How this calculator works
The spherical mirror relation uses positive object distance for a real object in front of the mirror. A concave mirror has positive focal length; a convex mirror has negative focal length. A real image in front is positive and a virtual image behind is negative. A plane mirror is treated separately. At the focal point the image is at infinity, so no finite image distance is returned. The paraxial model neglects aberrations.
Formula
1/f = 1/do + 1/di; m = -di/do. R = 2f; areal magnification = m².
Worked example
A 300 mm object distance with +100 mm focal length gives a +150 mm image distance and magnification -0.5. The image is real, inverted and half as tall.
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Frequently asked questions
What do negative image distances mean?
The image is virtual: rays appear to come from its location rather than meeting there.
Why is an object at the focal point rejected?
The outgoing rays are parallel and the image distance is infinite.
What does the sign of magnification tell me?
A negative value indicates an inverted image, while a positive value indicates an upright image.