There are two characteristics that greatly influence depth of field or sensitivity of the parallax adjustment.
1) magnification
2) objective lens size
Don't ask my how or why because I am not an engineer but these are the two main things that dictate depth of field of a scope. At least that's how I understand. I could very well be wrong but I don't know that FFP vs SFP has anything whatsoever to do with depth of field. Someone correct me if I'm wrong but I don't see how having the reticle in a different focal plane would effect depth of field.
As others have mentioned, setting up the ocular lens correctly to focus the reticle to your eye is always the first thing to do. Wherever that ocular setting is it does influence the parallax range and relationship of the markings on the knob/scope but again...I don't see how the ocular setting could significantly effect depth of field.(Once again I could be wrong I'M NOT AN ENGINEER)
One of my favorite hunting scopes is a 3-9x33 Leupold EFR because it has a really large depth of field...meaning I can set it to 6X magnification, the parallax to 30 yards or almost where ever(although I have found it better to stay on the high side on the parallax adjustment, it seems like the reticle gets out of focus more quickly when the parallax adjustment gets on the low side), and I can still very clearly see the reticle and the image at almost any distance. The low magnification and small objective lens size(33mm) make this possible. Perfect scope for very flat shooting high power rifles by the way! The SFP duplex reticle and large depth of field make it KISS.
This is why I like for a scope to have at least a 6X magnification on the bottom end. If you need to see something really close it still works...
This is why LVPO's have such a wide depth of field...1-6X is a common mag range for them and the image is relatively clear at all magnifications even with a fixed parallax because they have such small objective lens...20-28mm generally. KISS
What I've found is that scopes with objective lens sizes in the 40mm's...40mm, 42mm, 44mm are all common sizes really don't require parallax adjustment for a relatively clear image at less than 8X magnification. For a hunting/pesting airgun that has a an effective range of about 50 yards and doesn't require high magnification it is really hard to beat the 3-9x33 Leupold EFR. Leave it at 6X for a good magnification for hunting and forget about having to adjust the scope. One caveat, the reticle...the duplex reticle is only good for your PBR and Leupold didn't give us a tactical style turret. If Leupold would give us a good reticle with holdovers in this scope that were correct at 6X...WOW. Still, at 9X this scope does need parallax adjustment for the image to stay clear and the reticle crisp. It's amazing how precise you can be out to 50 yards with a crisp, clear, quality 9X scope. Again, Leupold dropped the ball bigtime with the turrets and reticle...but there is something to be said for a mid-power and narrow objective scope for the hunter/pester.
On the flip side of all this is that sometimes a large objective lens is desirable for the narrow depth of field. You can then use the parallax knob as a ranging tool with fairly good accuracy.
