I present here a quick and dirty comparison between the Spiratone 400mm f/6.3 preset lens, a Canon EF 300mm f/4.0 lens, and a Canon EF 28-135 IS lens.

All shots use the f-stops available on the preset lens. The Spiratone shots were made using -1 stop exposure compensation. For some reason, the camera overexposed with this lens.

All pictures were taken with the camera tripod mounted, IS turned off, and mirror lockup enabled, and exposure set to self-timer in order to minimize camera motion.