Someone asked why the view dropped off when moving from the center of the field. There are two possible reasons.
The first (and in my opinion, probably the most common) is vignetting when using lots of focal reduction. Reflectors in particular are generally only designed to fully illuminate maybe a 10mm image circle and this means that if you reduce the field by 50%, the fully illuminated image circle is now only 5mm. Many refractors running focal reducers will also suffer off axis illumination falloff because of vignetting in the front of the focuser tube, or in the light path.
I have a 6" reflector that works at f/2.8. It was designed to fully illuminate an APS-C size sensor when working at f/2.8 so my field is very bright all the way out to the edge with even a 7nm filter (have not tried it with my 5nm yet).
The other reason is that when the system gets very fast, the Ha filters can suffer from band shift. As the light entering from the objective comes in at steeper and steeper angles from the edges of the field, the narrow band pass starts to shift off of the Ha peak. Now again, I use an f/2.8 scope and even with an 7nm filter, the amount of band sift I see is very mild so this is why I think that many times the falloff people report is likely due to vignetting.
At 1x and 3x, though the light cone is usually between f/1 and f/2, and here, the 7nm and 5nm will show significent band shift. Under dark skies, I use a 12nm and this will show very little illumination falloff. At 1x under dark skies I can see the entire Barnard's loop and Angelfish Nebula in the same field.
So, falloff is often vignetting from the primary optics and even at f/2.8, a scope designed to fully illuminate APS-C size sensor should show little sign of falloff. Faster than this and the width of the band pass can start to be an issue.
This picture was taken by one of our members. This Cygnus like you can never see it using conventional eyepieces. Because it is f/1 and probably using a 5nm filter, you can see how the view looks vignetted, but again, this is actually more a function (in this particular case) of band pass shift as you get further off axis.
Someone asked why there are so many reports from 1x views.Look carefully at this picture! Can you see The North American Nebula? That should easy of course, but this is a 40 degree true field. The "Butterfly" in the middle is a super bright part of a nebula that most astronomers using standard eyepieces will never be able to see and probably only seen as Gamma Cygni Nebula on charts, but most of the nebula in the center of the field is the part of this complex.
Can you see the Pelican? The Veil? In the original, I could even pick out the Crescent Nebula (visible at 1x).
This is why so many people using NV work at 1x and 3x a lot. There are views so full of nebula that it simply won't all fit into smaller fields.