to return the lens as faulty, but there are a few things it could be on yours which you might want to check.
Have you got the D200 set to auto-focus? The selector (on the front of the camera, on the lower left) should be in the S position (as opposed to C or M).
Have you got the lens set to auto-focus? It should be in the M/A position, rather than M.
Have you tried cleaning the contacts? Pop the lens off and give the contacts a clean with the appropriate equipment. Dust on the contacts could be causing a problem.
Failing that, have you tried turning the camera on and off a few times?
I've obviously tried all the above on mine, and now suspect I've got a faulty lens that needs fixing ... which really sucks! Good luck with it, let me know if you get it working.
First make sure contacts on lens are clean ( see enclosed pic #2 ) . If this does not solve the problem the ultrasonic motor may be faulty pic #1.( sometimes switching on but sometimes not , will have to be replaced , unless there is a bad connection between the motor and lens pc board ) #1 #2