Looking to replace subject lens as fed up with slow focusing/failing to focus. Have considered new 24-120 F4G ED VR but its a bit over budget. Willing to consider Sigma/Tamron 24-70 or Nikon 24-85 f3.5-4.5G or indeed anything similar as have from 70- onwards covered with Tamron 70-300. Any suggestions/recommendations? Budget about £500.TIA