PhotoCalcs
Equivalent Focal Length Calculator — Cross-System Comparison
Find the equivalent focal length when switching between sensor sizes to maintain the same field of view.
Add the aperture multiplier needed to match depth of field on the target system
How We Calculate This
This calculator converts focal lengths between any two sensor sizes:
Conversion Formula
Full-frame equivalent = focal length × source crop factor
Target focal length = full-frame equivalent ÷ target crop factor
This works because crop factor represents the ratio of full-frame diagonal to sensor diagonal. The field of view is determined by the ratio of sensor size to focal length, so matching the field of view requires scaling the focal length by the ratio of crop factors.
Diagonal field of view
The diagonal angle of view on the source body is 2 · arctan(d ÷ (2f)), where d is the source sensor diagonal (full-frame diagonal ÷ source crop factor) and f is the focal length. A shorter focal length or larger sensor widens the angle.
DoF-equivalent aperture
To match depth of field across systems, multiply the source f-number by the conversion ratio (source ÷ target crop factor). For the same framing and depth of field, a smaller sensor needs a smaller f-number (wider aperture) — divide the f-number by the crop factor. For example, full frame at 85mm f/1.8 is matched by APS-C (1.5×) at ~56mm f/1.2. Put the other way round, a larger sensor gives shallower depth of field at the same f-number, which is why full frame is prized for background blur.
Frequently Asked Questions
When switching between camera systems with different sensor sizes, equivalent focal lengths tell you which lens on the new system gives the same field of view. For example, a 35mm lens on an APS-C body (1.5×) frames like a ~52.5mm lens on full frame (35 × 1.5 = 52.5), because the smaller sensor crops into the image — so to match that framing on full frame you would reach for about a 52mm lens.
First convert to full frame equivalent (multiply by the source crop factor), then divide by the target crop factor. For APS-C (1.5×) to MFT (2×): multiply by 1.5, then divide by 2. A 50mm APS-C lens is equivalent to 37.5mm on MFT.
Equivalent focal lengths give the same field of view, but perspective depends only on camera-to-subject distance. If you stand in the same position, you get the same perspective regardless of focal length — cropping simply selects a portion of the scene. Moving closer or further changes perspective.
Key focal lengths on full frame: 14-16mm ultra-wide landscape, 24mm wide reportage, 35mm street, 50mm standard/portraits, 85mm portrait, 135mm tight portrait/events, 200-400mm wildlife/sport. On APS-C, divide by 1.5 for the equivalent lens: 35mm FF ≈ 23mm APS-C.
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Last updated: February 2026
All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.