f Stop sharpness test
Here is a side by side comparison of the sharpness at the bottom and center of a .jpg image.  These images are straight out of the camera with no processing other than cropping.

 
 

This is the original picture

The two boxes show where the image was cropped. (You can ignore the over exposed sky.)

 

Here are the images from the bottom edge and center. 

f stop Edge Center
4.2
4.5
5
5.6
6.3
7
7.1
8
9
10
11
13
14
16
18
22
25
29

 

I would never allow such close zooming.  That is what gigapixel panoramas are for. This is simply a close up inspection.

The objective is to find the f stop that will give you the greatest depth of field.

The next step is to repeat this test varying only the focus.  You would use the f stop you find to work.

Shot with a Nikon D3s and a Tokina 10-17 set at 15mm focused at the right edge of  the infinity symbol (determined to be the sharpest setting from and earlier focus setting test).

Images usually start to get worse as the f stop goes up.  This is due to diffraction.  I am surprised that this lens does not show the typical diffraction at higher f stops.