Aperture / f-stop
The size of the lens opening, written like f/1.8 or f/11. Smaller number means more light and more background blur.
See it
What it is
Aperture is the adjustable iris inside the lens, and the f-stop is how wide it is open. The number is a ratio (focal length divided by the opening), which is why it feels backwards: f/1.4 is wide open and flooding light in, f/16 is nearly a pinhole. The classic full-stop series is f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, and each step halves or doubles the light.
Wide open (f/1.4 to f/2.8) buys you light in a dark room plus shallow depth of field, so it is the portrait, food, and product-isolation setting. Stopped down (f/8 to f/11) buys sharpness front to back, which is the setting for architecture, flat lays, and group shots where nobody is allowed to be soft. f/8 is the old 'just make it sharp' default for a reason.
Gotcha: lenses are rarely at their sharpest wide open, most peak about two stops down, and past roughly f/16 diffraction starts softening the entire frame, so cranking the number up to chase sharpness backfires. Aperture is also one leg of the exposure triangle: open it up and you have to speed up the shutter or drop ISO or the shot blows out.
Ask AI for it
Create a deep-focus landscape at f/11 shot from a tripod, focused near the hyperfocal distance so everything from the foreground rocks to the distant ridge reads as acceptably sharp. Even illumination corner to corner, crisp edge detail across the background, and natural depth cues (haze, overlap, scale falloff) still doing the work of distance. Keep the rendering honest rather than uniformly razor sharp at every plane.