How are the photos with out-of-focus background and clear foreground taken?

A photo with out-of-focus background and clear foreground is the effect of shallow depth of field. Three factors that affect the depth of field need to be understood when shooting:

1. The larger the aperture (the smaller the aperture value f), the shallower the depth of field, and the smaller the aperture (the larger the aperture value f), the deeper the depth of field.

2. The longer the focal length of the lens, the shallower the depth of field, and vice versa.

3. The closer the subject is, the shallower the depth of field is, and the farther the subject is, the deeper the depth of field is.