hi looking some help. im using a tamron af 70-300 mm 1:4-5.6. .most photos i take are of motorbikes, mostly road races,but occasionly short circuits where the bikes are farther away. i'm thinking of buying a better 2nd hand lenses. and to be honest i'm not sure what to go for . willing to spend around 400-450. would like sharper shots . and would a 200 mm lens of better quality be any use. i'm also not too sure about all tke jargon in the adverts thanks joe