Telescopes used: Borg 45ED II (f/7.2), C5 (f/10 and f/6.3), Tele Vue Ranger (f/6.86)
DSLR: Canon 450D
Coupling: 1.25" nosepiece + Canon EOS T-ring and EOS -> 2" nosepiece (ScopeStuff)
The exposures are fixed at 1/10s at ISO 200 for the Borg, taken on a tripod, and 2 second timer to reduce the shake. Live View with 10x is used for focusing and it's truely useful.
This is taken with the Borg 45ED II at prime focus, the coupling used is 1.25".
This is taken with the Borg 45ED II at prime focus, the coupling used is 2". To my big surprise, the whole field of view is brighter, not just the field edge. I don't expect it's so dramatic, I even re-do the exposures to ensure that I've done everything correctly.
Now we switch to a C5, the exposures are fixed at 2 seconds each at ISO 200:-
C5 at f/10 with 1.25"
C5 at f/10 with 2", notice that since the baffle tube of the C5 is smaller than 2", and actually, it's very close to 1.5" and thus, the increase in brightness is very minimal.
C5 at f/6.3 (with reducer), 1.25" coupling with the 1.25" visual back.
C5 at f/6.3 (with reducer), 2" coupling with the 2" visual back, but this time, the increase in brightness is not negligible.
Since the 2" visual back is longer than the 1.25" visual back, the reduction factor is slightly stronger since the focal reducer is placed before the visual back.
Finally, this is taken with my Ranger at prime focus. I have no choice for this one since it only has a 1.25" focuser. To my little bit of surprise, the image quality is quite nice. Diagonal is not required to reach focus, as oppose to visual observation.