The Thin Lens Equation is the formula that demonstrates the relationship between the focal length, f, the object distance, do, and the image distance, di. See below for their definitions.
do: the distance from the object to the optical centre
di: the distance from the image to the optical centre
f: focal length of the lens: the distance from the optical centre to the principal focus.
ho: height of the object
hi: height of the image
The aforementioned relationship is expressed through this formula:
1 + 1 = 1
do di f
This is another explanation.
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