Thank you all for your input, it's comforting to know that people are willing and anxious to help, and that some people are very much like me, doing things that require that help.

DutchNikon, you are my new hero. At first, I thought you meant the Fn button on the left side of the camera, but I knew I had assigned that to continual release. But, that started me thinking (seemingly a foreign concept to me from what I've done), and I went to the AE/L, AF/L, etc. area F and found it. I'm greatly relieved. I really liked this camera and thank you kindly for your part in renewing my enthusiasm for it.

Thanks too, to metalergr, that would have solved the "problem" as well, without the specific incite of DutchNikon, but also effective... And yes, I too, as is my wont, ignored the manual and had at it, because I was sure I'd run across all those buttons and choices before... I can, and will learn... someday... old dogs, new tricks... could happen...

Hi , I had the same problem yesterday. Reset your camera to factory setting and everything will go fine again. That works for me. I had this problem after so many changes I made to camera's menus.As a new buyer I tried to explore evething in one day.

Would it be possible that you have assingned the AEL/AF-L control ( f3) to be used for focussing ? . if so the camera will not autofocus using the release, but it will by pressing the AEL/AF-l button... ( read your manual about assingning the F(n) button.... ).

My first guess is that a setting on the camera got changed without you knowing (easy to do with a new camera). The first thing I would do is to put the camera into auto mode, set all the switches to autofocus, and try out the 18-55 again.

I would check lenses. My first suspect would be the Tamron lens. If the Nikon works and Tamron doesn't then that answers your question. I would also check the contacts and maybe clean them, but I can't imagine they are worn or dirty already. Obviously I would check the manual/autofocus switches.

Sounds weird though and not sure I really understand the problem. If all else fails return it or have it repaired...it will be under warranty. I however wouldn't mention you were using a third party lens on it.

Hi, new guy, new camera, hoping for some help... I got my D5100 a few days ago. For the first day the auto focus worked with both the kit 18-55, and the Tamron 18-270 in all auto focus modes. Then suddenly it stopped, nothing... manual focus is good and the continuous focus works but only if I use the monitor, it only blinks its little red center of the frame blink when I depress the shutter release half way when using the viewfinder, no movement at all. I've been to Nikon Support but so far no response. I was wondering if anyone out there has an idea. I really like the camera (or want to) but wonder if I should just send it back for a refund or replacement... or maybe even get a different camera... but ya know, this one really feels good...