Equivalence: Crop factor, aperture and sensitivity

Posted on by Mike Evans

We are most of us quite familiar with crop factor and the adjustment of focal length when using APS-C, Micro Four Thirds or 1in sensors. The maths is easy: 1.5, 2.0 and 2.7 covers these three options in relation to the 1:1 full-frame sensor. But what effect does sensor size have on aperture or sensitivity? It's all a question of equivalence.

Richard Butler, writing for DPReview, attempts successfully to explain equivalence, the way of comparing different sensor sizes in a readily understood manner. You can read the full article here. But just to whet your appetite, here are Richard's key points:

  • Multiply the actual focal length by the crop factor and you get the equivalent focal length.

  • Multiply the F-number by the crop factor and you get the equivalent aperture.

  • The equivalent aperture tells you what aperture on a full frame lens would give the same depth-of-field and the same total light as the one you're assessing.

  • F-numbers tell you about light intensity (how much light each square mm of the sensor sees). A larger sensor has more square mm collecting light.

  • F-numbers and ISO are sensor-size independent. Knowing the F-number is useful - but you need to remember that ISO100 on a small sensor won't be the same quality as ISO 100 on a larger sensor.