Crop Factor Calculator - Sensor Size, Crop Factor, and 35mm Equivalent Focal Length

A crop factor calculator turns a camera's imaging sensor width and height into the multiplier photographers use to compare lens behaviour across bodies. It also multiplies the actual focal length by that multiplier to give the 35mm equivalent focal length, so a 50mm lens on a 1.5x APS-C body behaves like a 75mm lens on full frame.

• • Compare lens field of view across bodies: Compute the 35mm equivalent focal length for a 35mm prime on APS-C, MFT, or 1 inch bodies so a photographer can pick a body that delivers the field of view they want.
• • Check a used camera sensor: Enter the sensor width and height from a spec sheet to confirm the crop factor against the manufacturer value before buying a lens.
• • Teach the focal-length multiplier: Demonstrate the diagonal ratio with a worked example so a classroom can see why a 25mm MFT lens and a 50mm full-frame lens share the same field of view.

Crop factor, also called the focal length multiplier, is the ratio between the 35mm full-frame diagonal and the diagonal of any other sensor, and the entry point to the equivalent focal length and equivalent aperture that set field of view and depth of field.

The crop factor is the diagonal ratio of a rectangle (the sensor), and the Rectangle Diagonal Angle Calculator shows the same Pythagorean diagonal math applied to any width-by-height rectangle.

The model is a right-triangle ratio: the sensor diagonal is the hypotenuse of a triangle whose legs are the sensor width and height, and the crop factor is the 35mm reference diagonal divided by that hypotenuse. Multiplying the actual focal length by the same crop factor gives the 35mm equivalent focal length.

• Sensor width and height in mm: Physical dimensions of the imaging sensor. Defaults to 23.5 mm by 15.6 mm (Nikon or Sony APS-C). Inches are converted to mm by multiplying by 25.4.
• 35mm reference diagonal: Fixed at sqrt(36^2 + 24^2), about 43.27 mm, the diagonal of the 35mm full-frame reference format from the 135 film cartridge.
• Actual focal length in mm: The lens's real focal length, printed on the barrel. Multiplied by the crop factor to give the 35mm equivalent focal length.

When solving for the equivalent focal length from a known crop factor, the same multiplication applies but the crop factor is the input. When solving for the sensor size, the diagonal ratio runs in reverse: sensor diagonal equals the 35mm reference diagonal divided by the crop factor, and the calculator recovers width and height from a 3:2 aspect ratio.

According to Wikipedia, the 35mm full-frame reference format has a width of 36 mm and a height of 24 mm, and the crop factor is the ratio of the 35mm diagonal of about 43.27 mm to the diagonal of the sensor under test.

When a sensor spec sheet lists dimensions in inches or in fractional inches, the Length Converter converts the values to millimetres before the crop factor diagonal math runs.

Four ideas come up every time crop factor is mentioned. Understanding them keeps the result grounded in what the lens and sensor actually do.

Once the equivalent focal length is on screen, the CM to Inches Calculator converts the same sensor and lens dimensions into inches for spec sheets that use the imperial system.

According to Wikipedia, Canon APS-C is about 22.3 mm by 14.9 mm and Nikon or Sony APS-C is about 23.5 mm by 15.6 mm, giving crop factors of about 1.61 and about 1.5, the two APS-C presets in this calculator.

The form is organised the way a photographer works: pick what to solve for, choose a sensor preset or enter the dimensions, then read the crop factor and 35mm equivalent focal length.

1. 1 Pick what to solve for: Use the Solve For menu to choose Crop factor, Equivalent focal length, or Sensor size.
2. 2 Choose a sensor preset or enter dimensions: Pick a preset to fill the width and height automatically, or choose Custom and enter the camera's spec-sheet dimensions. The Length Unit menu switches between mm and inches.
3. 3 Enter the lens focal length: Type the actual focal length in the selected unit; 50mm is the default and the classic 'normal' lens for a full-frame body.
4. 4 Read the crop factor and equivalent focal length: The primary result is the crop factor; the supporting rows list the 35mm equivalent focal length, the sensor diagonal, and the 35mm reference diagonal.

A photographer wants to use a 35mm f/1.8 prime on a Nikon APS-C body. They keep the Nikon or Sony APS-C preset (23.5 x 15.6 mm), type 35 in the focal length box, and read a crop factor of 1.53x with a 35mm equivalent of 53.6 mm, close to the classic full-frame normal lens.

If your sensor dimensions arrive in centimetres rather than millimetres, the CM to MM Conversion converts them in one step before the diagonal and crop factor run.

The crop factor calculator replaces the mental conversion photographers run every time they change bodies, with a single form that runs the diagonal math and the focal length multiplication in one step.

• • One-step crop factor and equivalent focal length: Read the crop factor and the 35mm equivalent focal length in the same panel, so you can compare lens behaviour across APS-C, full frame, and MFT bodies without switching tools.
• • Built-in presets for common sensor formats: Pick a preset for full frame, Canon APS-C, Nikon or Sony APS-C, MFT, 1 inch, 1/2.3 inch, or Medium Format 645 and the sensor width and height fill in automatically.
• • Millimetre and inch inputs accepted: Switch the Length Unit menu between millimetres and inches, so a single form handles European spec sheets in mm and US spec sheets in inches.
• • Reverse solver for sensor dimensions: Pick Sensor size from the Solve For menu to recover the sensor width and height from a known crop factor, useful when a spec sheet only quotes the multiplier.

Once the equivalent focal length is on screen, the Time Lapse Calculator turns the same lens and sensor data into the interval, total runtime, and storage budget for a time-lapse shoot.

Several real-world variables change the crop factor even when the body is the same. Knowing them explains why two cameras with the same marketing label can give different field of view.

• • The crop factor is a diagonal ratio, not an aspect ratio. A 1.5x crop factor on a 3:2 sensor and a 1.5x crop factor on a 4:3 sensor share the same diagonal but differ in width and height, so check the actual dimensions rather than relying on the multiplier.
• • The effective aperture is a depth-of-field equivalent, not a true optical property. The lens f-number does not change when the sensor changes; a 50mm f/1.8 lens still gathers the same amount of light, and the crop factor only changes how that light is projected onto the sensor.

According to Wikipedia, the Micro Four Thirds sensor measures 17.3 mm by 13.0 mm, giving a diagonal of about 21.64 mm and a crop factor of 2.0, the same value used as the MFT preset in the calculator.

Once the crop factor and field of view are on screen, the Data Storage Converter turns the resulting image dimensions into the raw file size budget for the shoot, so you know how many frames fit on a card.