The focal length listed on the lens is the effective focal length. Modern lens design let the lens be shorter then the focal length. The more elements a lens has then to more space it will take up I do not know of any other reason for lenses of the same Focal length to be different lengths.
Focal length vs lens length
OK, this may sound stupid but you all know me 
How does focal legnth relate to lens legnth?
I have noticed lenses that are half the legnth of another yet twice the focal legnth. Is the physical legnth determined by the number of elements which in turn determines space necessary to hold them inside the lens rather than by focal legnth?
Did that make any sense?
How does focal legnth relate to lens legnth?
I have noticed lenses that are half the legnth of another yet twice the focal legnth. Is the physical legnth determined by the number of elements which in turn determines space necessary to hold them inside the lens rather than by focal legnth?
Did that make any sense?
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