Depth of Field by Phil Gee

A lens can only focus points on the same Principle Plane of Focus (PPF) onto the Sensor as sharp points per exposure, the points in front of and behind the ones we are focused upon, will be out of focus by a greater or lesser degree.

We normally focus on the subject (the eyes if it is a head shot) and we have an expectation that the subject will be sharp i.e. in focus - all visible points of the subject fall within the Depth of field

What do we mean by being 'In or Out of Focus'?
Any point on the subject that appears sharp when we focus (is reproduced as a 'sharp' point on the sensor) is on the PPF points in front of and behind the PPF appear as progressively less sharp why? well its down to the size of the fuzzy blob (Blur Circle) they make on the Sensor which I have attempted to show by the dotted circles below.

If you move the cursor over the image you will see the affect of stopping down
has on the size of the fuzzy blob or Blur Circle to give it its technical name.

You can see from the schematic that as an aperture is introduced the angle of the rays decreases and so does the diameter of the Blur Circle on the sensor if this blur circle is less then the Circle of Confusion then it will appear sharp
( The term Circle of Confusion refers to a specific size of Blur Circle in given conditions)

At the position a point has grown to a size that the eye can see a difference i.e. a Fuzzy Blob rather than a 'sharp' point, we would say is the limit of the 'Depth of Field' (DoF) it is out of focus (soft); so the distance up to that position on either side of the PPF is the DoF for that lens setting check out: DoF & CoC notes in earlier section.

One must also remember it is not just the size the Blur Circle on the sensor, but rather its size in the final image and the viewing distance.
If your final image is to be 20x larger than the sensor size, the Circle of Confusion (Blur Circle) on the sensor will have to be 20x smaller. Because as you enlarge an image the Blur Circles get bigger and depth of field reduces, the last point to become 'soft' will be on the PPF.

In addition the closer the viewer is to the 'print' the smaller the value of the Circle of Confusion used needs to be if it is to appear sharp.

How to control DoF Requires a basic understanding of what we mean by 'Sharpness'

Lens Aberrations, diffraction, magnification, blur, atmospherics can

all have an effect on the apparent sharpness of an image

Circles of Confusion (Blur circle)

Optimum Aperture

Zone of Focus

Hyperfocal focusing (or why focus beyond infinity)