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# Thin Lenses; Ray Tracing
Definition of Focal Point
More precisely:
Focal Point: The image point of an object that is infinitesimally away on the lense that is on the lens axis.
Evidence of Focal Point
The third ray wouldn't normally do that unless it was infinitely thin.
**Thin Lense Equation**
$$
\frac{1}{f}=\frac{1}{d\_i}+\frac{1}{d\_o}
$$
Answer: As closer objects require the lense to be further from the sensor, the image will approach the focal length as the distance increases and vice versa as the image gets closer. ( think how the lenses move when a camera is focusing ).
**Combination of Lenses**
Object distance from second lens not equal to image distance from first lense