As to the viewfinder brightness:
The appeal of the SLR (single lens reflex) is due in no small part to its viewfinder system. The modern SLR maintains the lens aperture wide-open for viewing and composing. Thus the photographer is presented with a view that is at maximum brightness. Additionally, with the aperture open wide, depth-of-field (DOF) is at minimum. A shallow DOF is desirable for focusing as focusing is more critical. During the actual picture taking, the mechanism closes down the aperture to a pre-set f/number. This action expands the zone of DOF. Naturally, the larger the maximum aperture, the brighter the viewfinder view.
Let me add that landscape images likely only portray objects at an infinite distance. We set the focus at infinity ∞. Unless these vistas contain foreground objects, DOF is moot.
All that being said, there is truth and there is lore when it comes to lens sharpness. The old masters of landscape called themselves “Group 64”. They set their aperture to f/64 to gain maximum DOF and maximum acuity. The DOF part is correct, the acuity part is iffy.
A a rule-of-thumb: The camera lens is at maximum sharpness when stopped down about two f/stops. This is because at max aperture the outer perimeter of the lens is being used, this region has the most severe curve (figure), imaging forming rays from this region often go astray reducing acuity.
Thus as we stop down, acuity increases. Not so fast! As we stop down, twin demons of diffraction and interference begin to degrade the image. Due to the nature of light, some of rays hit the blades of the iris with a glancing blow. These ricochet and bleed into the path of the image forming rays. As we stop down the percentage of these glancing blows increase. The net result, the resolving power of the lens decreases as we stop down. We measure lens acuity in lines per millimeter. We are talking about the ability to distinguish spaces between closely ruled lines.
The Rayleigh Criterion conveys the theoretical resolving power of a lens as RP=1392 ÷ f/number. This value is usually given for green light; other colors will have a slightly different value.
Table of RP lines per millimeter vs. f/number:
f/1 = 1392
f/1.4 = 994
f/2 = 696
f/4 = 348
f/8 = 174
f/16 = 87
f/32 = 44
f/64 = 22
Note: f/8 value and above is higher than is pictorially useful for film.
How does this affect the final image? Generally we view small images and likely the RP is of little importance. However, when we make and view large images for display, this stuff becomes important. Especially true if the viewer is positioned near the enlarged displayed image.