As long as the eye, the pupil, is fully illuminated they will be equally bright, at least to the extent that you can likely notice.
This is true of galaxies, nebulae and planet's, "extended objects".
It is not true for stars which are point sources. They are dimmer when the exit pupil is larger than that your pupil. Stars act as points of light at anything but small exit pupils/high magnifications so that light that does not enter your eye, dims the star.
With an extended object, this loss of light is compensated for by the fact that the object is smaller, but a point cannot get smaller.
Close yourself up in a bathroom with a towel under the door as necessary and wait with a camera and a set of calipers preset to 7mm for 20 - 30 minutes. Take a pic in the mirror with the calipers held near your eye.
It only takes your pupil a short time to fully dilate. One minute is sufficient.
I do not recommend using calipers in the dark, they're sharp, too risky to hold near your eye.
When I've done this, I've used a thin strip of wood about 7 mm wide. I just need to measure it's width with the calipers.
Several attempts will be necessary to get the focus correct.
When you have a sharp photo, transfer the photo to a computer, zoom in on the image and measure the diameter of your pupil and the width of the stick on the computer screen with the calipers.
Your pupil diameter diameter is the ratio of the zoomed in measured diameters times the width of the stick. For example, if the stick is 9.1 mm wide and on screen, you measure your pupil to be 52.6mm and the stick to be 71.0 mm then your pupil diameter would be:
D = (52.6mm/71.0mm) x 9.1 mm = 6.74mm.
Even with a 6.7 mm dark adapted pupil, I'd choose a quality 30mm-31 mm 82 degree eyepiece for F/5 over a longer focal length eyepiece.
My dark adapted pupil is 7.7 mm and while I have very high quality 35 mm (7 mm exit pupil) and 41 mm (8.2 mm pupil), both 68 degree Panoptics, I rarely use them, the 31mm 82 degree Nagler provides nearly the widest view but with signifantly more magnification and is almost always my preference. In a 12 inch with a 1500mm focal length:
31 mm 82 degree = 1.60 degrees at 48.4x, 6.2 mm exit pupil.
35 mm 68 degree = 1.48 degrees at 42.9x, 7.0 mm exit pupil
41mm 68 degree = 1.75 degrees at 36.6x. 8.2 mm exit pupil.
It's unlike that your dark adapted pupil is as large as mine but even for me who can nearly take in that 8.2 mm exit pupil, the 48.4 x versus the 36.6 x is the important factor. I even choose the 35 mm over the 41mm for this reason.
This is based several hundred nights under dark skies with these three eyepieces. Under brighter skies, I'd forego these three for an even shorter focal length eyepiece.
I should add, these are expensive eyepieces, the least expensive the 35 mm Panoptic, currently on sale for $351. Explore Scientific offers very similar eyepieces that are not quite as perfect as these TeleVues but still quite sharp across the field and significantly less expensive. Their 34 mm 68 degree is $290.
There are eyepieces that cost less than $100 that are workable as finder eyepieces. Ed's 38 mm 70 degree eyepiece is one of them. It does offer essentially the widest field of view at the expense of the large exit pupil and a significant loss of sharpness away from the center of the field of view, stars towards the edge are far from tight, round points at F/5.