Understanding Crop Factor and Equivalency
In the world of digital photography, the 'full-frame' sensor (based on the historical 35mm film format) is the benchmark for focal length. However, many modern cameras use smaller sensors, such as APS-C (found in many DSLRs and mirrorless cameras) or Micro Four Thirds (MFT). When you use a lens on a smaller sensor, the 'field of view' is narrowed, making the lens behave as if it has a longer focal length.
The Crop Factor
The Crop Factor is the ratio of the full-frame sensor's diagonal to the smaller sensor's diagonal.
- Full Frame: 1.0x
- APS-C (Nikon/Sony/Fuji): 1.5x
- APS-C (Canon): 1.6x
- Micro Four Thirds: 2.0x
The Formula
To find the full-frame equivalent, you simply multiply the physical focal length of the lens by the crop factor of the camera sensor.
Equivalent Focal Length = Physical Focal Length * Crop Factor
Depth of Field and Equivalency
It is important to remember that while the focal length is effectively 'cropped' to a narrower field of view, the physical properties of the lens do not change. A 50mm f/1.8 lens on a Micro Four Thirds camera (2.0x crop) will give you the field of view of a 100mm lens, but it will still have the depth-of-field characteristics of a 50mm lens. For a true 'equivalent' depth of field, you would also need to multiply the aperture by the crop factor (e.g. f/3.6 equivalent).