OM System EE-1 Dot Sight

At 600mm equivalent the angle of view is about four degrees, and a bird that is obvious to your eye is nowhere in the finder. The EE-1 solves that the way a rifle scope does: it projects a reticle onto a clear screen you look through with both eyes open, so you keep the whole sky in view and put the mark on the subject before you commit to the viewfinder.

It sits in the camera's hot shoe, folds flat when not in use, and has adjustable reticle brightness for dawn through midday plus windage and elevation adjustment to align it with the lens. Any camera with a standard hot shoe can use it, which includes long-lens setups on spotting rigs as well as OM System bodies.