Long lenses can sometimes be hard to aim. It is hardest when you try to find a subject against a featureless backdrop, and even harder if the subject is moving fast. Think tracking a bird in the sky with a 500mm lens.
He clipped a red dot rifle sight onto the lens using a cardboard tube and uses the red dot to “acquire target”. Red dot sights do not emit any laser, so they are safe for the birds.
While this mechanism suffers from a slight parallax error it is still pretty useful for getting the “target” in the frame and then continue framing via the lens.
[does parallax error exists?] “For any particular distance, yes. But the error will then increase the further you move from that distance. I prefer the idea of keeping the axes of gunsight and lens aligned and making the adjustment by aiming a constant few inches low.
That few inches of parallax error makes no difference at all if what you’re doing is getting the target in the field of view of the lens, and then you shift to viewfinder or LCD to compose and focus. But it does make a difference when following a bird in flight and there’s no time to do that, so you’re completely relying on the red dot to give you sufficient accuracy to place the target on the central focus sensor. #
If you are wondering what Chris shoots with that contraption, see the (uncropped) picture below.