Do prime lenses really give shallower depth of field than zooms?

Not inherently. Depth of field depends on real focal length, real aperture and subject distance — none of which care whether the lens is a prime or a zoom. An 85mm prime at f/2.8 and an 85mm zoom setting at f/2.8, at the identical subject distance, produce the same depth of field. The real, common difference is maximum available aperture, not depth of field as a direct property of prime-versus-zoom design.

Why Primes Earned This Reputation Anyway

Prime lenses are typically easier and cheaper to design with a wide maximum aperture than a zoom covering the same focal length is — an f/1.4 or f/1.8 prime is common and comparatively affordable, while a zoom maintaining that same maximum aperture across a whole focal-length range is a much harder, more expensive optical engineering problem. Most zooms top out around f/2.8 at best for professional-grade models, or f/4-f/5.6 for consumer designs, which is where the real depth-of-field gap between typical primes and typical zooms actually comes from — the aperture ceiling, not the fixed-versus-variable focal length itself.

When a Zoom Genuinely Matches a Prime's Depth of Field

At the same real aperture, focal length and distance, a zoom lens produces identical depth of field to a prime — a photographer with a fast f/2.8 zoom at 85mm gets the same depth of field an f/2.8 85mm prime would give at the identical settings, even though the prime might offer a wider f/1.4 or f/1.8 option the zoom simply can't reach at all.

Compare any prime and zoom setting directly for depth of field in the DoF & Hyperfocal Calculator.