What the Crop Factor Calculator measures
Camera sensors come in many physical sizes, but focal length markings on lenses (like "50mm") are all measured the same way regardless of sensor size. Crop factor is the ratio between a full-frame sensor's diagonal and your sensor's diagonal, and it tells you how much narrower your camera's field of view is compared to shooting the same lens on a full-frame body. This calculator converts your lens's actual focal length into a 35mm-equivalent focal length — the number photographers actually compare when talking about "how zoomed in" a shot looks — and, if you provide an aperture, an equivalent f-stop that describes depth of field on an equivalent full-frame comparison.
Use it when you're used to full-frame focal length conventions but shooting on APS-C, Micro Four Thirds, or a compact/smartphone sensor, and want to know what focal length would frame a similar shot on a full-frame camera — useful for buying the right lens, or for comparing gear across different camera systems. It's also useful for understanding why a "50mm" portrait lens looks noticeably tighter on a crop-sensor body than on full frame.
The formula and its variables
Crop Factor = Full-Frame Diagonal (43.27mm) ÷ Your Sensor's Diagonal; 35mm-Equivalent Focal Length = Lens Focal Length × Crop Factor; Equivalent Aperture = Actual Aperture × Crop Factor.
- Full-Frame Diagonal: 43.27mm, the diagonal of a 36 × 24mm full-frame (35mm-format) sensor, used as the universal reference point.
- Sensor Type / Custom Diagonal: your camera's sensor size, either picked from common presets or entered as a custom diagonal in millimeters for less common sensors.
- Lens Focal Length: the focal length printed on your lens, in millimeters — this never changes regardless of what body it's mounted on.
- Equivalent Aperture: describes depth of field only, letting you compare how much background blur to expect versus the same framing on full frame — it does not change your actual exposure settings.
Worked example
With the defaults — APS-C (Nikon/Sony/Fuji/Pentax) selected at 1.5x crop factor, a 50mm lens, and f/2.8 aperture — the 35mm-equivalent focal length is 50 × 1.5 = 75.0mm, meaning the field of view looks similar to a 75mm lens on full frame. The equivalent aperture is 2.8 × 1.5 = f/4.2, meaning the depth of field (background blur) will look similar to shooting f/4.2 on a full-frame camera at the equivalent 75mm framing — even though your camera's exposure meter and settings still read f/2.8. For a custom sensor with a 28.2mm diagonal instead, crop factor would be 43.27 / 28.2 ≈ 1.53x, very close to standard APS-C.
Common mistakes and how to interpret the result
- Thinking the lens itself changes on a crop sensor. It doesn't — a 50mm lens is still physically a 50mm lens; only the portion of its image circle your smaller sensor captures changes, which narrows the field of view.
- Assuming equivalent aperture changes your exposure. It doesn't — your camera's actual f/2.8 still lets in the same amount of light and uses the same shutter speed and ISO relationship; equivalent aperture is purely a depth-of-field comparison tool.
- Mixing up sensor width/height with sensor diagonal when entering a custom value. The crop factor formula specifically needs the sensor's diagonal measurement, not its width or height alone.
- Forgetting which Canon APS-C variant applies. Canon's APS-C sensors use a 1.6x crop factor, while most other manufacturers' APS-C sensors (Nikon, Sony, Fujifilm, Pentax) use 1.5x — mixing these up gives a slightly wrong equivalent focal length.
Related calculators
Common Measurement Conversions for Everyday Use
| Quantity | US / Imperial | Metric Equivalent |
|---|---|---|
| 1 cup | 8 fl oz | 237 mL |
| 1 pint | 16 fl oz | 473 mL |
| 1 quart | 32 fl oz | 946 mL |
| 1 gallon | 128 fl oz | 3.785 L |
| 1 lb | 16 oz | 453.6 g |
| 1 ft | 12 in | 30.48 cm |