I hope you have your images ready to focus stack. For this exercise I just shot a few images of a camera and lens on a board. I took five shots each focused at a different place. Ready? Open Photoshop (I used Photoshop CC 2015). Do the following; File > Scripts > Load files into Stack:

Now the following dialogue box shows up:

Click on browse and select your images. It is crucial that you check the first box (Attempt to automatically Align Source Images). Your camera may have shifted just a bit during focus or between shots. This feature fixes that problem. Click OK. The following box shows up:

Choose the "Stack Images" option and click OK. Here is what you get. If you look to the right you will see the images loaded as a stack. If you look at the edges of the image you will also see different layers and how Photoshop aligned them. Either way, don't bother with anything now. Click on Edit > Auto-Blend Layers ...

Photoshop will now does its magic. Once done, you will see the following:

In the layers section you will now see the layers each with a mask. Simply flatten your image: Layer > Flatten image. Crop as you desire and you are done. See it is easy. Now you may notice some out of focus areas (on the lens on the right where the focal length is marked). The reason is because I did not shoot enough shots and I shot at F4. Shooting at F8 and taking more shots solves this issue.

          So you may say; "what is the big deal?" Let me show you the first shot and the last shot and you will see how amazing this really is. Just look at these two shots:

This first shot was focused on the first lens to the right (on the lens cap that says Sony). Look how out of focus the camera's logo (Sony) and the back of the neck strap is. Here is the last shot:

This image is focused on the back of the neck strap. You can clearly see how out of focus the front lens now is. I shot five images, focus stacked them and in a few seconds got the final image:

Now I call that remarkable. Apply this technique to macro photography or to landscape photography and you can get pin sharp images, front to back.