BIF photography is not necessarily expensive. I have an AF 500mm f8 reflex (mirror) lens, much smaller, lighter, and cheaper than any of the big refractive 500mm and 600mm lenses, but for distant birds it beats anything else I've got in terms of detail resolution. The trouble was that it was incredibly hard to locate a flying bird in the viewfinder, and if I ever did manage to get the bird in the viewfinder, it was incredibly hard to keep it there. The trouble was the narrow field of view of the 500mm lens on a crap sensor (APS-C) camera. I even found it difficult to aim the lens at stationary subjects if there were no good visual location aids such as a horizon I could track along. Once I tired to locate a particular rhododendron flower near the top of a very large rhododendron bush. The trouble was that when I had the wrong flower in the viewfinder I had no clue about in which direction to move locate the flower I wanted.
I decided I needed something like the gun sight ball and V of a rifle to aim the thing. I discovered that technology had moved on since the days of the school cadet force and that I could get a "red dot" optical gun sight quite cheaply, easier to use than the old traditional rifle sight, and requiring less DIY to add to my lens. Here's a photograph of the gun sight mounted on the lens.
Red Dot gun sight on 500mm reflex lens by
Chris Malcolm, on Flickr
This made it incredibly easy to aim the lens at a bird in flight and to track it as it moved. Here's an example of the kind of shot I'd previously found quite impossible to get.
MF 2012 Flying Seagull 6 by
Chris Malcolm, on Flickr
The oddly green underside of the bird is because it was flying over a large expanse of green lawn, a playing field.
Apart from birds against a featureless sky, another thing I'd always had trouble aiming this lens at was tiny distant boats in a large expanse of featureless water. With the aid of the gun sight I was able to follow this little two man kayak out into the sea (Firth of Forth) right out to a distance where it was quite tricky to see it with my (aging) naked eyes. It was just a tiny barely perceptible fleck of colour in the water when it was nearly run down by this large ship which couldn't see it because they'd erected a helicopter landing platform right across the low frontal view which the dropped bow had been designed to allow.
kayak & big ship: kayak turns by
Chris Malcolm, on Flickr
How did I mount the gun sight on the lens? I found a cardboard tube just a little too narrow to fir the lens body, cut a piece of suitable length, and slit id down the side so that it could be expanded to fit. That expansion also gave it a firm well aligned grip on the lens, and the slot gives space for the focus hold button. I simply glued the gun sight onto the tube, and used its calibration screws to line it up so that the red dot (or green if you like) is spot on what the central focus point in the viewfinder sees. The lens is aimed without the viewfinder, by simply looking with both eyes open, one looking through the sight, and moving the camera to place the red dot on the target.