Focus 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. 

              Focus Bottom Center


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 focus setting that will give you the greatest depth of field.

The next step is to repeat this test varying only the f stop.  You would use the focus setting here.

Shot with a Nikon D3s and a Tokina 10-17 set at 15mm shot at f11 (as determined from the f stop test)