If you want to know the math, wiki has explained it in detail which you can visit it HERE.
Here is the definition by wiki:-
depth of field (DOF) is the portion of a scene that appears sharp in the image. Although a lens can precisely focus at only one distance, the decrease in sharpness is gradual on either side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions
Im not going to talk about the term but i will show you one example of choosing right DOF is crucial in photography.
When the first time i have a fast prime (it’s 50mm f1.8) i always take picture with the largest aperture (1.8), due to the fact i am ‘jakun’ with the bokeh and DOF. then almost all of my portrait picture is have tack sharp focus on eye but the eye, hair and even nose has lost it sharpness due to the shallowness of the DOF.
Consider the following example, one taken with f=2.8 another one is f=5.6
f=2.8, shutter speed=1/30 seconds
f=5.6, shutter speed=1/30 seconds
i personally think the first pic look stupid because my intention is to photograph the whole car not the wheel! so lesson should be learn from now, so next time choose your DOF wisely.
P/s : Gyc, the example pic shown here is one of aaqil’s lori besar (big lorry). So now you know 🙂