Manual extension tubes with Nikon D7100?

I'm looking to purchase some inexpensive extension tubes off of Amazon to pair up with my 40MM Macro lens. Since the D7100 has an internal focusing motor will the camera auto focus both the rings and lens? Or will I have to do everything manually?

If you get the extension tubes with electronic interface, then you will be able to use the lens aperture controls from the camera, since the 40 mm macro does not have an aperture control ring.

As far as using auto-focus when shooting macro shots, it is not recommended if the lens has auto-focus.

At such high magnifications, you will be shooting at tiny lens apertures to get much depth of field at all and the shutter speeds will also be quite slow, meaning you will want the camera mounted on a tripod.

The best way to focus is to preset the image to subject ratio when manually focusing and then use something like a 4-way camera focusing rail to move the camera toward or away from the subject until it is in focus.

These things you will have learned eventually, but better to know those techniques now than to become frustrated by unrealistic expectations… Ie using auto-focus when shooting macro subjects

If you buy the cheap manual extension tubes, you will have no control over the lens at all - which includes focusing, and even the ability to set aperture (it will be permanently set to f/22). You will have to put the camera into manual exposure mode as well.

The reason for this is there's no jack-screw extension in the tubes (the thing that extends the camera's focus motor, or are their any electrical contacts in the tubes (which controls aperture among other factors).

You need the more expensive Kenko brand extension tubes, the ones that have both the electrical contacts and jack screw. But those are around $200, and if you already have a macro lens, you probably won't use such extension tubes much.

The 40mm is too short to use it with extension tubes

btw if you buy tubes with eletric contacts you will have both autofocus and lightmeter working