Having owned and run about every mid to high end optic out there except for USO extensively I will add :
As mentioned the tube diameter increase from 1" to 30mm has nothing to do with light gathering and every thing to do with both windage and elevation travel.
There is a formula for exit pupil but it doesn't fit every bill due to the variables in scopes due to the difference in glass quality, coating quality and the actual status of of the particular eye viewing through the scope. Since everyone of us has a different eye including different numbers of rods, cones and lense qualities this formula simply can't apply to all situations.
In general the best combo is to have great glass + great coatings + lower magnification + large objective.
For my eye great glass with 3x and 40 mm front objective doesn't come close to a great glass 3x with 56mm objective in low light, clarity or any other realm except for the weight savings.
Also someone brought up the point of the toilet paper roll.. the Europeans have known for years that if you block out the ambient light from entering your eye around the rear ocular that you will get a much better and brighter sight picture in low light situations. It will blow your mind if you have not tried it.. Web search "Rifle scope eye cups for something better than toilet paper rolls".
Until I find something better I will continue to run Kahles 3x12x56 on rigs where long shots are not the norm. On rigs that may see long range that need to retain the ability to shoot close in low light then the NXS 56mm in both 3.5x15 and 5.5x22 come into play. The Kahles beats the NF in Clarity and weight savings but doesn't dial.
The newer Tactical Kahles line loose against the NXS in weight and pricing.
I have found no better combinations out there and I truly am happy but bored with the optics that I currently run..
As I understand it the formula for determining exit pupil applies to all scopes regardless of glass quality or coatings. Divide the objective lens diameter by the magnification for exit pupil diameter.
If you have a variable with larger objective you can use a higher magnification to get the same width exit pupil. As mentioned, everyone's eyes are different and one person might be able to see better with a 50 MM objective at 8X while another may do better at 6X or 7X.
If I'm hunting late in the afternoon, I'll play with the power setting to see what magnification lets ME see best regardless of what theory says about how big a 65 year old's pupil will get in low light.
