"How to focus properly?", "What does DOF mean?", "Is auto-focus precise?", "What does back-focus and front-focus mean"?

Focusing is not as easy as it could seem, but it's one of the most important technique in photography, since a dark or too bright photo can be corrected via software, while an out of focus photo very hardly if not at all.

This lesson presents the basic principles of focus that are useful to be known.

Focal Plane

The "Focal Plane" is an ideal plane perpendicular to the camera lens and at the distance where the lens is focusing.

Example: if you are pointing the lens towards horizontally and focusing at 3 meters of distance, the focal plane is a vertical surface in front of you at 3 meters of distance in focus (if the lens is perfect).

This concept is very important and not considering it can cause out-of-focus (often abbreviated OOF) shots.

Two typical cases:

  • if you are focusing on a person A at distance of 3 meters, another person at the distance of 3 meters will NOT be in focus, since the focus plane is... a plane and not a sphere around the camera - see figure 1;

focus fig1Figure 1

  • if you focus on a person A at the distance of 3 meters and person B is also in focus (since he/she is on the focal plane of A), then focusing on person B will NOT imply that also person A is in focus (since he/she is not on the focal plane of B) - see figure 2.

focus fig2Figure 2

Depth of Field

"Depht of field" (often abbreviated DOF) is the range distance around the focal plane where subjects are acceptably in focus. Let's see an example: if everything between a minimum distance of 10 meters to a maximum distance of 16 meters is in focus, then we have a DOF of 6 meters around 13 meters (where the focal plane stands), or, in other words, we have a DOF of +/- 3 meters around 13 meters (13-3=10, 13+3=16).

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