Depth of field is the range within which focusing is resolved in a photo.
Aperture has a huge affect on to the depth of field.

Changing the f-stops (f/#) of a lens will change aperture and as such the DOF.

f-stops are a just certain number which is telling you the size of the aperture. That’s how f-stop is related to aperture (and DOF).

If you increase f-stops, it will increase DOF, the area in focus (and decrease the aperture). On the other hand, decreasing the f-stop it will decrease DOF (and increase the aperture).

The red cone in the figure is an angular representation of the resolution of the system. Versus the dotted lines, which indicate the aperture coverage. Where the lines of the two cones intersect defines the total range of the depth of field.

This image explains why the longer the depth of field, the greater the range of clarity.

http://gizmodo.com/5904107/100-tips-from-a-professional-photographer

http://digital-photography-school.com/digital-photography-tips-for-beginners

But wait, there’s more!

https://www.studiobinder.com/blog/flagging-lights/

http://erickimphotography.com/blog/diagonal/