And why would you shoot landscape wide open?
I have 3 main reasons for shooting landscape at a wide aperture:
- Waterfalls with stronger water flow can generate wind/spray around the waterfall if you are close up. If the foreground moves eg tree/leaves and then I would take a wide aperture shot (or high ISO) and layer blending the foreground with the longer exposure for smoothing the water or bubble trails in the water.
For landscapes with no moving foreground then I would use f9 or narrower for sharpness. For seascapes, I want the foreground to move/blur so use longer exposure times (and/or use filters). Similar for cloud movement for cityscapes. If I want light stars (sun stars or night street light stars) then I will use <f16 to generate them.
- Focus stacking a foreground panorama where there is limited light. eg foreground for a milky way shot during blue hour where the light is changing fast. It can be much faster to take 2 shots focused on near/far distance wide open and stack them rather than f9 and stack each panel before merging a panorama of say 7 panels for 180 degrees.
As
@neuroanatomist mentions, all astro landscape skies will be as wide as possible offsetting ISO. I use my Samyang 14/2.8 @f2.8 at 15-20 seconds per panel @ISO3200 which gives a reasonable ISO vs star trail balance. If using a star tracker then you can stop down eg 20/1.4 down to f2 to improve vignetting/sharpness with a 30sec-2minute exposure at low ISO.
Aperture is used creatively at the end of the day for what you want to achieve.