Shooting wide open means using a lens at its maximum aperture, the widest opening it offers, which corresponds to its smallest f-number. For an f/1.8 lens, wide open is f/1.8; for an f/4 zoom, it is f/4. The phrase is the opposite of stopped down, which means using a narrower aperture for more depth of field.
Wide open gathers the most light, allowing faster shutter speeds in dim conditions and lower ISO settings. It also produces the shallowest depth of field the lens can deliver, throwing backgrounds into the soft blur known as bokeh. This is why portrait, wedding, and low-light photographers reach for wide apertures: the combination of light-gathering and subject isolation is exactly what those situations demand, and it is the heart of creating a shallow depth of field.
There are optical trade-offs. Most lenses are at their softest wide open, especially in the corners, and may show vignetting and some aberration. Stopping down one or two stops usually sharpens the image noticeably and reaches the lens’s sweet spot, the aperture where it performs best across the frame. Modern fast lenses are increasingly designed to be excellent even wide open, but the tendency still holds for most glass.
The other challenge of shooting wide open is precision. With depth of field reduced to a sliver, focus must be exact, which is why portrait photographers focus on the near eye and why a tiny focus error becomes obvious. The old focus-and-recompose habit can shift the plane of focus enough to miss at f/1.4, which is why modern eye-detection autofocus is such a help when working wide open.
Video shooters face a particular issue wide open: in bright light the widest aperture can let in too much light even at the lowest ISO and a standard shutter angle, which is why they add a neutral density filter to keep shooting at f/1.8 while holding the correct exposure. Stills photographers solve the same daylight problem with a faster shutter speed.
Whether to shoot wide open is a creative decision, not a default. It is the right choice when you want maximum background separation or need every photon in low light, achieved through selective focus. When you need front-to-back sharpness, as in landscapes or group shots where everyone must be sharp, you stop down instead, accepting less light for greater depth.
A reliable habit when shooting wide open is to take more than one frame and confirm focus on the rear screen at magnification, since the thin plane is unforgiving and a slight sway forward or back between focus and shutter can be enough to miss. With practice and modern eye detection, the hit rate climbs quickly. It also helps to close down a third or half stop from the maximum when you want a touch more depth without abandoning the wide-aperture look.