For almost 5 years, my standard lens for aviation shooting has been my Canon 70-200mm F/4 L, which has been a trusty lens, very sharp, and a good length on the APS-C sensor on my 30D (112mm x 320mm equiv). However, switching to the 5D mark II has reduced the range. For a while now I've been contemplating the Canon 100-400mm F/4.5-5.6 IS L lens, which is very popular among planespotters for its range and sharpness.