Why do people say f/2.8 isn't "really" f/2.8 on a crop sensor?
This claim is misleading shorthand for a real but narrower idea. f/2.8 exposes identically on any sensor size — same shutter speed, same ISO, full stop. What people actually mean is that f/2.8 on APS-C (1.53x computed crop factor) produces roughly the same depth of field an f/4.3 lens would give on full frame, once you're framing the same shot. That's a real, useful equivalence; 'not really f/2.8' overstates it into something false about exposure.
The confusion has a specific, well-understood cause — full-frame and APS-C shots really do look different side by side, for reasons that have nothing to do with the f-number changing meaning. See why this mix-up is so persistent for the actual mechanism.
The More Accurate Way to Say It
'f/2.8 on APS-C gives roughly f/4.3-equivalent depth of field compared to full frame at the same framing' is accurate and specific. 'f/2.8 isn't really f/2.8 on APS-C' is not — it's a compressed, over-generalized version of the real claim that happens to be technically wrong about the one thing (exposure) it actually asserts, even though the underlying depth-of-field-equivalence math it's gesturing at is legitimate. The phrase gets repeated so often specifically because it sounds like it's explaining something real, and it is — it's just pointing at the wrong variable.
See the accurate depth-of-field-equivalent figure for your own aperture in the Crop-Factor & Equivalence Converter.
Frequently Asked Questions
Should I ever use the depth-of-field-equivalent aperture instead of my real aperture when setting exposure?
No — always use your lens's real f-number for exposure calculations (metering, flash, manual settings). The equivalent aperture is purely a depth-of-field comparison tool, not a substitute exposure value.
Does this same confusion happen with shutter speed or ISO?
No — this specific mix-up is unique to aperture, because aperture is the one exposure variable that also independently controls depth of field, giving it two genuinely different roles that get conflated. Shutter speed and ISO don't have an equivalent cross-format depth-of-field calculation to confuse with exposure.