A field changer device (4) comprising: a rotary magazine (5) of spherical general shape mounted so as to rotate about a rotation axis (R) coincident with an axis of symmetry of the magazine (5), which is inclined to a fixed principal optical axis (X); and at least three optical channels (A, B, C) provided in said magazine (5) and having concurrent respective optical axes (XA, XB, XC) inclined at the same angle (α) to the rotation axis (R), it being possible for said magazine (5) to rotate about its rotation axis (R) through predetermined rotation angles (β1, β2, β3) in order to place each of the optical channels (A, B, C) alternately along the principal optical axis (X), the optical channels (A, B, C) being arranged in the magazine (5) in such a way that at least two of the rotation angles (β1, β2, β3) of the magazine (5) are different.