[Don Pensack]

I recently switched from a 35 Panoptic to a 31 Nagler and noticed the image became noticeably brighter. The change from a 6.1mm exit pupil to a 5.4mm exit pupil was/is mostly responsible.

I used a Sky & Telescope Pupil Gauge to judge my exit pupil.
It consists of a series of small holes in two rows not quite parallel. Each pair of holes is 0.5mm closer than the pair above it, all the way from 10mm apart to just touching. You hold it up to the sky and move down the rows of pairs until the gap between the holes appears to disappear.

Using this, I discovered my pupil size, when looking at the sky, is 4.5mm.

Another technique is to take the magnitude scale from the bottom of a star atlas and cut it in half so you have a row of half-moon scallops. Hold it up to the eye so your eye's pupil makes the half moon into a round moon. Looking in a mirror, move the row of star images back and forth until it matches your pupil size. Darken the room completely and wait a couple minutes to turn on your red LED observing light. Illuminate the room just enough to see the pupil gauge near your eye and move the gauge back and forth until the half moon scallop just matches your pupil diameter. Turn the light off, wait a few seconds and turn it back on. Still matched? Good, that's your pupil diameter. In this manner, I obtained a 5.0mm pupil. which means that looking at the night sky actually causes my pupil to contract a little bit from looking at the ground (something I already knew).

Eliminating peripheral light can allow the pupil to dilate to maximum--hence, the black cloth over the head technique used by people in previous centuries.

At any rate, I had my answer as to why the 31 Nagler seemed brighter--the 35 Panoptic was stopping my scope down because its exit pupil was significantly larger than my dark adapted pupil.

For those of us who are older than a half century, matching the low power eyepiece to our pupil diameter makes a lot of sense.

Other advantasge of the 31 Nagler: it does have a larger true field of view than the 35 Panoptic (~10% wider), it has a darker sky image (higher magnification) which improves contrast, and it has less field curvature (so star images are tighter at the edge when the center is focused--the 35 Pan requires focusing on a star about 25% of the way out to get the entire field in good focus if your eye cannot accommodate the curvature when you focus on the center--this varies from person to person).

If your eye can accommodate the exit pupil, and the mild field curvature, the 35 Panoptic is a very nice low-power eyepiece. Though I ended up with the 31 Nagler, I used a 35 Panoptic for years without reservation. Either way, you don't lose.