When the object is outside the center of the concave spherical mirror, an inverted and reduced real image is formed, and the image is located between the focus and the center of the sphere; when the object is between the focus and the center of the sphere, an inverted and enlarged real image is formed, and the image is located outside the center of the sphere; when the object is inside the focus, an upright and enlarged virtual image is formed, and the image is on the other side of the mirror. No image is formed when U=f.

The focal length is behind the center of the mirror's front circle, but not at R/2. However, if the incident light is a paraxial light, the focal length can be approximately considered to be at R/2.

1/f = 1/d₀ + 1/d_i

f - focal length

d_i - Image distance

d₀ - Object distance