Depth of field is in relation to magnification and aperture size, or in our case objective lens size. A smaller objective lens with give greater depth of field due to the beam of light that emits being narrow, a larger objective lens creates a wider beam of light an a decreased depth of field. For example a scope with a 50mm front lens and 5mag will still produce a wider beam than that of a 40mm scope of the same magnification. Lowering the magnification creates the effect of greater depth of field by making the focal plane larger.
Therefore for HFT use a smaller objective is preferable but a balance between depth of field and adequate light transmission needs to be found hence 38mm-42mm scopes being the best of both worlds
To get a greater depth of field you can
Decrease aperature...smaller objective
Shorten focal length....Lower magnification
Thats my understanding of it anyway