I can remember from my distant school physics days that the image in a mirror lies beyond the mirror. But how does a modern camera sensor "know" this. If I face a mirror with my canon camera which has a cmos sensor and usm lens, it autofocuses correctly on the part of the reflection I select. Why does the sensor not see the mirror as a flat plane of light with varying degrees of intensity?. I have done multiple google searches and can't find an answer. Any light on this subject appreciated.