It works between the focal lengths of 200-300mm and is engaged by sliding the switch once the lens is extended past 200mm. This allows the focus ring to travel further, and the lens to extend ...

but the result is a near rectilinear lens. There is a small amount of residual barrel distortion, measuring -0.07% at 70mm, -0.06% at 100mm, -0.26% at 200mm and -0.05% at 300mm. Bokeh is very ...