Assuming a gnat is 1mm in size, that bring it to about 10.3m away...you would be able to see something (to a certain extent) at 10.3m while focused at infinite...I think. It's a Coronado brand, as he mentioned, so aperture is 40mm to 90mm. I've never compared the different apertures of Coronados so I can't tell that. More info here: http://www.coronadofilters.com/

The movement relative to a bird or bug's relative size will always remain the same.