Full-frame 35 mm is the common reference for describing lenses, so a lens on a smaller sensor is often quoted by its “35 mm equivalent” focal length. A smaller sensor records only the centre of the image circle, which narrows the view as if you had used a longer lens on full frame. The ratio of the two sensor diagonals (the crop factor) is the conversion: a 35 mm lens on a 1.5× APS-C camera frames like a 52.5 mm lens on full frame.
The same factor converts the f-number for depth of field and total light: f/1.8 on APS-C gives roughly the background blur of f/2.7 on full frame when the framing is the same. Exposure is different: f/1.8 lets through the same light per unit area on every sensor, so shutter speed and ISO settings do not change.
Teleconverters and focal reducers (speed boosters) multiply both the focal length and the f-number. Use the second mode to answer the opposite question: which lens and aperture on your camera will match a full-frame setup you know, such as a classic 85 mm f/1.8 portrait lens.
How to use the Crop Factor and Equivalent Focal Length Calculator
- Calculate
- Sensor size: Choose your camera’s sensor format. Crop factors are relative to full frame.
- Sensor width
- Sensor height
- Lens focal length
- Aperture f-number
- Full-frame focal length to match
- Full-frame f-number to match
- Teleconverter or focal reducer (×): 1 = none; 1.4 or 2 for teleconverters; 0.71 or 0.64 for focal reducers.
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How it works
The crop factor is the full-frame diagonal (43.27 mm) divided by your sensor’s diagonal. Multiply the real focal length and f-number by any converter and then by the crop factor to get the equivalents, or divide a full-frame target by them to find the lens you need.
Worked example
A 35 mm f/1.8 lens is used on a Sony APS-C camera (23.5 × 15.6 mm, diagonal 28.21 mm). The crop factor is 43.27 ÷ 28.21 = 1.534.
Equivalent focal length = 35 × 1.534 ≈ 53.7 mm, and the depth-of-field equivalent aperture = 1.8 × 1.534 ≈ f/2.8. In reverse, to match a full-frame 85 mm f/1.8 portrait lens you need 85 ÷ 1.534 ≈ 55 mm at f/1.2.
Sensor sizes and crop factors
| Sensor | Size (mm) | Diagonal | Crop factor |
|---|---|---|---|
| Full frame (35 mm) | 36 × 24 | 43.3 mm | 1.00× |
| APS-C (Nikon, Sony, Fujifilm, Pentax) | 23.5 × 15.6 | 28.2 mm | 1.53× |
| APS-C (Canon) | 22.3 × 14.9 | 26.8 mm | 1.61× |
| Micro Four Thirds | 17.3 × 13 | 21.6 mm | 2.00× |
| 1-inch type | 13.2 × 8.8 | 15.9 mm | 2.73× |
| 1/1.3-inch type (large phone main camera) | 9.6 × 7.2 | 12.0 mm | 3.61× |
| 1/2.55-inch type (typical phone camera) | 5.6 × 4.2 | 7.0 mm | 6.18× |
| 1/3-inch type (phone front or ultra-wide) | 4.8 × 3.6 | 6.0 mm | 7.21× |
| 1/1.7-inch type | 7.6 × 5.7 | 9.5 mm | 4.55× |
| 1/2.3-inch type (compact, action camera) | 6.17 × 4.55 | 7.7 mm | 5.64× |
| 1/3.2-inch type | 4.54 × 3.42 | 5.7 mm | 7.61× |
| 2/3-inch type | 8.8 × 6.6 | 11.0 mm | 3.93× |
Formula
Crop factor k = 43.27 mm ÷ sensor diagonal · Equivalent focal length = f × converter × k · Equivalent f-number (depth of field) = N × converter × k
Worked example
Inputs: Calculate: Full-frame equivalent of my lens; Sensor size: APS-C (Nikon, Sony, Fujifilm, Pentax) – 23.5 × 15.6 mm (crop 1.53×); Lens focal length: 35 mm; Aperture f-number: 1.8; Teleconverter or focal reducer (×): 1.
Results: Equivalent focal length (full frame) = 53.7; Equivalent aperture for depth of field = 2.76; Focal length needed on this sensor = 35; f-number needed on this sensor = 1.8; Crop factor = 1.534 ×; Diagonal angle of view = 43.89 °.
Key terms
- Crop factor
- Ratio of the full-frame diagonal to a sensor's diagonal.
- Equivalent focal length
- The focal length that gives the same angle of view on full frame.
- Equivalent aperture
- The f-number that gives the same depth of field and total light on full frame at the same framing.
- Teleconverter
- An optic between lens and camera that multiplies the focal length (and f-number).
Frequently asked questions
What is the crop factor of APS-C?
1.53× for Nikon, Sony, Fujifilm and Pentax (23.5 × 15.6 mm) and 1.61× for Canon (22.3 × 14.9 mm). They are usually rounded to 1.5× and 1.6×.
What is a 50 mm lens on APS-C equivalent to?
About 77 mm on Nikon/Sony/Fujifilm and 80 mm on Canon APS-C, a short telephoto portrait view.
Does crop factor change the f-number for exposure?
No. f/2.8 gives the same exposure on every sensor. The equivalent f-number only describes depth of field and total light gathered.
What is the crop factor of a phone camera?
Typical main cameras are 1/1.3- to 1/2.55-inch type sensors with crop factors of about 3.6× to 6.2×, which is why a 6.9 mm lens on a 1/1.3-inch sensor is marketed as about 25 mm.
Does a teleconverter cost light?
Yes. A 1.4× converter loses one stop and a 2× converter two stops, because the f-number grows by the same factor as the focal length.
What does a focal reducer do?
It shrinks the image circle onto a smaller sensor, cutting the effective focal length and f-number. A 0.71× reducer on APS-C nearly restores the full-frame view and adds one stop of light.
Is the 35 mm equivalent the same as the real focal length?
No. The real focal length never changes; the equivalent figure only tells you which full-frame lens would show the same view.