Hi Clive.

Basically, an AO scope focuses by moving the objective lens backwards and forwards by rotation. The big problem with this is that unless the optical centre and mechanical centre of the objective lens are absolutely identical, as you rotate the lens, the image (which is what you actually aim at, not the target) goes round in a circle. This is made even worse if the other lenses - there are at least two - are not also in perfect alignment. The tiniest misalignment through the system can give big problems at even medium distances. With a Side Focus system, the objective stays still and the focus lens is moved backwards and forwards. This is a more expensive mechanism, and far from foolproof, but it does mean that there is more chance of the effects of any alignment inconsistencies staying constant, as the whole thing is not rotating. So, in my opinion, SF systems are inherently better than AO systems, but whatever system you use, if you want the best precision you and your setup are capable of, you need to check it out at each distance you shoot.

Also remember that as well as optical precision, you also pay for robustness. I have had two scopes that started life pretty much perfect, but started zero-wandering after a couple of months of careful, real-world outdoor use.

As it happens, it looks like I have got lucky with a £175 Hawke Vantage SF 3-12 x 44 scope. At the moment, this particular scope is spot on at every distance I shoot, and compares very well on all counts with my £600 Sightron S111.

Good shooting

Alan