Circles of confusion.
In any given photo there is only one distance from the lens that is the sharpest, and thats the circle of least confusion.
Call it a pinpoint, forward and aft of that point the point grows larger and is called circle of confusion.
The range forward and aft of the circles of confusion wich is still acceptable sharp for the viewer is called the depth of field.
This depth of field largely depends on .........
-Focal length of the lens used (here 180mm)
-Aperture used (here full opening, of F2,8)
-And the distance from subject till the sensor (here about 2 meter)
See excellent online depth of field calculator for any given camera and lens combination >> [link]
(used lens is a Manual focus F2,8 180mm Leitz Elmarit from 1981 on a Canon 60D)
Calculated the Depth Of Field (DOF) in this photo is 1,21 cm only!
This foto of the lump of gras was taken against the light, so due to the reflection of the light , focal length, full open aperture and short distance for the 180mm, give a rapid growth of the circle of confusion.
The growth in diameter of the circle of confusion is linear with the distance further away from the point of least confusion.
The shape of the circle depend largely on the number of blades from the Aperture, ideal is a aperture with 8 or 9 blades or more, to keep this roundness even when the aperture is stopped down.
More about circle of confusion >> [link]
hope you like it , greetz len