Focal length of the normal lens
That is why many popular cameras do not offer lens interchangeability at all. They are equipped with just one lens of normal focal length which is installed at the factory and is not meant to be removable.
Even so, many photographers using such one lens
cameras find them more than adequate for the bulk of their picture
taking. Enough flexibility can be achieved with accessories such as
filters and close up attachments so that the need for another prime lens
is seldom felt.