MF 2012 Flying Seagull 9
Taken on a sunny afternoon at the Meadows Festival 2012 with a 500mm reflex. These reflex (mirror or Newtonian) lenses are usually considered to be absolutely useless for catching birds in flight for a number of reasons, the most important of which is the very narrow field of view, especially on a crop sensor camera. It's hard to aim the lens at what you want to look at, and it takes time to find it. So absolutely no chance of catching a bird in flight!
A red dot gun sight fitted to the lens as a sighter solves that problem. You keep both eyes open, and see a red dot superimposed on the scene. Move the camera to locate the red dot on your chosen target, and it's in the centre of view of the lens, i.e. on the central AF sensor. This can be done so fast that it's possible to track birds in flight. If you have continuous autofocus set and shutter fire only when focus found plus repeat while shutter held down the camera will take a photograph of the flying bird whenever you've managed to hold the red dot on it for long enough for the lens to autofocus.
My first effort at building the sight was slightly wobbly. Here's a photograph of it.
The lens and sight kept going out of alignment and had to be recalibrated. A gentle knock would throw calibration off. The application of more glue and some rubber bands firmed the thing up. This afternoon was the first trial of the toughened version. Excellent results! So here by way of demonstration are a dozen shots of seagulls wheeling over the Meadows Festival, attracted by the crowds of messy eaters. All the images have been cropped down to the bird, and size reduced to 67% of the original pixels. The depth of field of this lens is so shallow that it's impossible to get the whole bird in focus. In fact an image filling bird standing side on will have one leg in focus and the other out. So in this kind of wild flight tracking and shooting when focused the focus is more likely to be uselessly on a wing tip as usefully on the body. I'm encouraged by how many well enough focused shots came out despite my being a complete beginner at flying bird photography.
So I now have a rather good birds-in-flight lens, just by adding a cheap accessory (red dot gun sight) with some DIY bodgery to what is generally regarded as one of the worst possible lenses for catching birds in flight.