I second looksharp65 advice ....to use a 40X eyepiece as your main eyepiece is a compromise that you need to weight in terms of image brightness , FOV and steadiness..the DOF might be noticeable in the very near distance ,but I dont think over 20 or 30m should be a factor to decide upon..but real FOV should be noticeable ,and tracking and aiming would be more complicated at 40X than at 24X...then the use of available light is also limited ,in less bright environments and for digiscoping specially,when light means detail..and last ,the shake at 24x is of course less ,and that also means detail...