OK......I touched on it in your other post but will attempt to explain it fully.
First how to calculate exit pupil size in scopes.........divide the objective lens diameter by the magnification to arrive at exit pupil size for any power in a given scope. Variable scopes differ only in the fact that they vary as the power changes, decreasing as power goes up and increasing as power goes down. Take a 3x9x40 for example......it runs from 13.3mm at 3x to 4.4mm at 9 power. The important thing to remember here is that magnification being equal the extra light transmitted by a larger objective lens always creates a larger exit pupil whether its usable light or not.
Now how do you calculate the size of your eyes exit pupil to tell if it or your scope lens is the limiting factor in light transmission? The average person with normal eyes will have 2.5mm in bright noon day light - 3mm-4mm in average daylight - 5mm-6mm in dawn and dusk light periods - and 7mm in almost total darkness. As we age the eyes don't dialate as large and we don't get the 7mm and above pupil size of a younger person.
Now depending on the light conditions outside and the objective lens diameter of our scope we can determine the power that will meet but not exceed the exit pupil size of our eyes.
Here is a simple chart to use to determine exit pupil size at a glance. Remember we don't all have the same eye pupil size in differing light conditions and not all scopes are really the power they claim to be on the power ring so none of this is an exact science. It will however give you a pretty good understanding of exit pupil size and its relationship to objective size. The Conquest and 4200 Elite are very close to their power numbers advertised so it should help you to figure things out. Remember that the larger the objective bell gets the higher the scope needs to be mounted and the heavier the scope gets, two negative features in regards to an extremely large objective lens.
Now two final things..........a larger objective lens will allow you to use higher powers without exceeding your own eyes exit pupil.
The quality of the lenses and lens coatings can effect how much light entering a scopes objective lens will actually reach your eye after passing thru the 12 or so lenses in a variable power scope.Adjustable objective scopes add another lens so will be slightly dimmer due to the light loss of passing thru another lens. All scopes and lens coatings are not created equal and looking thru different scopes at low light times using charts like I posted in another thread will help you to compare and evaluate different scopes for brightness and clarity or resolution.
woods
