In transmitted light microscopy, the light intensity in the field of view varies directly as the square of the numerical aperture of both the objective and the condenser. It varies inversely as the square of the total magnification. The light travels once through the objective.
This is different for reflected light fluorescence microscopy since the light travels twice through the objective. The light intensity in the field of view varies directly as the fourth power of the numerical aperture of the objective. And as in transmitted light, is also varies inversely as the square of the total magnification.
Intensity = N.A. (to the forth power) / Magnification (to the second power).
In transmitted light, the light passes through a condenser then into the objective.
In reflected light, the first pass of light through the objective has the objective initially functioning as a condenser for concentrating the light on the specimen. The second pass of light through the objective is the same as in transmitted light, which has the objective acting as the normal light gatherer for image formation.
Example: A 40x objective with a N.A. or 1.0 will produce resulting images five times brighter than the same power objective with a N.A. of 0.65.
Reference: Book “Fluorescence Microscopy – The Essentials”. By Mortimer Abramowitz. 1993.