There are many terms relating to lenses that you should be familiar with.
The focal length, ƒ, is the distance between its optical centre and its principal focus.
Principal axis is the line which passes through the centre of the lens and which is perpendicular to the plane of the lens.
Optical centre is the point on the principal axis that is the midpoint between the surfaces of the lens.
Focal point (or principal focus) F is the point on the principal axis where all the rays parallel to the principal axis meet after passing through the lens
Focal plane is the plane perpendicular to the principal axis on which all parallel rays meet after passing through the lens.
The object distance, u, is defined as the distance from the object to the optical centre of the lens.
The image distance, v, is defined as the distance from the optical centre of the lens to the image.
Note: the image does not usually form at the focal point (do not confuse the two!)
The object height, ho, is the vertical height of the object, while the image height, hi, is the vertical height of the image.
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