Correction to: Nature Methods, published online 7 May 2018.

In the version of this Brief Communication originally published online, ref. 21 included details for a conference paper (Pegard, N. C. et al. Paper presented at Novel Techniques in Microscopy: Optics in the Life Sciences, Vancouver, BC, Canada, 12–15 April 2015). The correct reference is the following: Pégard, N. C. et al. Optica 3, 517–524 (2016). This error has been corrected in the print, HTML and PDF versions of the paper.