Equivalent f-numbers are a means of considering the combined effect of the aperture and sensor size.

In the same way that equivalent focal lengths describe the effect of sensor size on the field-of-view a lens gives, equivalent apertures describe the effect of sensor size on the aperture. (The underlying properties are not changed)

  • F-number = focal length/aperture diameter
  • Equivalent f-number = equivalent focal length/aperture diameter

Comparing equivalent apertures allows you to understand how much control a lens will give you over depth-of-field. It also gives a good idea of how low-light performance will compare between two cameras of different sensor sizes.

However, because the exposure and ISO system is, by design, independent of sensor size, equivalent apertures should only be used to understand camera/lens capability, not exposure.

For more information, read our more in-depth article on the subject.