Magnification of a mirror is \(m = -\dfrac{v}{u}\). A value of \(m = -1\) means the image is real, inverted, and exactly the same size as the object, so the image distance equals the object distance.

For a concave mirror, a real image of the same size as the object is formed only when the object is placed at the centre of curvature (C). The image then also forms at C, giving image size equal to object size.

If the object is beyond the centre of curvature, the image is real, inverted, and diminished (magnification between 0 and −1), not −1.

If the object is between the focus and the pole, the image is virtual, erect, and enlarged, giving a positive magnification greater than 1, not −1.

If the object is at the focus, the reflected rays are parallel and the image forms at infinity, so magnification is not a finite −1.

Hence, a magnification of −1 means the object is at the centre of curvature.