Why does full frame have shallower depth of field than APS-C at the same aperture?

This claim only holds when you're framing the same shot on both formats, not at the identical real focal length. Matching an APS-C 32.6mm f/8 composition at 5 meters, full frame needs roughly a 50mm lens — and that longer real focal length produces roughly 5.73 meters of total depth of field against APS-C's roughly 13.7 meters at its own equivalent focal length, the same aperture and the same distance. Full frame's depth of field is shallower here because of the longer real lens it needs, not because of anything about the sensor itself in isolation.

The Part That's Easy to Get Backwards

At the identical real focal length instead — both cameras with a genuine 50mm lens mounted — full frame's larger circle of confusion (0.0288mm against APS-C's 0.0188mm) actually gives it MORE depth of field, not less, since a more tolerant acceptable-blur threshold widens the sharp range on its own. That's a completely different, far less commonly discussed comparison, and it's the opposite direction from the popular 'full frame has shallower depth of field' claim — which is true, just specifically for the matched-framing comparison, not the matched-focal-length one.

Why the Matched-Framing Version Is the One People Actually Mean

Nobody switching from APS-C to full frame keeps using the identical real focal length and calls that a fair comparison — the field of view would be completely different. Photographers compare formats by the shot they'd actually take, and that's exactly the scenario where full frame's reputation for shallower depth of field, and the resulting stronger background separation, genuinely holds up.

Compare both scenarios directly for your own lenses in the DoF & Hyperfocal Calculator.