APS-C Crop Factor Calculator: Sensor Size and FOV

An APS-C crop factor calculator converts a camera’s sensor dimensions into a crop factor and can show the equivalent field of view (FOV) for a lens. Enter the sensor’s width and height, plus the lens focal length if you want the equivalent-focal-length result; the outputs are the diagonal-based crop factor and the full-frame-equivalent focal length.

Formula

The standard diagonal-based crop-factor formula compares the sensor’s diagonal with the diagonal of the 36 mm × 24 mm full-frame reference format:

• Sensor diagonal (mm) = √(sensor width² + sensor height²).
• Full-frame diagonal = √(36² + 24²) ≈ 43.266 mm.
• Crop factor = full-frame diagonal ÷ sensor diagonal.
• Full-frame-equivalent focal length (mm) = lens focal length (mm) × crop factor.

In the calculator, sensor_width and sensor_height are the active imaging area’s dimensions in millimetres, and focal_length is the lens’s stated focal length in millimetres. The output crop_factor is a ratio, shown as “x”; the equivalent focal length is stated in millimetres. The reference diagonal is calculated from a 36 × 24 mm full-frame sensor. Use the active image dimensions, not the outside dimensions of the sensor package.

Worked Example

Using the calculator’s defaults—23.5 mm sensor width, 15.6 mm sensor height, and a 35 mm lens:

1. Sensor diagonal = √(23.5² + 15.6²) = √(552.25 + 243.36) = √795.61 ≈ 28.21 mm.
2. Full-frame diagonal = √(36² + 24²) = √1872 ≈ 43.27 mm.
3. Crop factor = 43.266 ÷ √795.61 ≈ 1.53×.
4. Equivalent focal length = 35 × 1.53 ≈ 53.4 mm.

The calculator rounds the crop factor to 1.53× and the equivalent focal length to 53.4 mm. The latter describes approximate framing compared with a full-frame camera; it does not change the physical focal length of the lens.

What APS-C Crop Factor Means

APS-C is a family of sensor formats smaller than the 36 × 24 mm full-frame reference. APS-C sensors are not all exactly the same size, so there is no single crop factor that applies to every camera. A commonly quoted factor is around 1.5×; some APS-C systems are closer to 1.6×. The precise value depends on the imaging area being compared and the reference format.

A smaller sensor records a narrower portion of the image circle produced by a lens. That narrower framing is often described as a crop. It is a comparison of angle of view, not an optical magnification that physically changes the lens. A 35 mm lens remains a 35 mm lens on any camera; on a sensor with a crop factor near 1.5×, its framing is roughly like that of a 52.5 mm lens on full frame.

When to Use the Calculator

Use the crop factor to compare framing across camera formats, estimate what focal length would provide similar framing on a full-frame camera, or make sense of a lens recommendation written for a different sensor size. For example, if your APS-C camera’s crop factor is 1.5×, a 50 mm lens gives a full-frame-equivalent field of view of about 75 mm. Conversely, a full-frame-equivalent 24 mm view corresponds to about 16 mm on a 1.5× APS-C camera (24 ÷ 1.5).

Enter dimensions in millimetres. Sensor specifications may be published in other units, so convert them before using the calculator: 1 inch equals 25.4 mm. The calculator assumes the entered width and height describe the image area used for capture. If the camera crops video, uses a different aspect ratio, or applies a digital crop, its actual FOV can be narrower than this sensor-size estimate.

Field of View, Exposure, and Depth of Field

Equivalent focal length is useful for comparing field of view, but it is not a complete equivalence between camera systems. Framing also depends on the camera’s aspect ratio and any in-camera or post-processing crop. The calculator does not measure the scene’s angle of view in degrees, because that also requires the lens’s projection and the direction being considered; it reports the simpler focal-length comparison.

Crop factor alone does not change exposure. At the same aperture f-number and shutter speed, exposure per unit sensor area is not multiplied by the crop factor. A smaller sensor may collect less total light for the same framing and f-number, but that is a separate comparison involving sensor area, transmission, and image processing.

Depth-of-field comparisons require more than multiplying focal length. To match framing and viewpoint across formats, photographers may also change shooting distance or aperture. A larger format can produce shallower depth of field under comparable framing conditions, but the result depends on the lens, aperture, distance, and the criterion used to judge acceptable sharpness.

Common Mistakes and Limits

• Treating the result as a new focal length: equivalent focal length compares angle of view only. It does not alter lens markings or optical properties.
• Assuming every APS-C camera is identical: sensor dimensions vary. Use the camera maker’s active-area specifications where available rather than relying on a generic factor.
• Mixing units: width, height, and focal length must all use the same unit. This calculator expects millimetres.
• Confusing diagonal comparison with exact FOV: the formula uses sensor diagonals. Cropping to a different aspect ratio can change horizontal or vertical framing differently.
• Rounding too early: retain the calculator’s unrounded values for intermediate calculations; displayed results are rounded for readability.

The result is a geometric estimate, not a measurement of a particular photograph. Lens distortion, focus breathing, camera crop modes, stabilization crops, and the exact active sensor area can all affect the final captured frame. For a precise horizontal or vertical angle of view, use the relevant sensor dimension and the lens’s actual focal length in an angle-of-view calculation.

Frequently Asked Questions

What is the crop factor of an APS-C sensor?
It depends on the sensor’s actual dimensions and the reference format. Many APS-C cameras are commonly described as about 1.5× or 1.6× compared with 36 × 24 mm full frame. Calculate it from the camera’s sensor width and height for a more specific estimate.

How do I calculate APS-C equivalent focal length?
Multiply the lens’s actual focal length by the sensor’s crop factor. For example, a 35 mm lens on a camera with a 1.5× crop factor gives a full-frame-equivalent field of view of about 52.5 mm.

Does crop factor make a lens longer?
No. The lens’s focal length stays the same. The smaller sensor captures a narrower portion of its image, producing framing comparable to a longer lens on full frame.

Is APS-C always 1.5× crop?
No. APS-C sensor sizes differ by camera system and model, and some are often approximated as 1.6×. The exact diagonal-based factor comes from the sensor’s active width and height.

Does crop factor affect aperture or exposure?
It does not change the lens’s f-number or exposure setting. Crop factor is chiefly a framing comparison. Depth-of-field equivalence and total light comparisons involve additional factors and should not be inferred from crop factor alone.

Can I use this calculation for video?
Use it as a starting estimate if video uses the full sensor area. If the camera crops or reads a different portion of the sensor for video, use that video area’s dimensions to estimate the actual crop and framing.

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in sensor_width | Sensor width | mm | 23.5 | 0
in sensor_height | Sensor height | mm | 15.6 | 0
in focal_length | Lens focal length | mm | 35 | 0
out crop_factor | Diagonal crop factor | x | sqrt(43.266^2 / (sensor_width^2 + sensor_height^2)) | 2
out equivalent_focal_length | Full-frame-equivalent focal length | mm | focal_length * crop_factor | 1