Hi Gary, by the depth, or should I have said 'range of focus' I meant that the telescope could be focused on an object at infinity or on an object stuck to the front surface of the objective lens, a ball of blue tack in this case,but the OG aperture will be cut down to around 18mm if the scope is focused on an object at 1meter distance, and zero for the ball of blue tack; the focal shift of the eyepiece is around 50mm or so.