Shallow depth of field separates the subject from the background, which is desirable for a striking effect. A shallow depth of field means the lens is being used at its widest aperture. If the maximum aperture of a particular lens is f1.8, then that is a shallow as the lens will go.
People pay a huge premium for lenses with fast apertures, such as a 50mm f1.4, rather than the much more common 50mm f1.8.
It’s conventional wisdom to say that the way cameras focus means that about one third of the depth of field is in front of the subject and two thirds behind the subject.
Let’s say I am using a 50mm lens on an APS-C crop sensor camera and I am three metres from the subject. And I shoot with the aperture at f1.8. The distance from front to back that will be acceptably sharp is about 32cm. (I have put a link to a depth of field calculator at the end of this article.)
That means that if the subject moves 10cm forward, they are going to be out of focus.
Let’s see how much difference different sensor sizes make.
We start with a full frame sensor, a 50mm lens (that’s true length and not the 35mm equivalent), an f-stop of 1.8 and a subject distance of two metres. Using the depth of field calculator (see the link at the end), the depth of field will be 18.6cm
Now with an APS-C sensor and the same setup, the depth of field will be 14,.1cm
And with a Micro four thirds sensor the depth of field will be 9.3cm
So the bottom line is that the smaller the sensor the smaller the depth of field at a given distance to the subject.
Here is a link to Bob Atkins’ Depth Of Field Calculator.