Bokeh is the aesthetic quality of the blurred, out-of-focus areas in a photo, especially how out-of-focus points of light render as soft shapes.
Two lenses can throw the exact same amount of background blur and have it look nothing alike. That gap is bokeh, and it decides whether a soft background feels calm or restless. Budget zooms often render out-of-focus highlights with bright rims and hard edges, a look sometimes called nervous bokeh, which tugs the eye away from the subject instead of letting it settle. Lenses corrected for smooth rendering, including many 85mm and 105mm portrait primes, turn the same highlights into gentle discs that fade at the edge.
Blade count starts to matter once you stop down: a seven-blade diaphragm shapes background points into visible heptagons by f/4, while a rounded nine or eleven-blade design keeps them close to circular. For anyone shooting portraits, product or food against foliage or city lights, this is the difference between a background that supports the frame and one that competes with it.
Photograph someone at dusk with a string of cafe lights about ten metres behind them, using an 85mm lens wide open at f/1.4. A lens built for smooth rendering turns each bulb into a clean, evenly filled circle roughly the size of a coin in the frame, a little soft at the rim.
Swap to a 70-300mm kit zoom set to 85mm and f/5.6 and the same bulbs shrink, gain bright outlines around dim centres in what is often called soap-bubble bokeh, and pick up a faint seven-sided edge from the blades. Move the subject nearer the lights, or the camera nearer the subject, and those discs grow. Toward the corners they may also stretch into lemon or cat's-eye shapes, which is optical vignetting rather than the diaphragm, and it is strongest on fast lenses used wide open.
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