I will try to explain what is my simplified understanding about NV objective usage and telescopes, both prime and afocal.
This could be not 100% correct but I hope my learning from other forum's posts is good enough for real life usage.
Standard NV PVS-14 has ~27 mm objective, F1.2 (or close) and is presenting 40* AFOV at the eyepiece.
When I want to choose my target I need to take into consideration telescope or objective focal length (FL), speed (F:) and if I use it prime (with or without focal reducer/barlow) or afocal (which means keep original 27 mm FL objective and add one ocular in front of it).
Afocal usage is very similar with focal reducers/barlows for prime focus.
For example, if you keep 27 mm NV objective (afocal mode) and add 54 mm 2" ocular in front of it, this is similar with 0.5X focal reducer (27:54=0.5X).
Finally, the most important parameters in the NV ocular view is total magnification and speed.
If we take 100 mm F5 telescope and use it in prime mode (no NV objective), we have (100x5):27= 18.5x magnification and F:5 speed. FOV will be 40*/18.5 = 2.16*.
If we want a faster system, we can add a focal reducer or we keep 27 mm original NV objective and add for ex. a 2" 54 mm ocular (this is called afocal mode) for a 0.5X FL reduction (250 mm). This will result in 9.25X magnification, F:2.5 speed and 4.3* FOV.
In a similar way we can find the different telescope/focal reducers/barlows/objectives combination that will result in desired magnification, speed and FOV of the NV system.
This is just my understanding from reading other posts and from my own experiments, if something needs to be corrected please let me know.
Edited by t.mihai147, 15 May 2022 - 12:05 AM.