In a combination of lens experiment we measure U from the mean position b/w two lenses , Why ? If we measure U from the first lens or second lens will there be any difference b/w this case & the above mentioned one ?
Lens combinations are an approximation assuming the lenses are 'thin' and so have no thickness, to make the approximation slightly better you measure from the middle of them. If you measure form one of the lenses the positions will be slightly off - how much depends on the thickness + focal length.