Despite what the name suggests, the focal plane, or the sharpest plane of focus in an image is not the only part of an image that actually appears to be in focus.
Although the points adjacent to the focal plane are not in perfect focus, the brain registers them as being in focus if they lie within a certain range.
The definition of focal plane is the distance from the camera at which the sharpest focus is attained. Picture yourself in a field with a tree far off in the background and add a tape measure under your feet facing towards and away from the camera. Focus the camera upon you standing on the tape. You will then be in focus, even at f1.2. Now, if you step forwards, or backwards on the tape, you will be out of focus because they stepped out of the “focal plane” (i.e. your piece of imaginary tape).
At f1.2 your distance in focus is very thin. The higher you take your f-stop the more you can move forward or backwards.
You also need to be aware of where you are in relation to the subjects. When using a low f-stop you need to remain directly in front of your subjects… you can’t wonder off 8 feet to the left. You need to be centered and head on to your subject.
Here’s a quick guide on how you can deepen and reduce the depth of field:
Deep depth of field = Large f-stop (small aperture) + slow shutter speed Shallow depth of field = Small f-stop (large aperture) + fast shutter speed