My weapon of choice was to be my 400mm lens with a couple of extension tubes fitted to allow closer focus. Dragonflies in flight are not an easy subject to photograph. They move quickly and often only hover for a second or two. So often I found I was just getting the point of focus on the dragonfly and about to squeeze the shutter when they disappeared from my viewfinder. Nevertheless, over the space of approximately an hour I managed quite a few shots where the subject was in focus and sufficiently large in the frame.
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It's worth noting that I returned to the site a few days later with a view to having another go photographing the dragonflies. However, on this occasion they didn't co-operate at all. There only seemed to be 1 or 2 individuals around, they seemed to be patrolling a much larger area (regularly disappearing from view for minutes at a time) and they weren't hovering at all. The conditions were broadly similar to those on the first occasion so it's not clear why the dragonflies were behaving so differently. As is so often the case with wildlife photography you do need a bit of luck for all of the elements to come together.