My mm f/ lens, on a sensor, is equivalent to a 157.5mm f/2.1 lens. The crop factor for that sensor is 1.5x.

💡 Useful Info About Crop Factors:

  • ◉ A crop factor is the multiplier that needs to be used to compare the full-frame equivalent focal length and maximum aperture of a lens when used on a different-sized sensor. That means that sensors that are smaller than a full-frame (35mm) sensor will crop out a part of the image that's received by the lens, effectively cropping the image. Calculating the effective focal length using the crop factor therefore allows you to determine the effective field of view for that lens when used on a camera with a smaller sensor.
  • ◉ In that sense, full-frame sensors are the lingua franca of sensors, meaning that they are the sensor size used as the reference point.
  • ◉ Larger sensors than full-frame, like medium format sensors, have a reverse crop factor. Cropped medium format sensors include sensors for Pentax and Fujifilm medium format cameras as well as the Hasselblad X1D.
  • ◉ The crop factor is also used on the aperture to give us the maximum effective aperture equivalent on a full-frame camera. That means that we can have a better idea of the amount of background blur and depth of field that a lens will be able to produce.
  • ◉ If the topic seems a little confusing or you want to learn more, I recommend this video by Tony Northrup.