Apertia · Library
What does crop factor describe without changing the lens focal length?
Apertia shows APS-C as ×1.5 and a 33 mm lens as a 51 mm full-frame equivalent, but every depth-of-field and angle result still uses the real 33 mm.
What the crop label is
Next to each sensor choice Apertia shows a crop factor, 43.27 mm divided by that sensor's diagonal, where 43.27 mm is the full-frame diagonal. Under the focal-length ring the calculator adds a line such as 51 mm full-frame equivalent, which is the real focal length multiplied by that factor and rounded. Both are labels for comparing framing between formats.
The calculations themselves never use the equivalent value. Depth of field, hyperfocal distance and the horizontal angle of view take the focal length you selected and the preset's own width, height and circle of confusion. Entering an equivalent focal length as the lens input would apply the crop twice.
Worked example
The 23.5 × 15.6 mm preset gives a ratio of about 1.534. The angle calculation still uses the actual 50 mm focal length.
- Preset diagonal ratio
- 1.53404
- Horizontal angle with the actual 50 mm input (degrees)
- 26.449102
Set up a cropped-sensor kit
- Choose the sensor preset that matches the camera: APS-C, APS-C (Canon), Micro Four Thirds, 1-inch, medium format or iPhone main camera.
- Select the focal length written on the lens, not its equivalent.
- Read the equivalent line only to compare with a full-frame habit, for example knowing that 33 mm frames like a 50 mm.
- Save the body and lens as a kit so the preset and real focal length stay together.
Worked example: three presets
The APS-C preset is 23.5 × 15.6 mm, so its diagonal is about 28.21 mm and 43.27 ÷ 28.21 = 1.534, shown as ×1.5. A 33 mm lens there is labelled 51 mm equivalent, and its horizontal angle, calculated from the real 33 mm and 23.5 mm width, is 39.2 degrees, close to the 39.6 degrees of a 50 mm lens on full frame. Micro Four Thirds gives a factor of 2, so 25 mm reads as 50 mm. The iPhone main-camera preset gives 3.541, so a 7 mm focal length reads as 25 mm.
- APS-C crop factor
- 1.534 (×1.5)
- 33 mm on APS-C
- 51 mm equivalent · 39.2°
- 50 mm on full frame
- 39.6°
- 25 mm on Micro Four Thirds
- ×2 · 50 mm equivalent
- iPhone main camera, 7 mm
- ×3.541 · 25 mm equivalent
Common mistakes
- Typing 75 mm for a 50 mm lens on APS-C because the box says 75 mm equivalent.
- Assuming equivalent focal length also gives equivalent depth of field at the same f-number.
- Using the full-frame preset for a camera that records in a cropped video mode.
- Treating the rounded ×1.5 label as exact in a calculation; the preset uses its full dimensions.
What the factor does not change
A crop factor compares diagonals only and does not alter the lens. The preset sizes are planning values, and the Nikon guide to focal length and angle of view listed below explains the same relation between sensor size and field of view.