Effective Focal Length of Multiple Lenses

[RWard29]

In my Optics book (Hecht, 4th edition) the author discusses how, when dealing with a multiple lens system (say, with n many lenses), one can think of the system as a single lens system, with an effective focal length given by:

1/f(effective) = 1/f1 + 1/f2 + ... + 1/fn

My question is this: Where is the resulting "effective" lens located, with respect to the original source (or with respect to the original lenses)? I am dealing with a simple two lens system, and I've tried placing the lens at the midpoint between the original two, and at the location of lens 1, but neither gives the correct answer.

 

[RWard29]

Nevermind. I just answered my own question; I read what I wanted to see, not what was actually there. The formula I quoted applies only to thin lenses which are in contact with each-other, in which case the position of said "effective" lens is already well-defined.