What is the equivalent aperture on APS-C, and why does it matter for depth of field?
Multiply the real f-number by the sensor's crop factor: an f/2.8 lens on APS-C (1.53x computed crop factor for the Nikon/Sony/Fujifilm format) has a depth-of-field-equivalent aperture of roughly f/4.3 on full frame — meaning that once you're framing the same shot on both formats, full frame at f/4.3 produces roughly the same background blur APS-C gives at f/2.8.
This equivalence genuinely answers a depth-of-field question and only a depth-of-field question — it says nothing about exposure. See the one question crop factor never answers for why that specific distinction trips up so many photographers repeatedly.
Why It Matters in Practice
If you're comparing systems for portrait or product work where background blur is the priority, the equivalent aperture is the number that actually predicts what you'll see — an APS-C f/1.8 portrait lens (equivalent to roughly f/2.8 on full frame) won't isolate a subject as dramatically as a full-frame f/1.4 lens does, even though f/1.8 sounds close to f/1.4 on paper. Photographers switching systems use this specific calculation to find which lens genuinely replicates a favorite look, rather than assuming matching f-numbers alone will do it. It's also the number worth checking before assuming a cheaper crop-sensor lens can fully substitute for a specific full-frame look you already know and like.
The same multiplication holds at any real aperture. A fast f/1.8 prime, popular on APS-C for exactly this reason, works out to roughly f/2.8 equivalent — genuinely shallow, but nowhere near an f/1.4 full-frame lens's separation. Move to Micro Four Thirds' steeper 2.0x crop factor and that same f/1.8 lens becomes roughly f/3.6 equivalent, which is why background separation that reads as dramatic on Micro Four Thirds often looks comparatively modest once translated to a full-frame frame of reference.
Compute this for your own aperture and target sensor in the Crop-Factor & Equivalence Converter, pre-filled with f/2.8 on APS-C.
Frequently Asked Questions
Does the equivalent aperture change with focal length?
No — the multiplier is purely the crop factor, applied identically regardless of which focal length you're actually using; only the field-of-view equivalent, a separate calculation entirely, depends on focal length.
Is Canon's APS-C equivalent aperture different from Nikon or Sony's?
Yes, slightly — Canon's APS-C sensor has a computed crop factor of 1.61x against 1.53x for Nikon, Sony and Fujifilm, so the identical f/2.8 lens works out to a slightly narrower equivalent aperture, roughly f/4.5, on Canon's specific format.