Sigma 50-500mm auto focus problem

you need replace the focus ring, the camera can not without it especially taking photo.we are selling the camer parts in china. your visiting for our website is highly appreciated!
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Sounds like the autofocus motor is having issues, I'd suggest contacting Sigma.

How new is the lens? In or out of warranty? If out or warranty most likely solution is to find a Sigma authorized repair shop because your description sounds like the auto-focus motor gear has failed. If in warranty, check out Sigmaphoto.com for repair sites and also download the SIGMA_SERVICE_REQUEST.PDF. I've only had to deal with Sigma once on warranty matter and they were very helpful, provided UPS authorization for shipping on their nickel.

dos the manual focus work

Yes its a HMS lens it will work on most Nikons

Your Sigma 28-300mm doesn't have a HSM-motor as far as i know. When you switch it from MF to AF, you also have to disengage the manual focusing ring by pulling it towards the camera, or pushing it away from the camera. Using the AF without unlocking the manual focusing ring may damage the AF motor. 
I hope this helps, 
/Chris

The lens may not be a dynex type so it wont work on your 700

I was focusing to close to the subject & too much glare from a church window.

Hi    The switch is held on by two screws you can remove the screws    and the switch will fall off it will need cleaning 

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.

let me know how it works,
Caleb