The 21mm focal length completes the logical set of 21mm/13mm/8mm and nicely juxtaposes the 17mm/10mm/6mm combination. Each step within these two sets approaches a 3x field area gain. The 21mm's effective field stop diameter is 36.2mm, bringing its true field close to the 35mm Panoptic (38.7mm Field Stop diameter). In an f/4 Dobsonian, the exit pupil is just 5.25mm using the 21mm Ethos instead of 8.75mm with a 35mm Panoptic.