I've been thinking about how a lens focuses and how extension tubes work to allow you to focus closer, while giving up the infinity focus. I've read that focusing on subjects close to the lens presents optical challenges to lens manufacturers, and yet, it seems that the solution of simply increasing the distance between the lens and the film plane is so simple. I don't suppose anyone here has an insight into why lens manufacturers don't simply have a kind of sliding lens mechanism inside the barrel which can slide the whole lens forwards and backwards thereby enabling the photographer to shoot macro when he desires (giving up infinity focus and maybe a stop or two) and also to shoot at infinity when required, by sliding the glass back. It seems that it would not increase the cost of producing the lens very much, and would be quite a boon to macro photographers.