How can I get auto-focus to work?

I use a Nikon D3100 and I take portraits of teens. I recently invested in a AF Nikkor 50mm 1:1.8D lens but the auto-focus doesn't work. I didn't mind practicing with the manual focus but when I take portraits with that lens, sometimes they come out a little blurry. I wanted to try and use auto-focus again and had someone suggest to me that I buy a AF-S DX Nikkor 35mm 1/1.8G lens… It came in the mail today and I like how the pictures come out, but… Still no auto-focus. I have looked up both manuals, and I seem to be missing something!

On the 35mm AF-S, there's an M/A - M switch on the side of the lens. Make sure it's on M/A.

BTW, your camera will only autofocus with lenses that have built-in focus motors like the ones with the AF-S and AF-I spec. AF only is not enough.

If it still does not autofocus, try to use your 18-55mm lens again to verify that autofocus indeed still works with your camera. If it does not anymore, try cleaning the electrical contacts on the mounts of both lens and camera using a cotton tip. Be careful not to touch the mirror, focusing screen or the glass on the lens. Also try your best not to drop any fiber anywhere. If cotton does not work, try a soft pencil eraser making sure no particles fall inside the lens or camera. If nothing works, the camera needs servicing.

If your AF/M switch is properly set to AF, then you have the wrong lens. This is why I never recommend Nikon D3xxx series of cameras because they are the only Nikon DSLRs to not have internal focusing motors. As you're now experiencing, not having a motor makes buying lenses very confusing and in your case, costly because you now have two lenses that don't AF with your camera.

Since you're using such an old camera (D3100 was introduced Aug 19, 2010), you really should consider getting a new DSLR. Again, avoid the D3xxx series and go with anything in the D5xxx, or D7xxx series which is probably where you should be.

If you like the 35mm, then that's great. However, depending upon your style of shooting portraits, you should consider going with a portrait lens in the 50-100mm range. Sigma's Art 50-150mm f/1.8 is an amazing lens, and would be among the best for your situation.

Your camera does not have a focus motor inside, you will need to get lenses with a focusing motor. You can use the focus assist in the viewfinder to get sharp focus. The manual says which lenses work with your camera and how to use the manual focus assist.

It might be a manual only lens. Your picture can also be blurry if your shutter speed is too slow, picking up your hand shaking

As already mentioned the Nikon D3100 will only autofocus with the Nikon AF-S range of lenses. The older D lens will work fine in manual focus for you though and is an excellent lens for the money but if you need autofocus you will need to upgrade to the newer Nikon 50mm f/1.8G version which has a built in AF-S motor.

The Nikon 35mm f/1.8G should autofocus with your camera unless there's a fault or you have manual focus selected. It isn't a great focal length for portraits though the 50mm lens would be a better option.

Change your aperture to f22, flip the orange tab next to the aperture ring to lock in the position. Change your focus mode selector to S (Single-servo AF). Point and press your shutter 1/2 way down to focus and all the way down to shoot.

You have a nice camera. HOWEVER, in order to use the autofocus feature you must first set the camera to auto focus mode, and make certain you have the appropriate lens that has its own autofocus mechanism since that camera does not have the internal autofocus mechanism built in. You'll find this information right in your camera's Instructions Manual… That's why it's so important to read it before buying lenses for it or doing an important shoot… READ IT.

READ your camera's Instructions Manual, and you'll be able to find out which lenses are recommended for that camera. I'm surprised you did not mention to whomever you bought the lens from to ensure it would autofocus. That's why it's up to you to read the camera's Instructions Manual and be aware of what lenses are recommended, and what features and functions your camera offers you for better photography experiences.