Last night I was observing M83, seeing the three spiral arms get progressively easier to see as I increased power from 51x to 109x to 277x. While at low power this galaxy is a large bright round patch with just a hint of swirl, at high power the general brightness decreases, allowing detail to be seen better. Spaces between spiral arms become darker, and "gaps" in spiral arms contrast more with knots of brighter nebulosity.
I usually stop at 277x. My next eyepiece would be a 3.5mm Nagler with an 82 degree field. Things drift fast across such a field, at 435x, and good focus is hard to get unless seeing is good. Also, faint objects become much fainter, except for planetary nebulas, which tend to magically become brighter.
But last night I decided to pop in the 435x on M83. At first I was surprised by how small the core had become - at 51x the galaxy looked like 80% core, but now it was just a small ball of fuzz. The spiral arms seemed to have vanished. But as I improved the focus and jiggled the scope in order to track, the spiral arms popped into view, first in averted vision, then in direct vision. This was interesting - they started out barely detectable, but soon they were filling the field, and I could trace them much closer into the core than before. Just as far out, as well, but with much better resolution of the knots and gaps.
Encouraged by this, I decided to try it on another face-on spiral: M101. It was peaking at 25 degrees altitude. Maybe the seeing would not support it. But to my surprise, after a few moments, the spiral arms just popped, and again I could trace them further in, and see more detail in the outer reaches - all those HII knots!
I nudged the scope over to M51. I saw the "bridge" at 277x, at the limit between AV and DV. But at 435x, once it "popped" (again, that delay of a few moments during which only the reduced core draws attention, and the rest seems to have faded away), I could see the bridge easily with direct vision, and hints of the dark lane that accompanies it.
Finally, I looked at M104 at 435x as a control. On this bright edge-on the higher power did not show much more than 277x did. It did generate a pleasing observation as the galaxy "scrolled" through the field, letting me appreciate each detail in turn. But for the bright face-ons, I was surprised to find how much more detail I could see - how the dimming of the image actually made dim features more visible, once your eyes get used to it. Can't wait to try this on NGC 1365 and M33 later this year.